Statement on COVID-19 and International Travel

An Advisory Committee Statement (ACS)
Committee to Advise on Tropical Medicine and Travel (CATMAT)

Table of contents

Preamble

The Committee to Advise on Tropical Medicine and Travel (CATMAT) provides the Public Health Agency of Canada (PHAC) with ongoing and timely medical, scientific, and public health advice relating to tropical infectious disease and health risks associated with international travel. PHAC acknowledges that the advice and recommendations set out in this statement are based upon the best current available scientific knowledge and medical practices at the time of writing, and is disseminating this document for information purposes to the medical community caring for travellers.

Persons administering or using drugs, vaccines, or other products should also be aware of the contents of the product monograph(s) or other similarly approved standards or instructions for use. Recommendations for use and other information set out herein may differ from that set out in the product monograph(s) or other similarly approved standards or instructions for use by the licensed manufacturer(s). Manufacturers have sought approval and provided evidence as to the safety and efficacy of their products only when used in accordance with the product monographs or other similarly approved standards or instructions for use.

Key points/messages

Objectives

This statement is intended to provide guidance for health care providers advising patients on travel in the context of the COVID-19 pandemic.

Recommendations made in this guideline are done within the specific context of health protection for travellers. Policy and regulatory aspects of the COVID-19 response, such as requirements around vaccine passports and quarantine, are outside of the purview of this statement. Travellers should be made aware that factors outside of individual health might have a significant impact on their plans, and that these requirements are subject to change, sometimes with little notice. Travel advisors and their clients should, among other things, regularly verify any travel requirements in place at their destination(s) and for their return to Canada.

Methods

This statement was developed by members of a CATMAT working group (WG), none of whom declared a relevant conflict of interest. The WG and broader committee chose to develop the advice contained herein based on a narrative review of the evidence rather than a formal evidence-based methodologic approachFootnote 1. This reflects, among other things, the need for timeliness and the rapidly changing evidence related to prevention of SARS-CoV-2. The final statement and recommendations were approved by CATMAT.

Epidemiology

Coronavirus disease 2019 (COVID-19) is caused by the novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first recognized in Wuhan, China in December 2019. The epidemiology of COVID-19 continues to evolve. Part of this evolution has been the emergence of SARS-CoV-2 variants of concern (VOCs) and variants of interest (VOIs). It is expected that variants will continue to emerge as time goes on. Understanding the impact of these variants on transmissibility, COVID-19 disease severity, and the effectiveness of existing diagnostics, therapies, and vaccines is of crucial importance today, and will remain so into the future.

Up-to-date information on the global epidemiological situation is available through the World Health Organization (WHO)Footnote 2. The WHO COVID-19 Weekly Epidemiological Update also provides a summary on the global epidemiologic picture as well as current and emerging knowledge related to VOCs and VOIs.

Clinical manifestations

The clinical presentation of COVID-19 is highly variable and has been well described elsewhere.

Asymptomatic and pauci-symptomatic infections are common, and play an important role in transmission. However, the prevalence of such infections remains a subject of ongoing discussion/research, with significant heterogeneity between studiesFootnote 3. As a general rule, clinical illness is more likely to occur in older persons; indeed, many infections in younger people seem to be unapparent or associated with mild disease. Importantly, transmission is also very common in the days prior to symptom onset, regardless of the intensity of the eventual symptoms.

An area of significant uncertainty and interest is post-acute manifestations of COVID-19. Specifically, some persons, including those with mild illness during primary infection, suffer new or ongoing symptoms or clinical presentations following the acute phase. Current scientific knowledge on post-acute sequelae is limited, and research is ongoing to better understand the long-term effects of infection and possible risk factorsFootnote 4,Footnote 5.

Transmission

The median incubation period of COVID-19 (the time between exposure to SARS-CoV-2 and onset of symptoms) is 5 to 6 days, but can range from 1 to 14 daysFootnote 6,Footnote 7. The primary mode of SARS-CoV-2 transmission is through the respiratory routeFootnote 7,Footnote 8,Footnote 9,Footnote 10. Virus-laden respiratory emissions are produced through actions such as breathing, talking, coughing, sneezing, singing, shouting, or other vocalizations. Transmission can occur through the air, especially in poorly ventilated and highly contaminated environments, including over some distance. Nevertheless, proximity is a key factor in transmission risk. Contact with contaminated fomites (that is, contact with contaminated surfaces) is not considered to be important to the sustainment of pandemic transmissionFootnote 11. This conclusion is based largely on the success of interventions directed only at the prevention of inhaled particles, despite the demonstrated ability of the virus to survive in the environment. Even so, for the individual, the risk of fomite-associated transmission can be greatly reduced by undertaking routine sanitation measures, such as proper hand hygiene and routine cleaning of surfaces. Further, these approaches are considered important for reducing the risk of other travel-associated infections, particularly faecal-oral pathogens that can cause enteric illnesses.

Assessing individual travel associated SARS-CoV-2 risk

There are a litany of considerations that might impact the decision to travel (or not) in the face of the SARS-CoV-2 pandemic. Many are structural, including travel and border regulations, vaccination requirements, and pre-departure testing. This section does not consider these, nor does it consider indirect but very important impacts of individual travel on others, for example the situation where a healthy traveller might act as a SARS-CoV-2 vector for someone who is more vulnerable, or the potential for travel-associated importation of VOCs.

The focus of the present guidance is on individual-level risk assessment related to SARS-CoV-2 and related outcomes. Emphasis is placed on acute and serious COVID-19-related outcomes, such as hospitalization and/or death.

CATMAT acknowledges that there is a constellation of other outcomes of potential concern, including but not limited to long-term post-infection sequelae. These have been reported in children and adolescents as well as adults, both in persons who were very ill with COVID-19 and those who were not. Unfortunately, knowledge and evidence related to such conditions remains inconsistent and somewhat ill-defined, though this is changing as research interest intensifies. Importantly, while severe and acute outcomes are emphasized in this guideline, awareness that infection comes with other risks is something that the clinician and traveller should bear in mind. Further, while the focus of the current guidance is on individual-level risk to the traveller, responsible travel and consideration of the risks posed to destination countries and Canada (through introduction of variants, for example) and more vulnerable contacts should be included in the broader discussion about potential health risks and impacts when considering travel.

CATMAT suggests assessing risk by integrating a traveller-specific exposure assessment with a traveller-specific host assessment. Combined, these yield a risk estimate expressed below as an integrated risk matrix (Table 1). This methodology is generally consistent with approaches applied in other recommendations developed by CATMATFootnote 12,Footnote 13.

Table 1. Risk matrix for international travel in a COVID-19 environment
  Host Assessment
Exposure Assessment Low Moderate High Very high
Low Low Moderate Moderate Moderate
Moderate Moderate Moderate High High
High Moderate High High Very high

In Table 1, different factors can contribute to a lower or higher assessment of risk for the individual. For exposure assessments, health advisors should first consider the exposure risk inherent to the destination country or countries, and then may increase (shift down) or decrease (shift up) the exposure assessment as necessary, depending on the anticipated activities and other factors. The host assessment first considers the traveller's age and is adjusted from there based on other individual-level factors; for example, a traveller with diabetes would have their host assessment shifted to the right by one level from their initial age-based level (indicating an increase in the risk of severe SARS-CoV-2 related harms). In a very approximate fashion, each shift by one level represents a 2-4 fold change in risk. Adequate protection from vaccination (see Box 1 for definition) would generally shift the risk level for the traveller to the left by one level (indicating a decrease in the risk of severe SARS-CoV-2 related harms). A summary table of these factors, and how the host and exposure assessment levels may be adjusted accordingly, is provided in Table 6.

The risk matrix and accompanying recommendations are not a surrogate for local, regional, country-level or international travel or public health requirements. Rather they are intended as an adjunct to these, to be used by the health care advisor who is taking on the role of advising patients on travel during the COVID-19 era.

Risk levels on the matrix are expressly linked to recommendations (Table 7). To be clear, these are not GRADE-based recommendations, rather they represent expert opinion based on current knowledge related to SARS-CoV-2.

Finally, the matrix does not consider values and preferences of the traveller. Given that travellers will have divergent COVID-19 risk tolerances, it follows that their acceptance for travel recommendations will vary. It is in this decision-space that travel-related decisions will be made, and where health care providers can help travellers make informed choices.

Exposure assessment

Exposure assessment refers to the likelihood that travellers will be exposed to (but not necessarily infected with) SARS-CoV-2. A thorough review of the planned travel itinerary is necessary to appraise the likelihood of exposure. The baseline for the exposure assessment should be assigned based on the estimated transmission levels at the destination (low, moderate or high/unknown, see Table 2).

Table 2. Exposure assessment, baseline determined by the estimated SARS-CoV-2 transmission levels at the destination(s)
Exposure assessment Estimated SARS-CoV-2 transmission level at destination
Low Low level of SARS-CoV-2 transmission
Moderate Moderate level of SARS-CoV-2 transmission
High High or unknown level of SARS-CoV-2 transmission

Estimated transmission levels at the destination(s)

The CATMAT Secretariat provided technical support in developing an interim approach for estimating SARS-CoV-2 transmission levels at a destination. This approach uses a combination of indicators of the destination's epidemiological situation (for example, incidence rates, testing rates, percent positivity, trends) and additional situational awareness (see Appendix for more detail and results). The result of this assessment is the categorization of a destination's estimated SARS-CoV-2 transmission as being at a low, moderate, high, or unknown level. CATMAT recommends using the estimated transmission level at the destination as the baseline for the exposure assessment.

If several countries are to be visited, then the exposure assessment and resulting recommendations should reflect the entire planned travel itinerary. Recommendations might vary for different segments of the itinerary. In the case of cruise ship travel, CATMAT recommends that the baseline risk of exposure be based on the estimated transmission levels in the country from which the cruise is departing. While it is impossible to ascertain the exact level of transmission in this scenario, the best surrogate for the baseline risk of exposure is the country of departure. If travellers will be disembarking in other countries for significant periods of time, or participating in risky activities while travelling on a cruise, consider applying the same guidance as above regarding multiple itinerary segments.

The baseline risk of exposure may then be adjusted (shifted up or down) from the destination-based assessment, based on the factors identified below.

Estimated transmission in the sub-region(s) to be visited

Sometimes, there is subnational variation in transmission estimates for a country. For example, such variability is well monitored and data are easily available in Canada, the United States and the United Kingdom. As appropriate, this information can be applied to refine the exposure assessment. When the epidemiological situation for a sub-region is known to be worse than the transmission levels at the national level, providers may consider increasing the risk level for the exposure assessment by one level. In general, and recognizing this reflects a more conservative approach, CATMAT suggests the opposite not be applied; in other words, the exposure level should not be reduced if there is less transmission in a particular sub-region, relative to the rest of the country.

Duration of travel

Longer duration travel, other things being equal, will be associated with increased cumulative likelihood of exposure(s).

Epidemiologic trends

Where there is information that the epidemiologic picture in a location might be shifting in an important way, for example because of expanding spread of a VOC, consider increasing the exposure assessment by one level. In general, CATMAT currently advises against reducing the exposure level from the initial assessment, based only on promising information.

Types of activities

The types of activities and actions that individuals undertake will also influence the likelihood of exposure to SARS-CoV-2. In general, exposure likelihood increases with an increasing number of contacts, increasing duration of contact, decreasing proximity between individuals, and poor ventilation (in indoor environments). Proper ventilation involves the replacement of indoor air with outdoor air, which helps to reduce the concentration of viral particles, or recirculating air through a high-efficiency particulate air (HEPA) filterFootnote 14,Footnote 15.

For example, attending indoor mass gatherings would be associated with a greater risk of exposure than would hiking with a small group of co-travellers. Likewise, if indoor (or outdoor) gatherings involved actions that were more likely to broadly disseminate virus (for example, singing, shouting, cheering, exertion during exercise) they would be considered to pose a greater exposure likelihood than actions that did not (for example, reading the newspaper, standing quietly in line, shopping in a supermarket).

Table 3 provides a non-exhaustive list of activities characterized by their anticipated exposure level. Where possible, the full spectrum of activities planned by the traveller should be explored. If most or all of their activities are likely to place them at a lower likelihood for exposure, then consider decreasing the exposure assessment by one level from the initial destination-based exposure level. Conversely, the opposite applies if some or many of their activities could place them at relatively higher risk for exposure (i.e. increase the exposure assessment by one level).

Table 3. Types of activities that are considered to be at relative lower or greater likelihood for exposure to SARS-CoV-2
Lower likelihood Medium likelihood Higher likelihood

Lower density outdoor environment. Distancing possible.

Examples: Trekking in a wilderness area, playing golf, individual exercise outdoors, walking in uncrowded areas.

Impact: If most exposure will be of this type, consider reducing the exposure estimate by one level from the destination-based exposure risk (e.g., from high to moderate) [Note: individual events with "high" potential for exposure obviate this, see adjacent]

Lower density indoor activities. Distancing possible, perhaps improved ventilation (can include conveyances).

Examples: Shopping at a mall, attending a small language class, visiting an office building, purchasing groceries, travel on a major/modern airline/aircraft, travel in an uncrowded taxi with open windows, a tour on an uncrowded open-topped bus.

Higher density outdoor activities. Potentially limited distancing.

Examples: Attending an open-air party/concert, visiting a crowded beach, swimming in a public pool, crowded open air bus tours, shopping at a crowded outdoor market, walking in a busy urban environment.

Impact: If most exposure will be of this type, no change to exposure estimate [Note: individual events with "high" potential for exposure supersede this, see column to the right]

Higher density indoor activities. Often limited distancing, many people, limited ventilation (can include conveyances), actions that facilitate virus dispersal (e.g., singing, shouting, cheering, loud talking, exertion during exercise).

Examples: Eating at a buffet, attending a large conference, attending a large indoor music concert, attending a sports event at an indoor stadium, attending a large indoor service, going to a busy indoor bar/restaurant, attending an indoor fitness class/crowded gym, travel in a crowded and closed and/or poorly ventilated conveyance (e.g., a local bus/minibus).

Impact: If there is potential for a significant amount of exposure of this type (even if it reflects a single event, e.g., attending a "crowded" indoor wedding), consider increasing exposure estimate by one level from the destination-based exposure risk (e.g., from moderate to high)

Vaccination status of likely contacts

In some circumstances, there will be confidence that most or all potential contacts are adequately protected with vaccine (see Box 1 for definition), for example, on a cruise where travellers and crew are obligated to provide documentation of vaccination as a condition of embarkation, or at venues where proof of vaccination is required for entry. In this situation, it is reasonable to decrease the exposure assessment by one level (as in, shift up one row in the risk matrix). This should be considered an exceptional circumstance, in that it only applies where there is not regular mixing with persons outside of the vaccinated "cohort" at the "event".

Box 1. Defining adequate protection, for the purposes of the risk assessment

For the purposes of risk assessment, an individual may be adequately protected by vaccine if:

  • They have received a full series of COVID-19 vaccine, according to recommended schedules.
  • They have received a partial series with a Health Canada authorized vaccine following a previous SARS-COV-2 infection (confirmed by PCR).

By COVID-19 vaccine, CATMAT is referring to any COVID-19 vaccine products authorized for use in Canada according to recommended schedulesFootnote 16or a non-Health Canada authorized vaccine (one or two doses) followed by the provision of an additional dose of mRNA vaccineFootnote 17. Medical advisors should keep apprised of the latest guidance from NACI/PHAC, as considerations for adequate protection may evolve over time.

The term adequately protected by vaccine used in this document for the purpose of travel risk assessment may not be entirely consistent with NACI guidance, with the vaccination requirements in other countries, nor with requirements for exemption from border measures such as quarantine upon return to Canada, particularly for individuals with a previous PCR-confirmed SARS-CoV-2 infection who have received a partial series of vaccine. When advising patients on travel, providers should apprise travellers of this distinction, and that they will need to monitor the requirements for vaccination at their destination(s) and for their return to Canada.

Judgement

CATMAT acknowledges that while the baseline for the exposure assessment is based on quantitative estimates of transmission level, the assessment is then adjusted based on various qualitative factors, and should always be applied respecting the specific context. Judgement, common sense, and flexibility are required. For example, travelling to an area where there is virtually no SARS-CoV-2 transmission should not normally require further activity-based adjustments to the exposure assessment. Conversely, engaging in activities with a lower likelihood of exposure to SARS-CoV-2 should not necessarily prompt a decrease in the exposure risk assessment if travel will be to an area experiencing a surge in transmission. Indeed, in the latter situation the health care advisor might consider increasing the exposure assessment by one level based on the shift in the epidemiologic picture. Box 2 provides examples of exposure assessments.

Box 2. Examples of exposure assessments

Exposure likelihood decreased from the initial destination-based exposure assessment

A couple is travelling to a South American country by air for a two-week trekking trip. They plan to stay in the country's populous capital for several days, after which they will join a cohort of travellers (from a low transmission country) that will remain together for the trek. Accommodations will not be shared with others in the travel cohort, but there might be shared indoor meals.

In this example, the country is considered a high-risk country. The planned activities involve actions that potentially could include increased exposure likelihood, particularly to other participants on the tour. The tour group effectively comprise an imperfect cohort (some mixing with local populations) however, much of the trip will be spent outside in a "low density" environment.

Air travel in a modern commercial aircraft is not considered to be a higher likelihood exposure.

Consider decreasing the exposure assessment by one level to moderate, if the large majority of exposure will be with others in the low risk travel cohort, in a low density setting and there are no planned higher likelihood activities (see Table 3).

Exposure likelihood increased from the initial destination-based exposure assessment

A family is travelling to a Western country to visit relatives. They plan to attend indoor sporting events and expect to take advantage of the largely re-opened economy to enjoy themselves at venues such as restaurants and amusement parks.

In this example, the country is considered a moderate-risk country, with respect to SARS-CoV-2 transmission levels. The planned activities involve actions that potentially could include increased exposure likelihood.

Consider increasing the exposure assessment by one level to high.

Host assessment

Host assessment refers to the likelihood that travellers will suffer significant SARS-CoV-2 infection-related harms.

Vaccination status

Setting aside host-based predispositions towards serious harm if infected, the most important modifiable risk factor for protection against COVID-19 is vaccination. In this respect, the current evidence suggests that COVID-19 vaccines approved and used in Canada provide high levels of protection against severe diseaseFootnote 18. Protection against symptomatic infection also appears to be robust, but might be more variable, especially in the context of VOCsFootnote 18.

For most persons deemed to be adequately protected through vaccination (see Box 1 for definition), CATMAT suggests that the host assessment be reduced (shifted to the left) by one level, from the baseline age-based assessment (see Table 4).

This would mean, for example, a person otherwise considered at high likelihood of significant SARS-CoV-2 infection-related harms based on the host assessment, would be considered at moderate likelihood, if adequately protected by vaccine.

CATMAT recommends that, in general, NACI guidance on the use of COVID-19 vaccines be followed, including for travellers. However, there are unique travel-related circumstances related to vaccination. These are considered in the sections below.

In addition to protection against disease, vaccination acts to reduce transmission, both by preventing infection altogether and by reducing onwards transmission in the case of breakthrough infectionsFootnote 19. This remains an area of active research interest, with overall transmission reduction being lower for the Delta variant, but higher in those receiving an additional dose of vaccine after the initially recommended seriesFootnote 20. For these reasons, letters for exemption from vaccination are discouraged. This is not only for the safety of travellers and their companions, but also as this represents a risk of importation into the destination country.

Variants of concern (VOCs)

At the time of writing, and recognizing that there are many unknowns related to vaccine performance against VOCs, the evidence is suggestive that a full vaccine series with a Health Canada authorized COVID-19 vaccine provides a high level of clinical protection against severe disease associated with most VOCs, including Delta. As this is an evolving area of research, NACI guidance on use of COVID-19 vaccines should continue to be monitored, as the epidemiology and evidence pertaining to VOCs and COVID-19 vaccines changesFootnote 16,Footnote 18.

Full or partial series

CATMAT acknowledges that a partial vaccine series provides substantial protection against earlier SARS-CoV-2 strains, such as the ancestral strain, as well as the Alpha variant (previously called B.1.1.7). However, there is less certainty about protection against VOCs such as Delta, particularly regarding symptomatic illness and asymptomatic infection, as well as the ability to prevent transmission if infectedFootnote 18. For this reason, for the sake of simplicity, and in recognition that the likely policy framework for many/all jurisdictions will be based on receipt of a full vaccine series, CATMAT suggests that the host assessment generally only consider persons as adequately protected by vaccine if they meet the criteria outlined in Box 1 (see below for more detail on individuals with a previously confirmed SARS-CoV-2 infection).

How to advise travellers who have been confirmed to be previously infected with SARS-CoV-2

Current evidence suggests that individuals known to be previously infected with SARS-CoV-2 are likely to have at least moderately durable protection, for six months or longer, against the SARS-CoV-2 variant associated with the original infectionFootnote 21,Footnote 22. There is less certainty about protection against VOCs. At this time, there are little data to allow a comparison of the duration or effectiveness of vaccine induced immunity versus natural immunity. In vitro, post-vaccination antibody levels to spike protein are usually much higher than in convalescent plasma, while those with natural immunity have a broader range of antibody response.

At the time of writing, NACI continues to recommend that a complete series of a COVID-19 vaccine may be offered to individuals in the authorized age group, without contraindications to the vaccine, who have had previous PCR-confirmed SARS-CoV-2 infectionFootnote 16,Footnote 18. CATMAT endorses this guidance, and further suggests that such persons not be considered as adequately protected for the purposes of the host assessment based solely on evidence of previous infection. CATMAT also recommends that previously infected individuals with a partial series should continue to be advised to complete their COVID-19 vaccine series, according to recommended schedulesFootnote 16, prior to travel.

There is a growing body of evidence demonstrating that individuals with previous infection who receive a single dose of vaccine generate a comparable immune response to individuals without previous infection who receive two doses of a vaccine. As such, NACI acknowledges that based on current immunogenicity evidence, it is possible that one dose of COVID-19 vaccine for individuals who have had a previous SARS-CoV-2 infection may adequately protect against COVID-19Footnote 18.

For this reason and for the purposes of the risk assessment only, CATMAT considers that previously infected individuals who have received a partial series of Health Canada authorized COVID-19 vaccine may be adequately protected in the host assessment (see Box 1 for more detail). In other words, CATMAT judges that this provides sufficient protection to justify, for the individual traveller, decreasing the host assessment (shifting to the left) by one level. This guidance may change if supported by information on clinical protection, particularly as comparative vaccine effectiveness data between these groups (one dose in previously infected individuals compared to a complete series in individuals without prior infection) are lacking, and protection against most VOCs is unknown at this timeFootnote 18. Of note, SARS-CoV-2 infection following a first dose of vaccine provides an unknown degree of protection. A second dose of vaccine would be required in these circumstances to be considered adequately protected at this time.

Though individuals who have received one dose of COVID-19 vaccine following a previous confirmed SARS-CoV-2 infection may be adequately protected for the purposes of the risk assessment, travellers should be advised that this may not be consistent with the vaccination requirements in Canada and other countries, nor with requirements for exemption from border measures such as quarantine upon return to Canada. Travellers will need to monitor the requirements for vaccination at their destination(s) in addition to the latest Canadian requirements.

Vaccination and quarantine/testing requirements in Canada

The definition of what constitutes adequate protection by vaccine, that is applied in this guidance, may differ from that which will be applied to policy around travel and quarantine requirements or other border measures for vaccinated individuals departing from or returning to Canada. Travellers should be made aware of the current requirements in Canada, including relevant provincial/territorial and local legislation, regulations, and policies, and that these could change during their travel period.

Host "risk" factors

Age

Age, as a risk factor, has the most profound impact on the likelihood of developing the most serious COVID-19 outcomes (for example, ICU admission, death). Importantly, age is confounded with other risk factors, particularly co-morbidities such as diabetes, obesity, and cardiovascular disease, such that the independent impact of age is reduced in adjusted analysesFootnote 23,Footnote 24,Footnote 25. Nonetheless, public health authorities (including Canadian at the federal, provincial, and territorial levels) use age as a fundamental baseline for assessing host vulnerability to, and consequent recommendations for prevention of, COVID-19; CATMAT has adopted this approach (see Table 4).

Though presented as finite groupings in the host assessment table, there is also within group variation in the likelihood of severe disease, with the likelihood of severe outcomes being generally higher among older age groups, relative to younger ones (for example, the likelihood of severe disease is substantially lower among healthy 30-year-olds as compared to healthy 59-year-olds). Given the evolving evidence regarding risk of severe outcomes from COVID-19 in infants under 1 year, this pediatric population should be excluded from the age-based risk assessment. Health professionals should consider these limitations when advising individuals travelling with children under 1, and take care to remind travellers that travel with infants may be higher risk for several reasons, including lower immunologic development and experience in this population and access to appropriate care and treatment should they become ill (COVID-19 related or otherwise).

An excellent analysis of the relationship between age and COVID-19 is available in the NACI Recommendations on use of COVID-19 Vaccines. A relevant excerpt from the guideline is:

"The review by the Alberta Research Centre for Health Evidence (ARCHE) found strong evidence (of moderate or high certainty) for at least a 2-fold increase in mortality from COVID-19 with age 60-69 years versus <60 years.

A previous review by ARCHE found a moderate certainty of evidence for at least a 5-fold increase in mortality and hospitalization with age over 70 years (versus 45 years and younger).

Studies treating age on a continuum or across small increments consistently found that risks for hospitalization and mortality increased with increasing age (e.g., approximately 2-6% and 5-10% relative increase in risk per year)".

Table 4. Host assessment, baseline level determined by age (in years) at the start of travel
  Host Assessment
Low Moderate High Very high
Age (in years) at start of travel 1 to 29 30 to 59 60 to 79 80+

In general, if a person is considered to be adequately protected by vaccine (see Box 1), CATMAT suggests that the host assessment be reduced (shifted to the left) by one level, from the baseline age-based assessment. For persons with identified risk factors (see Table 5), increase the host assessment (shift to the right) by one level from the age and vaccination-based level.

Children less than 12 years of age

At the time of writing, none of the COVID-19 vaccines currently authorized in Canada are indicated for use in children less than 12 years of age, though authorization for use in children aged 5 to 11 years is forthcoming. Barring any pre-existing medical conditions, this group is considered to be at very low risk for severe COVID-19 associated outcomesFootnote 26,Footnote 27,Footnote 28. Even though children are at very low risk of severe outcomes, even if unvaccinated, they may act as reservoirs for transmission. Furthermore, the risks of prolonged symptoms (also referred to as "long COVID") following infection are not well quantified in children at this time. Hence, care should be taken to protect children and others around them from infection through assiduous use of personal preventive measures, when in situations with higher transmission risk, such as crowded or poorly ventilated settings. In the absence of vaccination, recommendations as outlined in Table 7 continue to apply to children as well as adults. Where mixed age groups are travelling together, vaccination of those who are eligible acts as an important adjunctive measure in providing some indirect protection to younger children and any other groups that are unable to be vaccinated or, that will not derive the same degree of individual protection from vaccination.

Though many gaps remain in understanding COVID-19 in children, current evidence suggests that risk factors for severe disease include underlying medical conditions such as type 1 diabetes, cardiac and circulatory congenital anomalies, and obesityFootnote 29; some of which are similar to those risk factors observed in adults (see Table 5)Footnote 24,Footnote 25. As such, CATMAT recommends that the same approach (to increase the host assessment level from the initial age-based assessment based on presence of other risk factors) be applied when assessing children.

In addition to the aforementioned health considerations, families travelling with children should also be aware that countries may have differing requirements for children under 12, and in some cases, the same entry, quarantine, and testing requirements may apply as for other unvaccinated travellers.

Other risk factors

The impact of other risk factors on the most serious COVID-19 outcomes is an area of ongoing investigation with a plethora of available peer-reviewed and pre-peer review evidence available. A rapid review commissioned by NACI and undertaken by the ARCHEFootnote 24,Footnote 25 synthesized the available evidence based on the quality of evidence (using a GRADE-like approach) and magnitude of effect. In their guidance, NACI has extracted the ARCHE review and summarized host-based factors as per the below Table 5. Importantly, as pointed out by NACIFootnote 18:

"The ARCHE review found strong evidence (of moderate or high certainty) for at least a 2-fold increase in mortality from COVID-19 with a small number of medical conditions (classified as Level 1…). The review found a low certainty of evidence for at least a 2-fold increase in mortality from COVID-19, and/or a low or moderate certainty of evidence for at least a 2-fold increase in hospitalization for a longer list of medical conditions (classified as Level 2).

A moderate certainty of evidence of at least a 2-fold increase in hospitalization and mortality from COVID-19 in people living with two or more medical conditions was found. However, there is no direct evidence on the combination of medical conditions that increase this risk.

Caution should be taken when interpreting evidence of low certainty (e.g., for medical conditions listed as Level 2). As evidence accumulates, observed associations may change. For example, a previous rapid review by ARCHE found low certainty evidence for at least a 2-fold increase in hospitalization or mortality for males, people with liver disease, and people with heart failure. As evidence has accumulated, there is now stronger evidence for little-to-no increased association of severe outcomes in these populations. The list of medical conditions included [see Table 5] may not be comprehensive as it is based only on evidence from published studies included in the ARCHE review."

Table 5. Host factors associated with increased risk for severe outcomes from COVID-19Footnote 24,Footnote 25 (extracted from the NACI Recommendations on use of COVID-19 Vaccines)
Increased risk of severe outcomes from COVID-19 (hospitalization/mortality)

Increasing age (strong evidence)
(based on moderate certainty of evidence of ≥2-fold increase in mortality)

  • ≥60 years (particularly ≥ 70 years)

Medical conditions - Level 1 (strong evidence)
(based on moderate or high certainty evidence of ≥2- fold increase in mortality)

  • Down syndrome
  • End-stage kidney disease
  • Epilepsy
  • Motor neuron disease, multiple sclerosis, myasthenia gravis, Huntington's diseaseFootnote a
  • Type 1 and 2 diabetes

Medical conditions - Level 2 (limited evidence)
Level 2a (based on low certainty of evidence of ≥2-fold increase in mortality)

  • Cerebral palsy
  • Major psychiatric disorder (schizophrenia, schizoaffective disorder, or bipolar disorder); in combination with prescription drug use for the condition in the past 6 months
  • Obesity class III (BMI ≥40 kg/m2)
  • Parkinson's disease
  • Sickle cell disease or severe immunodeficiency, transplant (any type)
  • Solid organ transplant
  • Recent bone marrow or stem cell transplant
  • Metastatic cancer
  • Recent/current chemotherapy or radiotherapy

Level 2b (based on low or moderate certainty of evidence of ≥2-fold increase in hospitalization)

  • Previous cerebrovascular accident
  • Pregnancy (any stage)
  • Frailty (among community and non-community dwelling people; measured on scales that include items such as weight loss, exhaustion, physical activity, walking speed, grip strength, overall health, disability, presence of disease, dementia, falls, mental wellbeing)
  • Vasculitis
  • Obesity - all classes (BMI >30 kg/m2)
Footnote a

These conditions were grouped within a single study; evidence for the individual conditions is either unavailable or of lower certainty.

Return to footnote a referrer

CATMAT suggests that the risk factors identified in the NACI guidance be used to modify the host assessment in much the same way as was used for the exposure assessment. However, for the host assessment, the adjustment will be unidirectional (as in, it may only result in a shift towards a higher risk level). Generally, CATMAT recommends increasing the host risk assessment (shifting to the right) by at least one level if the traveller falls into any of the categories identified in Table 5. It is recognized that this is a conservative approach, as some or all of the risk associated with the other identified individual risk factors is likely already factored into the age-based groupings.

In the situation that the health advisor considers the traveller to be especially vulnerable to COVID-19 (independent of age), such as due to the presence of multiple risk factors or particularly severe medical conditions, consider shifting the host assessment to the right by two levels (indicating a larger increase in the risk of severe SARS-CoV-2 related harms). Box 3 provides examples of host assessments.

Due to the paucity of evidence on the efficacy and effectiveness of vaccination in preventing severe outcomes from COVID-19 among populations with known immune suppression, CATMAT recommends a conservative approach in applying the risk matrix assessment to these individuals. Many or all will derive some protection from vaccination and it is strongly advised that NACI guidance on the use of COVID-19 vaccines in this population continue to be followed and closely monitored as the evidence evolves on how to optimize protection in this diverse populationFootnote 16,Footnote 30.

If the traveller has any of the immune suppressive conditions included in Table 5, CATMAT maintains the recommendation to increase the host risk assessment (shift to the right) by at least one level. For immune suppressive conditions, or for immune suppressing medications, the clinical implications will vary significantly among travellers and should again be subject to individual assessments and clinical discretion. The effectiveness of COVID-19 vaccines is not well known and may be reduced in these individuals. As such, immune suppressed travellers may be at risk both due to their underlying disease and because their protection from vaccine may be reduced. Accordingly, providers may consider during an individual assessment whether the host risk assessment should be increased (shifted to the right) by more than one level. In some cases and based on consultation with a medical expert, it may be appropriate to exercise caution, and treat these individuals in the host assessment as not being adequately protected by vaccine, regardless of their actual vaccination status. Though not specific to COVID-19 vaccines, health care advisers may also refer to Immunization of immunocompromised persons in the Canadian Immunization Guide, Part 3 - Vaccination of Specific Populations, for further discussion and considerations regarding immunization of immunocompromised individuals.

Box 3. Examples of host assessments

Likelihood of severe outcomes decreased from the initial age-based host assessment

A 70-year old individual who is healthy and without identified risk factors is planning to travel. The patient has received a complete series of COVID-19 vaccine, with a product authorized for use in Canada and thus, is considered adequately protected (see Box 1).

This patient is at increased risk for severe COVID-19 if infected, by virtue of their age. They should be carefully screened for other risk factors. If none are identified, due to their vaccination status, consider reducing their host assessment by one level from their aged-based risk (moving them from high to moderate).

Likelihood of severe outcomes increased from the initial age-based host assessment

A 40-year old person with Down syndrome is planning to travel. The patient also has diabetes, and is adequately protected by vaccine (see Box 1 for definition).

This patient is at increased risk for severe COVID-19 if infected based on identified medical conditions. As identified in the ARCHE review, there is moderate quality evidence to indicate that Down syndrome is associated with a large relative risk increase for COVID-19 associated death. Given this, and the presence of diabetes, consider adjusting the host assessment by two levels based on risk factors. However, as the individual is adequately protected by vaccine (which can reduce the risk by one level), full adjustment will result in an increase of only one level, from moderate to high.

Access to appropriate medical care

Travellers, particularly those individuals deemed to be at moderate or high risk of severe outcomes who must travel, should also consider the availability and potentially high cost of adequate medical services in the destination country, should they become infected and require urgent care. Travellers may experience difficulties acquiring transportation if they must be transported elsewhere to receive suitable care. Furthermore, any travel and treatment costs incurred might not be covered by travel-specific insurance.

Furthermore, should travellers become infected while travelling, they may experience delays in returning home to Canada, which may be costly. In the scenario of poor or limited access to these services, health care professionals advising these individuals may consider recommendations from a higher risk category, where appropriate.

Integrated risk assessment

Combining the exposure and host assessments yields a risk assessment matrix as shown in Table 1. Risk matrix for international travel in a COVID-19 environment. Host and exposure assessments consider a variety of factors (see above) and risk levels can be adjusted (as in, increased or decreased) based on the attributes of the traveller and their travel plans (for example, vaccination status, risk factors for severe disease, and exposure in environments conducive to transmission). A summary table of these factors and how the host and exposure assessment levels may be adjusted accordingly is provided in Table 6.

Table 6. Summary of traveller (host) attributes and travel plans (exposures), and their application in the risk matrix
Exposure assessmentFootnote a,Footnote b,Footnote c Host assessmentFootnote e,Footnote f

Consider increasing exposure assessment by at least one level, if:

  • The epidemiological situation for a sub-region is known to be worse than the national level transmission.
  • There is a shifting epidemiological situation that could increase risk of SARS-CoV-2 exposure (e.g. increasing spread of VOCs)
  • Some/many activities in the planned itinerary place the traveller at a relatively higher risk for exposure (see Table 3).

Consider increasing host assessment by at least one level, if:

  • The traveller has any of the medical conditions listed in Table 5 or, is otherwise assessed by a medical professional as being at increased risk of severe outcomes (e.g. due to other immunosuppressive conditions)Footnote g.
  • The traveller is at moderate or high risk of severe outcomes, and will have poor or limited access to health care services at their destination.

Consider decreasing exposure assessment by one level, if:

  • Most/all activities in the planned itinerary place the traveller at a lower likelihood for exposure (see Table 3).
  • There is confidence that most potential contacts will be adequately protected by vaccine (for example, events/activities where proof of vaccination is required)Footnote d.

Consider decreasing the host assessment by one level, if:

  • The traveller is adequately protected by vaccine (see Box 1 for definition)Footnote h.
Footnote a

Exposure assessment refers to the likelihood that travellers will be exposed to (but not necessarily infected with) SARS-CoV-2.

Return to footnote a referrer

Footnote b

Baseline for the exposure assessment will be assigned based on the estimated transmission levels at the destination(s).

Return to footnote b referrer

Footnote c

The exposure assessment, while based on quantitative estimates of transmission level, is qualitative, and should always be applied respecting the specific context. Judgement, common sense and flexibility are required.

Return to footnote c referrer

Footnote d

This should be considered an exceptional circumstance, and only applies to situations where there is not regular interaction with persons outside of the vaccinated "cohort" or event.

Return to footnote d referrer

Footnote e

Host assessment refers to the likelihood that travellers will suffer significant SARS-CoV-2 infection-related harms.

Return to footnote e referrer

Footnote f

Baseline level for the host assessment is based on the age (in years) at the start of travel (see Table 4).

Return to footnote f referrer

Footnote g

If the traveller is considered to be especially vulnerable to COVID-19 (independent of age), e.g., due to multiple or particularly severe risk factors, consider increasing the host assessment (as in, shifting right in the matrix) by two levels.

Return to footnote g referrer

Footnote h

The effectiveness of current vaccines in certain immune suppressed individuals is currently unknown and may be suboptimal. To account for uncertainty in the effectiveness of current vaccines in these travellers, in some cases and based on consultation with a medical expert, it may be appropriate to exercise further caution, and treat these individuals in the host assessment as not being adequately protected by vaccine, regardless of their true vaccination status.

Return to footnote h referrer

Other considerations

Vaccination outside of Canada

In general, CATMAT advises against receipt of vaccines not authorized in Canada, or offered in international locations where vaccine storage and handling may not reflect Canadian standards. Rather, emphasis should be placed on receiving a full series with (a) vaccine(s) authorized for use in Canada before leaving Canada. This recommendation reflects, among other considerations: uncertainty related to the effectiveness or safety of products that are not authorized for use in Canada; uncertainty about the robustness of vaccination protocols (including post-vaccination monitoring and management of acute adverse events); and the ease of vaccination in Canada. Furthermore, given the time required to mount an immune response between doses, and the recommended intervals between doses to optimize immune response to the vaccineFootnote 18, for many travellers the benefits of vaccination will not accrue until after travel has been completed.

For some travellers, for example those who will be outside of Canada for a longer period of travel (more than a month) and cannot complete a series before departure, consideration should be given to receipt of vaccine outside of Canada. This may include children who were less than 12 years of age before travel, but become eligible for vaccination while abroad, and will remain outside of Canada for more than a month. This is the less preferred option, compared to pre-departure completion.

Assuming vaccination outside of Canada is being considered, then the factors mentioned above should be considered as part of the risk-benefit analysis. If the traveller decides to pursue vaccination outside of Canada, then they should be advised to opt for a vaccine product that is authorized for use by Health Canada and follow NACI guidance. If this is not possible, then preference is for vaccines that have met the necessary safety and efficacy criteria of the WHOFootnote 31. In addition to vaccines authorized for use in Canada, additional COVID-19 vaccine products have been given authorization for emergency use by the WHO. However, not all of these products will necessarily meet the same efficacy standards as has been demonstrated for Canadian-licensed products, nor will they necessarily meet regulatory requirements for country entry, quarantine modification, etc. Travellers who receive a COVID-19 vaccine product that is not currently authorized for use in Canada should also be made aware that additional doses of mRNA vaccine may be offered upon their return to Canada, in accordance with current guidance from the Public Health Agency of Canada, in order to optimize protection.

Serologic testing

Serologic testing is not routinely recommended for travellers. Commercial tests are increasingly available, and are generally of two types. Antibodies to the nucleocapsid of SARS-CoV-2 occur during natural infection to a variable degree, and after vaccination with some vaccines that use whole virus as an antigenFootnote 32. Antibody to the spike protein occurs after both infection and vaccination. In general, high titres of neutralizing antibodies are relatively predictive of protection against a given strain of virus, however an accurate correlate of protection has not been determined. Cell mediated immunity appears to also be an important factor in vaccine protection, but tests are not standardized or commercially available. Commercial serologic kits have been of highly variable accuracy, correlate only partially with neutralizing antibody assays, and a given assay may not indicate protection against a given viral variant. Therefore, while the absence of antibody to spike protein in a recently vaccinated individual might raise concerns regarding vaccine response and protection, the problems with sensitivity and specificity, and lack of validation as good correlates of protection, render testing of little practical utility in the individual traveller.

Irrespective of their utility, travellers should be aware that countries may still choose to use serologic testing as part of the regulatory requirements for entry or in determining exemptions to other border measures such as quarantine.

Recommendations

It should be made clear to any individuals planning to travel outside of Canada that they may often be increasing their risk of COVID-19 by travelling, even while taking the recommended precautions. This should be factored into any discussions between practitioners and their patients, when considering travel. Recommendations based on the integrated risk assessment are provided in Table 7. However, the following broad guidance applies to all individuals intending to travel outside of Canada:

Table 7. Recommendations for travellers based on the integrated risk assessment
Integrated risk assessment Recommendations
Low Comply with all local COVID-19 regulations and practice basic hygiene principles applicable to all situations.
Moderate Recommendations as per low risk, as well as non-medical public health measures (mask wearing, physical distancing), if not already specified by local regulations.
High Recommendations as per moderate risk as well as limiting exposure length to as short as reasonably possible / choosing alternate activities when possible.
Very high Recommendations as per high risk and consider deferring travel plans altogether. If travel must occur, recommend testing during travel based on contacts or symptoms, and routine testing and self isolation after return to Canada even when not required by regulations.

Acknowledgements

This statement was prepared by the COVID-19 Working Group: Libman M (Chair), Lagacé-Wiens P, Boggild A, Vaughan S, Lee J, Bui Y, Schofield S, Rossi C, Tunis M and Jensen C (National Advisory Committee on Immunization Secretariat), and Farmanara N (CATMAT Secretariat) and was approved by CATMAT.

CATMAT would like to acknowledge the technical and administrative support from the Centre for Border and Travel Health at the Public Health Agency of Canada for the development of this statement.

CATMAT members: Libman M (Chair), Lagacé-Wiens P, Boggild A, Bui Y, Vaughan S, Greenaway C, Acharya A, Lee J, Bogoch I, Plewes K.

Liaison members: Angelo K (Centers for Disease Control and Prevention), Pernica J (Association of Medical Microbiology and Infectious Disease Canada), Viel-Thériault I (Canadian Paediatric Society).

Ex officio members: Marion D (Canadian Forces Health Services Centre, Department of National Defence), Plamondon M and Kerr P (Biologics and Radiopharmaceutical Drugs Directorate, Health Canada), Rossi C (Medical Intelligence, Department of National Defence) and Schofield S (Pest Management Entomology, Department of National Defence).

Conflict of interest

None declared.

Appendix: Estimated SARS-CoV-2 transmission levels by destination

Description

This appendix summarizes the estimated SARS-CoV-2 transmission levels at a destination. Destinations are categorized as either having low, moderate, high or unknown levels of transmission based on several indicators. Some of the indicators that the assessment process considers include:

If there are no, or insufficient data available to estimate the level of SARS-CoV-2 transmission for a particular destination, they are included in the unknown transmission category. From a functional perspective, these destinations should be categorized as High in the exposure assessment.

Considerations

While this appendix will be updated regularly, it is possible that it will not reflect the most up-to-date information on the level of SARS-CoV-2 transmission at a particular destination. Furthermore, while adjustments will be made where possible, this approach relies on reported COVID-19 case data. Data limitations, such as underreporting of cases, may result in an underestimation of transmission levels for a particular destination. Results are presented at a country/destination level, and as such do not account for sub-national variation in transmission.

Appendix: Estimated SARS-CoV-2 transmission levels by destination
Date of assessment: January 03, 2022
Estimated SARS-CoV-2 transmission levels Country/destination
Low level of SARS-CoV-2 transmission

Antarctica
Benin
Bhutan
Cambodia
China
Cook Islands
El Salvador
Eritrea
Falkland Islands
Hong Kong

Indonesia
Iraq
Japan
Kiribati
Kosovo
Macau
Marshall Islands
Micronesia
Montserrat
Nauru
New Zealand
Niue
Palau

Saint-Pierre-et-Miquelon
Samoa
Solomon Islands
Taiwan
Tokelau
Tonga
Tuvalu
Vanuatu
Moderate level of SARS-CoV-2 transmission

Brunei Darussalam
Costa Rica
Cuba
Gambia

India
Iran
Oman

N/A
High level of SARS-CoV-2 transmission

Albania
Andorra
Angola
Antigua and Barbuda
Argentina
Armenia
Aruba
Australia
Austria
Azerbaijan
Azores
Bahamas
Bahrain
Barbados
Belarus
Belgium
Belize
Bermuda
Bolivia
Bonaire
Bosnia and Herzegovina
Botswana
Brazil
British Virgin Islands
Bulgaria
Burkina Faso
Canary Islands
Cape Verde
Cayman Islands
Central African Republic
Chile
Colombia
Comoros
Cote D'Ivoire
Croatia
Curacao
Cyprus
Czech Republic
Democratic Republic of Congo (Kinshasa)
Denmark
Djibouti
Dominica
Dominican Republic
Ecuador
Egypt
Equatorial Guinea
Estonia
Eswatini
Ethiopia
Fiji
Finland
France
French Guiana

Gabon
Georgia
Germany
Ghana
Gibraltar
Greece
Greenland
Grenada
Guadeloupe
Guam
Guinea
Guyana
Honduras
Hungary
Iceland
Ireland
Israel
Italy
Jamaica
Jordan
Kenya
Kuwait
Kyrgyzstan
Laos
Latvia
Lebanon
Lesotho
Libya
Liechtenstein
Lithuania
Luxembourg
Madagascar
Malawi
Malaysia
Maldives
Mali
Malta
Martinique
Mauritania
Mayotte
Mexico
Moldova
Monaco
Mongolia
Montenegro
Morocco
Mozambique
Namibia
Nepal
Netherlands
New Caledonia

Nigeria
North Macedonia
Northern Mariana Islands
Norway
Pakistan
Panama
Paraguay
Peru
Philippines
Poland
Portugal
Puerto Rico
Qatar
Réunion
Romania
Russia
Rwanda
Saint-Barthélemy
Saint Kitts and Nevis
Saint Lucia
Saint Martin
Saint Vincent & the Grenadines
San Marino
Sao Tome and Principe
Saudi Arabia
Serbia
Seychelles
Sint Maarten
Slovakia
Slovenia
Somalia
South Africa
South Korea
South Sudan
Spain
Sri Lanka
Suriname
Sweden
Switzerland
Thailand
Togo
Trinidad and Tobago
Tunisia
Turkey
Turks and Caicos
Uganda
Ukraine
United Arab Emirates
United Kingdom
United States
Uruguay
Vietnam
Virgin Islands (U.S.)
Zambia
Zimbabwe

Unknown level of SARS-CoV-2 transmission

Afghanistan
Algeria
American Samoa
Anguilla
Bangladesh
Burundi
Cameroon
Chad
French Polynesia

Guatemala
Guinea-Bissau
Haiti
Kazakhstan
North Korea
Liberia
Mauritius
Myanmar
Nicaragua
Niger
Papua New Guinea

Republic of Congo (Brazzaville)
Senegal
Sierra Leone
Singapore
Sudan
Syria
Tajikistan
Tanzania
Timor-Leste
Turkmenistan
Uzbekistan
Venezuela
Yemen

References

Footnote 1

Committee to Advise on Tropical Medicine and Travel (CATMAT). Evidence Based Process for developing travel and tropical medicine related guidelines and recommendations. 2017; Available at: https://www.canada.ca/en/public-health/services/publications/diseases-conditions/evidence-based-process-developing-travel-tropical-medicine-guidelines-recommendations.html. Accessed July 5, 2021.

Return to footnote 1 referrer

Footnote 2

World Health Organization. WHO coronavirus (COVID-19) dashboard. 2021; Available at: https://covid19.who.int/. Accessed June 18, 2021.

Return to footnote 2 referrer

Footnote 3

Buitrago-Garcia D, Egli-Gany D, Counotte MJ, Hossmann S, Imeri H, Ipekci AM, Salanti G, Low N. Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis. PLoS medicine. 2020 Sep 22;17(9):e1003346.

Return to footnote 3 referrer

Footnote 4

Centers for Disease Control and Prevention. Post-COVID conditions: Overview. 2021; Available at: https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-care/post-covid-conditions.html. Accessed July 7, 2021.

Return to footnote 4 referrer

Footnote 5

Nalbandian A, Sehgal K, Gupta A, Madhavan MV, McGroder C, Stevens JS, et al. Post-acute COVID-19 syndrome. Nat med. 2021 Mar 22;27(4):601-15.

Return to footnote 5 referrer

Footnote 6

Government of Canada. COVID-19 for health professionals: Transmission. 2021; Available at: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/health-professionals/transmission.html. Accessed June 18, 2021.

Return to footnote 6 referrer

Footnote 7

World Health Organization. Transmission of SARS-CoV-2 - Implications for infection prevention precautions: Scientific brief. 2020; Available at: https://www.who.int/publications/i/item/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations. Accessed July 7, 2021.

Return to footnote 7 referrer

Footnote 8

Government of Canada. COVID-19: Main modes of transmission. 2021; Available at: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/health-professionals/main-modes-transmission.html. Accessed July 7, 2021.

Return to footnote 8 referrer

Footnote 9

O'Keeffe J, Freeman S, Nicol A. National Collaborating Centre for Environmental Health (NCCEH). The Basics of SARS-CoV-2 Transmission. Vancouver, BC: NCCEH. 2021 Mar.

Return to footnote 9 referrer

Footnote 10

Centers for Disease Control and Prevention. Scientific brief: SARS-CoV-2 transmission. 2021; Available at: https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/sars-cov-2-transmission.html. Accessed: July 15, 2021.

Return to footnote 10 referrer

Footnote 11

Centers for Disease Control and Prevention. Scientific brief: SARS-CoV-2 and surface (fomite) transmission for indoor community environments. 2021; Available at: https://www.cdc.gov/coronavirus/2019-ncov/more/science-and-research/surface-transmission.html. Accessed: July 15, 2021.

Return to footnote 11 referrer

Footnote 12

Committee to Advise on Tropical Medicine and Travel (CATMAT). Chapter 2 - Prevention and risk assessment: Canadian recommendations for the prevention and treatment of malaria. 2019; Available at: https://www.canada.ca/en/public-health/services/catmat/canadian-recommendations-prevention-treatment-malaria/chapter-2-prevention-risk-assessment.html. Accessed: July 5, 2021.

Return to footnote 12 referrer

Footnote 13

Boggild AK, Libman M, Greenaway C, McCarthy AE, on behalf of the Committee to Advise on Tropical Medicine and Travel (CATMAT). CATMAT statement on disseminated strongyloidiasis: Prevention, assessment and management guidelines. Can Comm Dis Rep 2016;42:12-19. doi: https://doi.org/10.14745/ccdr.v42i01a03.

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Footnote 14

Government of Canada. COVID-19: Improving indoor ventilation. 2021; Available at: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/prevention-risks/covid-19-improving-indoor-ventilation.html. Accessed: July 10, 2021.

Return to footnote 14 referrer

Footnote 15

Centers for Disease Control and Prevention. Ventilation in Buildings. 2021; Available at: https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html?s=09. Accessed July 10, 2021.

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Footnote 16

National Advisory Committee on Immunization. Statements and publications: COVID-19. Available at: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci.html. Accessed September 10, 2021.

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Footnote 17

Government of Canada. COVID-19: Recommendations for those vaccinated with vaccines not authorized by Health Canada for those staying in Canada to live, work or study. 2021; Available at: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/guidance-documents/recommendations-those-vaccinated-with-vaccines-not-authorized-health-canada-staying-canada-live-work-study.html. Accessed: August 18, 2021.

Return to footnote 17 referrer

Footnote 18

National Advisory Committee on Immunization. Recommendations on the use of COVID-19 vaccines. 2021; Available at: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci.html. Accessed: July 23, 2021.

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Footnote 19

Regev-Yochay G, Amit S, Bergwerk M, et al. Decreased infectivity following BNT162b2 vaccination: a prospective cohort study in Israel. Lancet Reg Health Eur. 2021 Aug 1;7:100150-100150. doi: https://doi.org/10.1016/j.lanepe.2021.100150.

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Footnote 20

Eyre DW, Taylor D, Purver M, Chapman D, Fowler T, Pouwels K, et al. The impact of SARS-CoV-2 vaccination on Alpha and Delta variant transmission. medRxiv. 2021 Jan 1. doi: https://doi.org/10.1101/2021.09.28.21264260.

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Footnote 21

Krutikov M, Palmer T, Tut G, Fuller C, Shrotri M, Williams H, et al. Incidence of SARSCoV-2 infection according to baseline antibody status in staff and residents of 100 long-term care facilities (VIVALDI): a prospective cohort study. Lancet Healthy Longev. 2021 Jun;2(6):e362,e370. doi: https://doi.org/10.1016/S2666-5247(21)00219-6.

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Footnote 22

Townsend JP, Hassler HB, Wang Z, Miura S, Singh J, Kumar S, Ruddle NH, Galvani AP, Dornburg A. The durability of immunity against reinfection by SARS-CoV-2: a comparative evolutionary study. Lancet Microbe. 2021 Oct 1. doi: https://doi.org/10.1016/S2666-5247(21)00219-6.

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Footnote 23

Romero Starke K, Petereit-Haack G, Schubert M, Kämpf D, Schliebner A, Hegewald J, Seidler A. The Age-Related Risk of Severe Outcomes Due to COVID-19 Infection: A Rapid Review, Meta-Analysis, and Meta-Regression. Int J Environ Res Public Health. 2020 Aug 17;17(16):5974. doi: https://doi.org/10.3390/ijerph17165974.

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Footnote 24

Gates M, Pillay J, Wingert A, Guitard S, Rahman S, Zakher B, et al. Risk factors associated with severe outcomes of COVID-19: An updated rapid review to inform national guidance on vaccine prioritization in Canada. medRxiv. 2021 May 22. doi: https://doi.org/10.1101/2021.04.23.21256014v2.

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Footnote 25

Wingert A, Pillay J, Gates M, Guitard S, Rahman S, Beck A, et al. Risk factors for severe outcomes of COVID-19: a rapid review. medRxiv. 2020 Sep 1. doi: https://doi.org/10.1101/2020.08.27.20183434.

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Footnote 26

Smith C, Odd D, Harwood R, Ward J, Linney M, Clark M, et al. Deaths in children and young people in England following SARS-CoV-2 infection during the first pandemic year: a national study using linked mandatory child death reporting data. medRxiv, 2021 July 7. doi: https://doi.org/10.1101/2021.07.07.21259779.

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Footnote 27

Ward J, Harwood R, Smith C, Kenny SE, Clark M, Davis PJ, et al. Risk factors for intensive care admission and death amongst children and young people admitted to hospital with COVID-19 and PIMS-TS in England during the first pandemic year. medRxiv. 2021 Jan 1. doi: https://doi.org/10.1101/2021.07.01.21259785.

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Footnote 28

Harwood R, Yan H, Da Camara NT, Smith C, Ward J, Tudur-Smith C, et al. Which children and young people are at higher risk of severe disease and death after SARS-CoV-2 infection: a systematic review and individual patient meta-analysis. medRxiv. 2021 Jan 1. doi: https://doi.org/10.1101/2021.06.30.21259763.

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Footnote 29

Kompaniyets L, Agathis NT, Nelson JM, Preston LE, Ko JY, Belay B, et al. Underlying medical conditions associated with severe COVID-19 illness among children. JAMA network open. 2021 Jun 1;4(6):e2111182. doi: https://doi.org/10.1001/jamanetworkopen.2021.11182.

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Footnote 30

National Advisory Committee on Immunization. Rapid response: Additional dose of COVID-19 vaccine in immunocompromised individuals following 1- or 2- dose series. 2021 Sept 10; Available at: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci.html. Accessed: September 13, 2021.

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Footnote 31

World Health Organization. Regulation and prequalification, emergency use listing: COVID-19 vaccines. 2021; Available at: https://www.who.int/teams/regulation-prequalification/eul/covid-19. Accessed September 10, 2021.

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Footnote 32

Ong DS, Fragkou PC, Schweitzer VA, Chemaly RF, Moschopoulos CD, Skevaki C. How to interpret and use COVID-19 serology and immunology tests. Clinical Microbiology and Infection. 2021 May 8:27(7):981-986. doi: https://doi.org/10.1016/j.cmi.2021.05.001.

Return to footnote 32 referrer

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