Rapid risk assessment: the risk of Yellow fever (YF) to Canadians

The risk assessment is reviewed on a regular basis, and is updated as required.

Disease Background

Organization:

Published: 2016-08-17

Yellow fever (YF) is a vaccine-preventable, vector-borne illness caused by a virus from the family Flaviviridae. It is transmitted to humans through the bite of an infected mosquito, primarily Aedes aegypti, and possibly Aedes albopictus or Haemogogus species (along with an array of forest-dwelling mosquitoes in South America)Reference 1. There is no specific treatment for YF. Vaccination is the most important preventive measure, and confers long-lasting immunityReference 1. In its most severe form, YF causes a hemorrhagic fever which has a high case fatality rate, even with aggressive supportive measures intended to limit symptoms.

Yellow fever is endemic in 34 Sub-Saharan African and 13 South American countriesReference 2. In 2013, there were an estimated 130,000 (95% CI 51,000 to 380,000) severe cases and 78,000 (95% CI 19,000 to 180,000) deaths in Africa, accounting for about 90% of the global casesReference 3. All countries where Aedes aegypti (and possibly Aedes albopictus) is present (i.e. all countries endemic or previously-affected by outbreaks of Dengue, Chikungunya, Zika, or other related arboviruses) are at risk of local YF transmission. However, we note that such authochthonous transmission has never been reported in Asia despite widespread populations of competent YF vectors.

Following the International Health Regulations (IHR)Footnote 4, countries where YF may occur and conditions conducive to establishing local transmission are present require proof of vaccination prior to entry into the country. Proof of YF vaccination remains a requirement for entry to Angola in accordance with IHR. However, cases of Chinese migrant workers who were not vaccinated but working in Angola have been reported by the WHO. Beginning July 11, 2016, the WHO will stipulate that the term of validity of the YF vaccination certificate will change from 10 years from the date of the vaccination to the duration of life of the person vaccinated against yellow fever. It is expected that Angola will adopt this changeFootnote 5.

Event background information

On January 21, 2016, the Ministry of Health of Angola notified the WHO of an outbreak of YF. The first case was detected December 5, 2015 in Viana municipality, with subsequent spread throughout the country. Cases related to Angolan travel have been reported by China, Kenya, and the Democratic Republic of the CongoFootnote 6. As of May 19, 2016, Angola has reported 2420 suspected and 736 lab-confirmed cases (including 298 deaths). It has been suggested that sub-optimal surveillance implies that this is an underestimate of actual casesFootnote 7. An extensive WHO-supported vaccination campaign centered in Luanda (the capital and largest city), has seen more than 6 million Angolans (of a total population of 24.3 million) vaccinatedFootnote 6. Vaccination coverage outside the capital is not known, but is thought to be much lowerFootnote 7.

Globally, there is a YF vaccine shortage following the emergency vaccination of more than 6 million Angolans, from a global emergency stockpile of 11 million doses. YF-VAX® from Sanofi Pasteur (manufactured in the U.S.) is the only yellow fever vaccine authorized for sale in Canada.

Risk Assessment

In this section, we consider i) the likelihood of infection with YF for Canadians in Canada, ii) the potential for local transmission (i.e., in Canada) of YF, and iii) the likelihood of infection with YF for travelling Canadians.

Likelihood of infection for Canadians who do not travel

Mosquito-related transmission routes are the dominant source of YF infection noted in the literature. Based on current knowledge and available evidence, there are no other non-mosquito associated transmission routes.Footnote *

Given the short infectious period (estimated between 3 and 6 daysFootnote 1), and the lack of observed blood-associated transmission, the risk of YF infection for Canadians remaining in Canada is Very Low (high confidence).

Likelihood of local transmission in Canada

Local transmission would require the presence of a species of mosquito that can be infected with and transmit YF to human hosts. Ae. aegypti is the principal vector, with Ae. albopictus and Haemogogus mosquitoes possibly playing a role. The distribution of Ae. aegypti is largely limited to tropical and subtropical areas and, while Ae. albopictus can occur in temperate regions, it also is limited by climateFootnote 9Footnote 10Footnote 11. Haemogogus mosquitoes are thought to be strictly limited to Central or South America and have never been detected further from their home range (Northern Argentina).

The likelihood of Ae. aegypti, Haemogogus, or other tropical forest-dwelling mosquitoes becoming established in any area of Canada under current climatic conditions is estimated as Very Low (high confidence). For Ae. aegypti, this assessment is supported by a recent studyFootnote 9.

Ae. albopictus occurs in the United States, including the southern parts of some Eastern and upper Midwest states that border Canada. The likelihood that this species will become established in any area of Canada under current climatic conditions is considered Low (medium confidence). This assessment is based on recent workFootnote 9Footnote 10Footnote 11 that estimated the current probability of this species occurring in most of Canada as at, or approaching, zero. However, one study did report that small areas in southern Ontario, southern Nova Scotia, as well as southern coastal British Columbia had low to moderate climatic suitability for this speciesFootnote 11.

Self-sustaining populations of the mosquitoes that transmit YF are thought not to occur in Canada. Based on this assumption of absence of Ae. aegypti and Haemogogus species, or absence of Ae. albopictus from all but a very few limited locations in Canada, there is Very Low likelihood (high confidence) of epidemic spread and subsequent endemic establishment of YF in Canada.

Likelihood of infection for travelling Canadians

YF is rare among Canadian travellers, with 5 cases reported to the Agency since 2006.

The increasing number of YF cases among unvaccinated Chinese citizensFootnote 6 visiting Angola indicates an ongoing risk for unvaccinated travellers. Any unvaccinated traveller to an endemic or outbreak region could become infected.

Angola is not a major travel destination for Canadians (1,200 visits by Canadian residents in 2014). According to IHR and Angolan government requirements for travellers over 9 months of age, travellers must present proof of vaccination status upon arrival (as in all YF endemic countries). Considerations related to vaccination include time between vaccination and possible exposure, vaccine failure, and vaccine availability/supply.

At the present time, due to the low numbers of Canadian travellers to the outbreak region (Angola) and mandatory YF vaccination requirement for entry to Angola, the likelihood of infection for Canadian travellers is assessed to be Low (high confidence). It should be noted that this assessment would change if exported YF lead to widespread mosquito-borne transmission in other, more frequently-visited countries, a possibility that has not been assessed here.

Public Health Agency of Canada's response

The Agency is closely monitoring the situation.

The Agency has a travel health notice Yellow Fever in Angola reminding Canadians to get vaccinated before travelling to Angola and consider not travelling there if they have not been vaccinated against YF. Travellers should consult this notice before travel for the Agency's latest recommendations, which may change as the event evolves. In addition, there is information about the risk of yellow fever transmission and whether countries require a YF vaccine certificate for entry on the Government of Canada's travel.gc.ca website.

The Agency provides recommendations for health care professionals through the guidance Statement for Travellers and Yellow Fever developed by the Committee to Advise on Tropical Medicine and Travel and a Yellow Fever Vaccine chapter in the Canadian Immunization Guide.

Bibliography

Appendix 1 - Definition of technical terms

Table 1: Working definitions for likelihood levels
Level Definition
Very low Could occur only under exceptional circumstances
Low Could occur some of the time
Medium Will occur some of the time
High Is expected to occur in most circumstances
Table 2: Working definitions for confidence levels
Level Definition Examples of information/evidence

Low

Little or poor-quality evidence, significant uncertainty, conflicting views amongst experts, no experience with similar incidents. Further research is likely to have significant impact on the results of the assessment.

Further research is very likely to change the results of the assessment and the confidence in the assessment and the information used.

Individual case reports

Grey literature

Individual, non-expert opinion

Medium

Adequate quality of evidence, including consistent results, reliable source(s), and assumptions made on analogy. Agreement between experts or opinions of two trusted experts. Further research may necessitate some changes to the assessment.

Further research is likely to have an impact on the confidence in the assessment and information used. It may change the results of the assessment.

Non-peer-reviewed published studies/reports

Observational studies

Surveillance reports

Outbreak reports

Individual, expert opinion

High

Good-quality evidence, multiple reliable sources, verified, multiple expert opinions concur, experience with previous and similar events. Further research is unlikely to change the results of the assessment.

Further research is unlikely to change the confidence in the assessment.

Peer-reviewed published studies where design and analysis reduce bias (i.e. systematic reviews, RCT, outbreak report).

Textbooks regarded as definitive sources.

Expert group risk assessment, or specialized expert knowledge, or consensus opinion of experts.

Footnotes

Report a problem or mistake on this page
Please select all that apply:

Privacy statement

Thank you for your help!

You will not receive a reply. For enquiries, contact us.

Date modified: