Evidence on the virulence, transmission and impact of B.1.617.2 (Delta) among children: update 1
October 2021
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Table of contents
- Introduction
- What's new
- Key points
- Overview of the evidence
- Methods
- Evidence tables
- References
Introduction
What is the evidence on the virulence, transmission and impact of the Delta variant among children?
The SARS-CoV-2 variant of concern (VOC) B.1.617.2 known as Delta by the World Health Organization (WHO) naming system is currently the predominant variant in many countriesFootnote 1. VOCs are of concern when compared to the original SARS-CoV-2 variants, as their complement of mutations lead to increased transmissibility, increased virulence (morbidity or mortality), changes in clinical disease presentation, immune evasion, reduced effectiveness of treatments, vaccines and/or public health measures (PHMs) and/or are associated with diagnostic detection failuresFootnote 1Footnote 2Footnote 3.
Delta has been reported to have higher transmissibility than the Alpha VOC and original variant in the general populationFootnote 4Footnote 5Footnote 6Footnote 7Footnote 8Footnote 9. Delta has outcompeted other variants including Alpha in several countries and is the dominant variant at >99% of sequenced VOC cases in Canada since September 5, 2021Footnote 10Footnote 11Footnote 12. Emerging evidence also reports higher risk of severe outcomes such as hospitalization, ICU admission, and death associated with Delta compared to non-VOCs or Alpha in the general populationFootnote 13Footnote 14Footnote 15Footnote 16. A risk profile of the evidence on Delta is available upon request at ocsoevidence-bcscdonneesprobantes@phac-aspc.gc.ca. Vaccination has been shown to protect against serious outcomes of COVID-19 and Canada has one of the highest vaccination coverage rates and currently 81% (October 8, 2021) of the population ≥12 years of age has received two doses of vaccineFootnote 10Footnote 17. However, COVID-19 vaccines are not yet authorized by Health Canada for those <12 years old.
As of October 2021, children are now more than a month into their school year and Canada is in the fourth wave of the pandemicFootnote 18Footnote 19. Delta is the dominant variant across Canada and the proportion of Delta has remained around >99% of VOC cases with sequencing data reported across the country. Recent models predict that COVID-19 case counts have slowed and in many parts of Canada they appear to be decreasingFootnote 19. Evidence is beginning to emerge on the effects of the Delta variant on children. To further inform public health strategies to protect children including in school settings, this evidence brief summarizes what is known on the virulence, transmission and impact of Delta among children aged 0-18 years old including the impact of public health interventions on the spread of Delta among children. This report presents evidence up to October 8, 2021.
What's new
This update identified 16 new studies since September 15, 2021, on the virulence, transmission and impact of Delta among children. This included 1 cluster-randomized control trial, 1 surveillance data analysis, 1 ecologic study, 6 outbreak investigations, and 7 predictive models. The new studies did not include virulence data but rather there were:
- Additional outbreak data on transmission among children and lack of transmission when public health measures (PHMs) (e.g., masking, cohorting, screening and quarantine) were implemented.
- One ecological study examined the impact of when mask use was mandated or not in schools.
- Predictive models that explored the impact of mask mandates as well as other PHMs in schools and/or the community on COVID-19 cases attributable to schools being open both within school populations and in the community.
- Predictive models that examined the impact of extending vaccination to younger children on number of predicted cases, hospitalizations and deaths in a community.
Key points
This review includes 23 studies pertaining to the virulence, transmission and impact of Delta among children with the majority focused on the impact of public health measures (Table 1, Table 2, and Table 3). Of these, one UK cluster-randomized control trial (RCT) in secondary schools, April–June 2021, examined the impact of PHMs, four general population surveillance studies (three US and one UK) focused on Delta cases in children between June and August 2021 when children were not in school. One US ecological study evaluated the impact of masks August–September 2021, and seven outbreak investigations (six US, one France) reported on transmission events and the prevention of transmission events May-August 2021. Six predictive models (four US, one Austria, and one Germany) evaluated Delta transmission in schools or the impact of opening schools on community transmission and the impact of public health measures and four predictive models (US, Australia, UK, and China) reported on the impact of vaccinating children. The level of evidence on all parts of this review is considered low as most outcomes are reported in one or two studies, thus we caution the readers that this review reports preliminary evidence and the conclusions, and strength of them, may change as more studies and evidence become available.
Virulence
Highlights from the current literature include:
- Current evidence suggests that Delta is not more virulent in children than the original variant or Alpha. Two surveillance studies in the US and one in the UK reported that although the incidence and hospitalization rates have increased, the proportion of COVID-19 cases with severe outcomes (e.g., ICU admission, invasive mechanical ventilation and mortality) has not changed among children during June-August 2021 compared to historical timeframes when Delta was not dominantFootnote 20Footnote 21Footnote 22.
Transmissibility
Highlights from the current literature include:
- Without vaccinations and other public health measures such as physical distancing, cohorting and masking, Delta has high transmissibility in children.
- There were high attack rates across seven outbreak investigations when PHMs were not implemented: an attack rate of 50% in an elementary classroomFootnote 23; the odds of a school outbreak was 3.5 times more likely in schools with no mask requirementFootnote 24; an attack rate of 20% in a gymnastics facilityFootnote 25; camp outbreaks with an attack rate of 26% with no PHMsFootnote 26, 0.08% cases in a camp with multiple PHMsFootnote 27; and a 31-fold increase in infections in camps during Delta circulation compared to the original variantFootnote 28. In a household outbreak, six siblings were infected by their 13 year old sibling (75% attack rate)Footnote 29.
- High viral load (identified by low cycle threshold (Ct <20) value, where Ct is a proxy for viral load) in 18.3% of children, both symptomatic (30%) and asymptomatic (70%) was reported at community testing sites in California, where 20% of vaccinated adults also had Ct<20 at the time of testingFootnote 30.
- The RCT study and three predictive models showed that infection rate of cases in schools closely follow those case rates of the community and that transmission in schools will not drive Delta transmission in the communityFootnote 31Footnote 32Footnote 33Footnote 34. A UK predictive model suggests with high vaccination coverage in adults that the peak infections in fall 2021 may be identified in schools before other settingsFootnote 35.
- Delta was estimated to result in nearly 10 times the cases attributable to being in school compared to Alpha in a model when keeping all other parameters constantFootnote 34.
Impact of public health measures
Vaccination
Highlights from the current literature include:
- In the US, vaccinated adolescents (12-17 years old) had 10.1 times lower hospitalization rates compared to unvaccinated adolescents during the beginning of a Delta dominated surge in COVID-19 cases (June-July 2021), suggesting that vaccination prevents severe illness and US states with the lowest vaccination coverage had higher adolescent emergency department visits (3.4x) and hospitalizations (3.7x) compared to US states with the highest vaccination coverageFootnote 20.
- Several models from the UK, US, Australia and China look at the increases in infections, hospitalizations and deaths after school re-opening in fall 2021 and the potential impact of vaccination coverage in different child age groups (16+, 12+ and 5+, 3+ and all ages). Predictions show increasing reductions (10-90%) with increasing vaccine eligibility in more pediatric age groupsFootnote 35Footnote 36Footnote 37Footnote 38.
- Both in the community and in schools, most models showed that very high vaccination coverage of the population is needed (>80%) before other PHMs (e.g., mask policies, quarantine policies, social distancing, cohorting in schools) added no additional benefitFootnote 34Footnote 35Footnote 37Footnote 38.
Impact of public health measures on school transmission
Quarantine policies
Highlights from the current literature include:
- An RCT in England secondary schools and colleges found no difference in infections or COVID-19 related days absent between 10 day self isolation for contacts vs. a rapid antigen test for 7 days where those who test negative remain in schoolFootnote 31. A model predicted the test to stay protocol would result in slightly more cases than quarantineFootnote 32.
- Whole class quarantine for 10 days following discovery of a case had the same impact on total number of cases and deaths (~50%) in the community attributable to school opening as a high stringency PHM scenario with masking, cohorting and increased ventilation in schools in a UK modelling studyFootnote 35.
Masking in schools
Highlights from the current literature include:
- An ecologic study reported that pediatric case rates in counties with school mask requirements experienced 18.53 cases per 100,000 per day lower than counties without school mask requirementsFootnote 39.
- A US predictive model estimated the peak hospitalization level could exceed January 2021 (when Alpha was dominant) if kindergarten to grade 12 (K-12) were opened without mask policiesFootnote 40. If schools had hybrid learning or there was high mask-efficacy, there were fewer excess deaths (7%) and a 71% reduction in peak excess hospitalizations in the general population attributable to school opening. Overall the number of excess cases were reduced by 23-35% with masks and by an additional 11-13% with hybrid learningFootnote 40.
Combinations of public health measures in schools
Highlights from the current literature include:
- Five predictive models examined the potential reduction in Delta cases attributable to in-school transmission in elementary schools, middle schools, and high schools that implemented different combinations of PHMs (masking, testing, cohorting, and improved room ventilation) with different levels of vaccination coverage. In different models vaccination coverage ranged from 30-85% in the eligible population (e.g., children ≥12 years, teachers and community). Baseline scenarios with no public health measures estimate a high level of Delta infections among susceptible children in all school environments (e.g., 75%), and excess infections, hospitalizations and deaths in the communityFootnote 33Footnote 40. The trade-offs between different risk reduction strategies are highlighted below:
Elementary school (4-11 years) / middle school (12-13 years) / high school (14-18 years)
Highlights from the current literature include:
- Without testing and masking (Delta R0=4.0), more than 75% of susceptible (e.g., unvaccinated or not previously infected) students will get infected within three monthsFootnote 33. With masks (R0=2.0), the proportion infected drops to 50%/35%/24% and routine testing further reduces infections to 22%/16%/13% for elementary/middle/high schools respectivelyFootnote 33.
- Higher vaccination coverage within the school population was associated with fewer cases and less additional impact from PHMs in the school population. However, only in high schools where all students were eligible for vaccination and vaccination coverage exceeded 90% was the impact of other PHMs found to be negligibleFootnote 34.
- With universal mask use and school vaccination coverage of 70% of the eligible population, the excess symptomatic infections attributable to school reopening was far less in high schools with only 0.4% excess infections compared to elementary and middle schools with 2.0%, and 3.0% excess infections within the school population, respectivelyFootnote 34.
- Using remote learning as the baseline scenario, universal screening (testing twice a week) was estimated to have averted 57% of excess cases compared to no additional PHMs in both elementary and middle schoolsFootnote 32. Active testing was shown to be a necessary component of an Austrian model to get Ro≤1 in all school typesFootnote 41.
Higher vaccination coverage among eligible individuals in school (e.g., students ≥12 years, teachers and staff) was associated with a decrease in school-attributable transmission in all schoolsFootnote 34. In elementary and middle schools, a hybrid schedule of in-person learning where ≥60% of the time students receive remote instruction and are not in school, could prevent much of the excess transmission by reducing the number and duration of contactsFootnote 32. The strictest combination of interventions tested (masks, cohorting and 70% vaccination coverage), would result in the largest reduction in excess infections. For example, strict PHMs would result in excess infections among 1.7% of elementary students compared to 6.6% with 70% vaccination coverage onlyFootnote 34. Lower risk tolerances of <5 excess infections per 1,000 students or teachers could be achieved in middle schools with cohorting and a 70% vaccination coverageFootnote 34.
Overview of the evidence
Twenty-three studies pertaining to the virulence, transmission and impact of Delta in children were identified and included in this review. Virulence studies included observational studies of surveillance data from mainly large national databases. Studies reporting transmissibility were from outbreak investigations and predictive modelling studies. Studies reporting PHMs including the impact of vaccinations were mainly predictive models with some evidence obtained from a cluster-RCT and an ecologic study.
A formal risk of bias assessment was not conducted. One RCT was identified, although generally considered the 'gold-standard', they are not always feasible and the evidence may not be representative of real-world settings, thus limiting the generalizability of the findings.
The observational studies included surveillance and ecological studies obtained from large national databases with data pertaining to children. Due to the nature of surveillance data, the evidence is at high risk of bias as the sample may not be representative of the population and may have insufficient detail to answer the research question. In addition, these studies are subject to missing information, selection bias and confounding factors.
The quantitative predictive models in this review do not identify actual outcomes of strategies that have been tested, but rather present a range of plausible outcomes based on theoretical scenarios. Their results are useful to compare different options as part of a decision-making process, however the results need to be interpreted with caution as the models will vary based on the assumptions, input values and region-specific parameters used.
A key knowledge gap in this research is the lack of high-quality studies reporting evidence on the transmission and virulence of Delta in children or infants compared to the original SARS-CoV-2 variant or other VOCs. A comparison against other VOCs and the original variant is needed to contextualize and understand the difference in the impact of Delta on transmission and severity among children, and in adults vs. children, including in school settings. Real-world studies that assess the impact of different PHMs, including vaccination coverage in school and community settings on transmission, virulence, and impact of the Delta variant in children, are also needed.
Overall, the level and quality of evidence on the Delta variant and children is low and there are knowledge gaps in the existing literature base. As more studies and evidence emerge, the conclusions and/or strength of the findings of this review is likely to change.
Virulence of Delta among children
Two surveillance studies in the US and one from the UK reported cases and hospitalization rates increased in children and adolescents age 0-17 years coinciding with Delta becoming the dominant variant in May-August 2021Footnote 20Footnote 21Footnote 22 Table 1.
- There was no difference in ICU admission, invasive mechanical ventilation and mortality between March 1, 2020, and June 19, 2021 (before Delta was dominant) and June 20–July 31, 2021 (after Delta was dominant) indicating the proportion of cases that lead to severe outcomes in children 0-17 years has not changed in the USFootnote 20.
- The US papers describe increased incidence rates and hospitalization rates in children given the current epidemiological situation of increasing COVID-19 cases.
- Incidence rates in the US between August 14–27, 2021 have increased among children and adolescents aged 0–4, 5–11, and 12–17 years: 16.2, 28.5, and 32.7 per 100,000 persons compared to 1.7, 1.9 and 2.9 in June 2021, respectivelyFootnote 21. In the UK cases 5-12 years old increased from 0.35% to 1.05% test positivity and 13-17 years cases increased from 0.16% to 1.33%Footnote 22. In August 2021 approximately 50% of adolescents (12-17 years) in the US were vaccinated, whereas the UK has just approved 1 dose for 12-15 year olds in October 2021.
- Hospitalization rates in the US increased to 1.4 per 100,000 children (0-17 years) in the population the week of August 14, 2021 compared to 0.3 per 100,000 during the week of June 26, 2021, representing a 4.7-fold increase. The increase is similar to the increase in peak hospitalization rate in January, 2021 of 1.5 per 100,000 children. The highest increase in hospitalizations occurred in children 0-4 years with 1.9 per 100,000 compared to 0.2 per 100,000, representing a 10-fold increaseFootnote 20.
Transmission of Delta among children
Seven outbreak investigations Table 2, three surveillance studies Table 1, and three predictive models Table 3 provide some evidence for Delta's increased transmissibility in children. However, cases in children have been shown to be linearly correlated with community case rates, suggesting increased transmissibility across all age groups for Delta, however no age specific differences have been reportedFootnote 31Footnote 32Footnote 33. The models identified that transmission of Delta in schools contributes to the community transmission, but that children and school settings will not drive Delta transmission in the communityFootnote 34. With high vaccination coverage (80%) in adults, it is possible that surveillance data moving forward may detect peaks of community infection first in schools and then other settings due to the large proportion of the school population (i.e., children <12 years of age) remaining ineligible for vaccination at the present timeFootnote 35. Compared to Alpha the increased transmissibility of Delta is estimated to result in 10 times the school attributable excess casesFootnote 34.
High viral load and incidence of asymptomatic cases in children was reported in a surveillance study where 18.3% of children under 12 years of age had Ct (cycle threshold value used as a proxy for viral load) values of <20 (suggesting high viral load), and 70% of these children were asymptomatic at the time of testingFootnote 30. There was no comparator group to evaluate whether there was a change from previous variants in circulation.
Delta outbreak investigations in different settings
There were two studies that reported high transmissibility of Delta in school settings. The first was an outbreak investigation in an elementary school in the US May 2021 that describes an unvaccinated teacher who taught until 2 days post symptom onset. There was a 50% attack rate in the classroom despite high adherence by the students to wearing a mask and being seated 6 feet apartFootnote 23. The teacher occasionally took off her mask, and the positive case pattern in the classroom and epidemiological investigation was consistent with the teacher as the source of exposureFootnote 23. The second is an analysis of all school associated COVID-19 outbreaks during the first 3-6 weeks of school in the US for K-12 schools between July-August 2021 that reported outbreak risk was 3.5 times higher in schools with no mask requirement than in those with a mask requirement implemented at the time school startedFootnote 24.
There were three camp related outbreak studies, two of which reported on multiple camps during summer 2021. One study reported on the successful prevention of outbreaks at nine overnight camps due to implemented PHMs which included testing (before and during camp), masking, and physical distancing, as well as vaccination of staff and campers aged 12+. Six cases in campers ages 8-14 years and three in staff were reported among 7,173 persons attending these camps and no secondary transmission was identified during campFootnote 27. In an evaluation of outbreaks at summer camps in Louisiana when there was a thirty-one-fold increase in confirmed camp-associated cases statewide (June-July 2021 vs. June–July 2020)Footnote 28, the mean outbreak size was 11.5 cases (range: 2–59 cases) among the 28 camps with outbreaks and PHM strategies ranged from none to some preventative measures (e.g., masking indoors, vaccination of staff)Footnote 28. A single outbreak at a 5-day overnight church camp had an attack rate of 26% where there was no requirement for proof of vaccination or masking and none of the campers were symptomatic at the beginning of the campFootnote 26.
Another outbreak occurred in a gymnastics facility resulting in 47 linked-cases and an overall attack rate of 20% in the gymnastics facility (staff and gymnasts) and a household attack rate of 20%Footnote 25. Within a household cluster the index case (a 13 year old) spread Delta to six unvaccinated siblings (100%) and not to their vaccinated parentsFootnote 29.
Taken together the outbreak investigations show that Delta can result in a high attack rate, particularly when PHMs are not employed.
Impact of public health measures
One UK cluster-randomized controlled trial, one US surveillance study, one ecologic study and seven predictive models parameterized to mimic different areas in the US, Austria, and Germany are included in this section. The surveillance study and one predictive model evaluated hospitalizations and deaths attributable to transmission in schools. Six predictive models, one RCT, and an ecologic study examined transmission in school settings, and one model examined transmission while children were on summer vacation. Two modelling studies and one RCT reported school absences as an outcome. One predictive model compared the impact of transmission of Delta versus Alpha in a school setting.
Impact of vaccinating children
There are few data points on the protective effect of extending vaccination to children and youths against Delta, one study analysed surveillance data from the US recently highlighted that vaccinated adolescents had a 10.1 times lower risk of hospitalization compared to unvaccinated adolescents from June 20-July 31, 2021Footnote 20. In the same analysis, the percent of emergency department visits and the rate per 100,000 of hospitalizations in August 2021 when Delta was the dominant variant in the quartile of states with the lowest vaccination coverage was 3.4 times and 3.7 times that in the quartile of states with the highest vaccination coverage, respectivelyFootnote 20. Suggesting both direct and indirect protection from vaccine coverage in the whole population. There was no further analysis of potential differences in vaccine protection by variants that caused infection.
Four predictive models (one each in the US, UK, Australia, and China) that were parameterized to assess different levels of vaccination coverage in children are included in this section, Table 3. Overall, the predictive models indicate that extending vaccination coverage to younger children reduces pediatric and general population cases, hospitalizations and deaths. Most models look at different scenarios of vaccine coverage and other PHMs (e.g., masking, social distancing and vary the stringency of other PHMs) to explore how well different combinations control the number of COVID-19 cases.
- In the US, with 85% vaccination coverage of 12+ year olds, school opening with no other PHMs is estimated to lead to a 2-fold increase in pediatric hospitalizationsFootnote 38. Reducing contacts in schools by 25% or 50% through masking, ventilation and distancing is expected to decrease the overall median cumulative hospitalizations in 0-19 year olds by 8% and 23%, respectivelyFootnote 38. Extending vaccination eligibility to children aged 5-11 years regardless of level of physical contacts, reduces hospitalizations in 0-19 year olds by ~50% compared to no vaccinationFootnote 38. Early vaccination in 5-11 year olds and reaching a 90% overall vaccination coverage in 12+ year olds, 60% of remaining hospitalizations will be averted and the need of extra mitigation measures may be avoidedFootnote 38. Delaying COVID-19 vaccination among children by 3 months, vaccination occurs January 2022 instead of October 2021, will void most of the benefits measured over the school yearFootnote 38.
- A UK model estimated a 5-fold increase in cases with school reopening despite 80% vaccination coverage among adults. Extending vaccination coverage (80%) to 16+ years/ 12+ years / all ages reduces the total number of infections by 10%/ 40% / >90% and deaths by ~ 1000 / 5000 / nullifies impact of opening schoolsFootnote 35.
- An Australian model which assessed the impact of extending vaccination coverage to 5+ year olds from 15+ year olds report using a Pfizer or Mixed vaccination program leads to fewer infections when Reff is high (Reff 5-7)Footnote 36. Vaccine coverage >85% including those 5+ years is needed to achieve herd immunity for a Reff of 5 (Delta scenario), where a Reff of 7 (more transmissible than Delta scenario) will not achieve herd immunity regardless of the programFootnote 36.
- A modelling study from China reported high attack rates given low efficacy of vaccines (54% VE, vaccine not stated) despite good vaccine coverage in individuals 3+ yearsFootnote 37. Extending vaccinations to 3-17 year olds achieved the highest reduction in infections (55%-57%)Footnote 37. High vaccine effectiveness (VE) and high coverage (e.g., 90% VE and >93% coverage) or moderate to strict PHMs with lower vaccine effectiveness and coverage are needed to reach vaccine-induced herd immunityFootnote 37. A 90% reduction in infections is possible with a "US-like" scenario (VE of 79% and natural immunity of 22%) by vaccinating individuals 3+ yearsFootnote 37.
Lifting community public health measures and vaccination coverage
A German modelling study assessed the impact of COVID-19 by age group (including children) and showed that lifting PHMs (e.g., masking) too early with insufficient vaccination coverage results in high community case incidence and the highest incidences are in the unvaccinated 0-14 year old age groupsFootnote 42. The longer PHMs are maintained as vaccination coverage increases, the lower the incidence of community infections including in the unvaccinated children (0-14 years)Footnote 42.
Impact of public health measures on K-12 schools
There were few empirical studies on the impact of PHMs with Delta in circulation. One study reported schools in the US with mask requirements had 18.53 cases per 100,000 per day lower than the average change for counties without school mask requirements (p<0.001), which was significant in an ecological analysis after controlling for covariates (p<0.001)Footnote 39.
The remaining studies were predictive models that indicate PHMs including vaccination coverage in students, teachers, staff and the community reduced the transmission of Delta among school-aged children and the burden of COVID-19 cases, hospitalizations and deaths in the community. The models identified that transmission of Delta in school contributes to the transmission of infection, but does not drive Delta transmission in the community.
As each model is parameterized a little differently and considers different PHMs, vaccination coverage and outcomes, it is difficult to directly compare across models. Each of the seven predictive models examine the scenario where Delta is dominant, with the general assumption that Delta is more transmissible and thus has a higher R0 (range 4.0-6.0) compared to previous variants or VOCs. Scenarios include different levels of vaccination coverage (30-85%) and PHMs (e.g., masking, testing, quarantine policies and cohorting) for comparison. Outputs are presented by type of school (i.e., elementary, middle, and high school) to account for varying levels of vaccination coverage, size of schools and mixing patterns of staff and students in these schools or for all K-12 schools. The summary below highlights outcomes related to the number of infections, however some models also report on school absences, hospitalizations and deaths, these are detailed in Table 3. Overall, a reduction in the number of cases was correlated with a decrease in all other outcomes.
Elementary schools
Elementary schools generally include children 4 up to 11 years old, thus none of the children are eligible for vaccination in these settings. Key findings on elementary schools from the predictive models are listed below:
- The baseline scenario of 30-50% immunity (from infection or vaccination) and no PHMs estimated more than 75% of susceptible students will get infected within three monthsFootnote 33. The addition of masks dropped the proportion infected to 50% and testing further reduces infections to 22%Footnote 33.
- Compared to fully remote instruction, 5-day in-person attendance with no in-school testing (90% of teachers and staff were vaccinated with 80% vaccine effectiveness) was associated with a 40% projected increase (excess cases attributable to school transmission) in infections among students at a community case rate of 10 cases/100k/day and a 38% increase at 50 community cases/100k/dayFootnote 32.
- In a community with 10 cases/100k/day, weekly screening averted 57% of excess incidence (cases attributable to school transmission) relative to remote learningFootnote 32.
- If students with known exposures were allowed to stay in school with daily testing (the "test to stay" strategy), slightly more transmission occurred compared to isolation of exposed cases (quarantine)Footnote 32. With "test to stay" compared to quarantine and 10 community notifications/100k/day, weekly screening prevented 46% rather than 57% of excess transmission, and weekly surveillance of 20% of a random sample of unvaccinated students and teachers prevented 17% rather than 25%Footnote 32.
- A 70% vaccination coverage without additional PHMs resulted in 6.6% excess symptomatic cases in elementary schools across a 128-day semester, compared to 15% with 60% vaccination coverage and 18% with 50% vaccination coverageFootnote 34.
- With universal mask use, community and school vaccination coverage of 70%, will result in 2.0% excess symptomatic casesFootnote 34.
- With increasing vaccination coverage from 50% to 70%, there is a 24% decline in school-attributable transmission, suggesting that adult to child transmission represents an important source of school-attributable illnessFootnote 34.
- Increasing vaccination coverage of teachers from 70% to 90% reduced the estimated excess rate of infection from 6.6 to 3.9 symptomatic cases per 100 elementary students across the four-month semester, representing a reduction of 41%Footnote 34. This suggests that increasing vaccination coverage among elementary school teachers can reduce infection among their studentsFootnote 34.
- The strictest combination of interventions tested (masks + cohorts, 70% vaccine coverage), would result in excess infection among 1.7% of elementary students assuming they are equally as susceptible as older children and 0.4% of elementary students assuming students are half as susceptible as older children compared to 6.6% with only 70% vaccination coverage in >12 yearsFootnote 34.
- In a modelling study in Austria with 80% vaccination coverage in teachers and 60% in family members reduced the cases in different school types. For elementary schools R0 could be reduced to <1 with a combination of PHMs (room ventilation, cohorting, mask policies) or testing twice per week or a combination of bothFootnote 41.
Middle schools
Middle schools generally include children 11-13 years old, thus some of the children are eligible for vaccination in these settings. Key findings on middle schools from the predictive models are listed below:
- The baseline scenario of no PHMs and with 30% of middle school children vaccinated, estimated more than 75% of susceptible students will get infected within three monthsFootnote 33. The addition of masks dropped the proportion infected to 35% and testing further reduces infections to 16%Footnote 33.
- Compared to fully remote instruction, a 5-day middle school attendance (assuming 90% of teachers and staff were vaccinated with 80% vaccine effectiveness, and 50% of middle school students were vaccinated, and quarantine of known close contacts) increased incidence (excess cases attributable to school transmission) by 72% at a community case rate of 10 cases/100k/day and by 60% at 50 community cases/100k/dayFootnote 32.
- In a community with 10 cases/100k/day, universal weekly screening averted 57% of excess incidence (cases attributable to school transmission) relative to remote learning and weekly surveillance of 20% of a random sample of unvaccinated students and teachers prevented averted 34% of the excess transmission associated with school attendanceFootnote 32.
- The "test to stay" strategy increased transmission slightly compared to the remote-only baseline (e.g., a 72% increase with quarantine to an 82% increase with test-to-stay at 10 community notifications/100k/day)Footnote 32.
- A hybrid schedule of in person learning with ≥60% remote instruction, could prevent much of the excess transmission by reducing the number and duration of contacts for both elementary and middle schoolFootnote 32.
- Under a 70% vaccination coverage without additional PHMs, there were 8.8% excess symptomatic cases in middle schools across a 128-day semester, compared to 11% with 60% vaccination coverage and 13% with 50% vaccination coverageFootnote 34.
- With universal mask use, community and school vaccination coverage of 70%, an estimated 3.0% excess symptomatic infection attributable school transmission is predicted.
- Achieving lower risk tolerances, such as <5 excess infections per 1,000 students or teachers, required high vaccination (70%) and a cohort approachFootnote 34.
- Given 45% vaccine effectiveness, masking all middle school students would avert symptomatic infection for 3.9% of students compared to masking only unvaccinated students and teachers. At 85% VE and above, there was little difference in school-attributable transmissionFootnote 34.
High schools
High schools generally include children 14-17 years old, although some studies included students 11-18 years, thus most children are eligible for vaccination in this settings.
A UK cluster-randomized controlled trial in secondary schools in children 11-18 years reported that there was no difference in symptomatic PCR-confirmed infection between self-isolation of school-based COVID-19 contacts for 10 days and voluntary daily lateral flow (LFD) device testing (aIRR 0.94, p=0.61) or days absentFootnote 31.
Key findings on high schools from the predictive models are listed below:
- The baseline scenario (Delta R0= 4.0) of no PHMs and assuming 40% of high school children were vaccinated, estimated more than 75% of susceptible students will get infected within three monthsFootnote 33. The addition of masks dropped the proportion infected to 24% and testing further reduces infections to 13%Footnote 33.
- With greater transmissibility, Delta R0=5.0, 88% of susceptible students can be infected without public health measures and with masking this would be reduced to 41%Footnote 33.
- With a 70% vaccination coverage without additional PHMs, there will be 4.4% excess symptomatic cases in elementary schools across a 128-day semester, compared to 7.2% with 60% vaccination coverage and 10% with 50% vaccination coverageFootnote 34.
- With universal mask use, community and school vaccination coverage of 70%, the excess symptomatic infection attributable to school transmission is reduced to 0.4% in high schoolsFootnote 34.
- At 70% coverage without additional PHMs, an excess of 4.0 (89% HPDI: 0, 7.1) symptomatic cases per 100 students is estimated across the 128-day semester, and at 95% vaccination coverage an excess of 0.2 (89% HPDI: -0.2, 0.6) cases per 100 students was estimated. High school students could achieve a transmission tolerance of fewer than 10 excess cases per 1,000 population without PHMs if vaccination coverage is >90%Footnote 34.
- Given 45% VE, masking all high school students would avert symptomatic infection for an additional 6.1% of students compared to masking only unvaccinated students and teachers. At 85% VE and above, there was little difference in school-attributable transmissionFootnote 34.
- In a modelling study from Austria, 80% vaccination coverage in teachers, 60% in family members, and 50% in eligible students reduced the cases in different school types. The largest impact was in secondary schools given they are eligible for vaccination. Other PHMs were still needed, but control of Delta spread could be achieved with fewer or less stringent other PHMs (active testing, room ventilation, cohorting, or mask policies)Footnote 41.
K-12 schools
These schools include students from kindergarten to grade 12 which includes children from 5 to 18 years old, some of which are eligible for vaccination.
A model of re-opening schools in the US with 75% vaccination coverage and no masks, reported the excess infections in the general population due to school opening could be reduced by 23%-36% with masks and an additional 11-13% reduction with hybrid learningFootnote 40.
Comparison of impact of transmission in schools of Delta compared to Alpha
One predictive model ran scenarios for Alpha (R0=2.5) and reported that with a 70% community vaccination coverage, universal masking and vaccine effectiveness of 85%, school attributable excess transmission would be nearly ten times lower (<1 infection per school) for Alpha than Delta. This scenario estimates fewer than 25% probability of an in-school transmission per monthFootnote 34.
At a 70% community vaccination coverage with no additional PHMs, there was a higher number of infections (between 1-5 cases per school) for Alpha but it was still lower than for Delta (4-13 cases per school).
Methods
A daily scan of the literature (published and pre-published) is conducted by the Emerging Science Group, PHAC. The scan has compiled COVID-19 literature since the beginning of the outbreak and is updated daily. Searches to retrieve relevant COVID-19 literature are conducted in Pubmed, Scopus, BioRxiv, MedRxiv, ArXiv, SSRN, Research Square and cross-referenced with the COVID-19 information centers run by Lancet, BMJ, Elsevier, Nature and Wiley. The daily summary and full scan results are maintained in a refworks database and an excel list that can be searched. One of the foci is to identify studies as variants of concern or under investigation. Studies identified under this foci were further characterized in our VOC/VOI database. Targeted keyword searching was conducted within these repositories to identify relevant citations on COVID-19 and SARS-CoV-2.
Search terms used included:
- School terms: (Delta or B.1.617) and school and/or (transmission or severity)
- Daycare terms: (Delta or B.1.617) and (daycare or ECEC)
- Children terms: (Delta or B.1.617) and (children or adolescent or youth or pediatric) and/or (transmission or severity)
- Age terms: (Delta or B.1.617) and (age or years)
This review contains research published up to October 8, 2021.
Grey literature
A grey literature search was conducted to compliment the database search. The grey literature search focused on targeted governmental agencies. A detailed list of websites searched is available upon request. The grey literature search was conducted October 8, 2021.
Each potentially relevant reference was examined to confirm it had relevant data and relevant data was extracted into the review.
Acknowledgments
Prepared by: Kusala Pussegoda and Lisa Waddell, NML, Emerging Science Group, Public Health Agency of Canada.
This document underwent peer-review by a subject matter expert, editorial review and science to policy review coordinated through the Office of the Chief Science Officer.
Knowledge mobilized by the Office of the Chief Science Officer: ocsoevidence-bcscdonneesprobantes@phac-aspc.gc.ca
Evidence tables
Study | Methods | Key outcomes |
---|---|---|
Surveillance data analysis (n=4) | ||
Riley (2021)Footnote 22 Preprint Surveillance data analysis UK Jun-Jul 2021 |
Real-time Assessment of Community Transmission-1 (REACT-1) study conducted throat and nose swabs from a representative sample of people in England aged 5 years and older. Test positivity is calculated. Round 13 commenced on 24 June 2021 and swabs were collected up to and including 5 July 2021 (round 13 interim). The results from round 13 interim and complete results for round 12, in which swabs were collected from 20 May to 7 June 2021, were compared to measure the rate of change of the epidemic in England and identifying key drivers of that change (growth or decline). |
UK surveillance data reported Delta infections by age group between round 12 (May 20-Jun 7) and 13 (Jun 24-Jul 5) when the proportion of Delta cases rose from ~60% to ~90%.
|
Delahoy (2021)Footnote 20 Surveillance data analysis US Mar 2020–Aug 2021 |
This analysis uses Coronavirus Disease 2019–Associated Hospitalization Surveillance Network (COVID-NET) data to describe COVID-19–associated hospitalizations among U.S. children (0-11 years old) and adolescents (12 - 17 years old) during March 1, 2020–August 14, 2021. |
|
Siegel (2021)Footnote 21 Surveillance data analysis US Aug 2020–Aug 2021 |
This analysis uses daily COVID-19 case data were obtained from CDC's case-based surveillance system and daily emergency department (ED) visits were obtained from the National Syndromic Surveillance Program Surveillance to analyze COVID-19–associated hospitalizations among U.S. children and adolescents aged 0–17 years between August 2020–August 2021. |
|
Archarya (2021)Footnote 30 Preprint Surveillance data analysis US Jun- Aug 2021 |
Surveillance data from free symptomatic and asymptomatic community PCR testing sites in San Francisco at Unidos en Salud and UC Davis, California during a two-month period from June 17 to August 31, 2021 (n=869 samples) when Delta was dominant. Ct values were calculated and compared. Lower Ct values equate to higher viral loads and increased likelihood of being infectious. |
|
Study | Methods | Key outcomes |
---|---|---|
Outbreak investigations on transmission (n=7) | ||
Lam-Hine (2021)Footnote 23 Outbreak investigation US May- Jun 2021 |
This is an outbreak investigation of 27 Delta variant cases that occurred in Marin County, California following an exposure to an unvaccinated teacher in an elementary school during May-June 2021. Approximately 72% of eligible individuals in the city where the school was located were vaccinated. Whole genome sequencing (WGS) of all 18 available specimens identified the Delta variant. The specimen from the teacher was unavailable for WGS, and it is not known whether the teacher was infected with Delta. |
|
Nathan (2021)Footnote 29 new Outbreak investigation France Aug 2021* |
This is an outbreak investigation in six siblings infected with the Delta variant that occurred in a family setting. The parents were both fully vaccinated with BNT162b2 (Pfizer). |
|
Dougherty (2021)Footnote 25 Outbreak investigation US Apr – May 2021 |
This is an outbreak investigation in a gymnastics facility in Oklahoma, USA that occurred during April 15 to May 3, 2021. There were 47 cases linked to the gymnastics facility, including household contacts. Forty (85%) persons in the outbreak were unvaccinated. |
|
Matthias (2021)Footnote 26 Outbreak investigation USA Jun 2021 |
COVID-19 outbreaks at two events sponsored by the same organization. This is an outbreak investigation in a 5-day overnight church camp for 14-18 year olds (n=335 campers and staff) and a 2 day men's conference (n=530 attendees and staff). No proof of COVID-19 vaccination or SARS-CoV-2 pretesting or testing on arrival, or masks were required. An overnight church camp housed campers in large, shared boarding facilities of approximately 100 campers each, dined in a cafeteria together, participated in indoor and outdoor small group activities in which campers were with the same persons during program events, and participated in activities with all campers during all 5 days. Several camp staff also attended a men's conference June 18-19 at a different location than the camp. |
|
Braun (2021)Footnote 27 Outbreak investigation US Jun-Aug 2021 |
This study investigates multicomponent prevention strategies for outbreaks at an overnight camp in the US. The camps implemented multiple prevention strategies including vaccination, frequent testing, cohorting, masking, physical distancing, and hand hygiene during June–August 2021. All camps requested that staff members and campers adhere to masking and physical distancing when interacting with persons outside their immediate family for 10–14 days before arrival at camp. Vaccination coverage was 93% among eligible persons aged ≥12 years. Campers across all nine camps were required to submit at least one negative SARS-CoV-2 RT-PCR test result from a test performed within 72 hours before the start of camp, regardless of vaccination status. |
|
Jehn (2021)Footnote 24 new Outbreak investigation US Jul-Aug 2021 |
This is a summary of outbreak investigations to assess school mask policies in K-12 Schools in Maricopa and Pima Counties, which account for >75% of Arizona's population. Schools resumed in-person learning for the 2021–22 academic year during late July through early August 2021. Schools that were included had a no mask requirement, early mask requirement that was in place when school began, or a late mask requirement that was implemented any time after school began. A school-associated outbreak was defined as the occurrence of two or more laboratory-confirmed COVID-19 cases among students or staff members at the school within a 14-day period and at least 7 calendar days after school started. Logistic regression analyses adjusted for school county, enrollment size, grade levels present, Title I status, and 7-day COVID-19 case rate in the school's zip code during the week school commenced. |
|
Tonzel (2021)Footnote 28 Outbreak investigation US Jun-Jul 2021 |
This is an outbreak investigation in Louisiana youth summer camps. Delta variant became predominant during June–July 2021. This period also coincided with apparent underutilization of preventive measures such as vaccination, masking, and physical distancing. |
|
*Publication date is used to estimate when the study was conducted. |
Study | Methods | Key outcomes |
---|---|---|
Randomized control trial (n=1) | ||
Young (2021)Footnote 31 RCT UK Apr- Jun 2021 |
An open-label, cluster-randomised, controlled trial in secondary schools (11-18 years) and further education colleges in England. Schools were randomly assigned (1:1) to self-isolation of school-based COVID-19 contacts for 10 days (control) or to voluntary daily lateral flow device (LFD) testing for 7 days with LFD-negative contacts remaining at school (intervention).
Analysis: |
Infections:
School absences:
|
Ecologic studies (n=1) |
||
Budzyn (2021)Footnote 39 Ecologic study US Jul – Sep 2021 |
This ecologic study assessed impact of masking in schools on incidence of infection among K-12 students across the US using data from July –September 4, 2021. Counties with 1) a valid school start date, and MCH Strategic Data included a known school mask requirement for at least one district; 2) in districts with known school mask requirements, a uniform mask requirement for all students or no students; and 3) at least 3 weeks with 7 full days of case data since the start of the 2021–22 school year were included. County-specific pediatric COVID-19 rates (number of cases per 100,000 population aged <18 years) from CDC's COVID Data Tracker were tabulated and aggregated by school start week. A multiple linear regression was constructed that adjusted for age, race and ethnicity, pediatric COVID-19 vaccination rate, COVID-19 community transmission, population density, social vulnerability index score, COVID-19 community vulnerability index score, percentage uninsured, and percentage living in poverty. |
|
Predictive models on transmission and impact of public health measures (n=6) |
||
Lasser (2021)Footnote 41 Predictive Model Austria Sep 2021* |
Cluster Analysis: Austrian data up to December 22, 2020 on 616 clusters was included to calibrate the model. Model: Outcomes:
Assumptions:
|
Infections with varying PHMs and no vaccination coverage scenarios:
With vaccine coverage of 80% in teachers/60% in household adults reduces the number of cases and vaccination in 50% of eligible students further reduces the number of cases, making it easier to control spread with other PHMs. |
Mele (2021)Footnote 40 Preprint Predictive Model US Sep 2021* |
Model:
Outcomes: additional community infections hospitalizations, deaths due to school opening. Scenarios:
Assumptions:
|
Infections by varying public health measures:
Masking or hybrid learning scenarios:
Hospitalizations and deaths:
Masking and hybrid learning scenarios:
|
Koslow (2021)Footnote 42 Predictive Model Germany Jul 2021* |
Model:
Outcomes:
Scenarios:
Assumptions:
|
Baseline scenario: lifting all PHMs in July with no masking or testing:
Lifting all PHMs in July with the exception of masks:
Delay in opening to August and lifting masking and testing:
Delay in opening to August and keeping masks and testing:
|
Zhang (2021)Footnote 33 Predictive model US Aug 2021* |
Model:
Scenarios:
Assumptions:
|
Baseline scenario:
Elementary school:
Middle school:
High school:
School absence:
|
Bilinski (2021)Footnote 32 Predictive model US Aug 2021* |
Model:
Outcomes:
Scenario without testing:
Scenario with diagnostic testing:
Assumptions:
|
Infections by varying public health measures:
Middle school:
Elementary and Middle school:
School absences:
Middle school:
|
Head (2021)Footnote 34 Predictive model US Aug 2021* |
Model: Outcomes:
Scenarios:
Assumptions/Parameters:
|
Elementary schools:
Infections with varying PHMs:
Hospitalizations:
Middle School
Infections with varying PHMs:
Hospitalizations:
High school
Infections with varying PHMs:
Impact of transmission of Delta versus Alpha:
Hospitalizations
|
Predictive models on vaccination coverage in children (n=4) |
||
Bracis (2021)Footnote 38 Predictive Model US Oct 2021* |
Model: All simulations included in this analysis started on June 1st, 2021. Scenarios:
Outcomes:
Scenarios:
Assumptions:
|
Hospitalizations:
Different proportions of physical interaction (PPI):
Extending vaccination to 5-11 year olds and changing PPIs:
Delaying vaccination:
Increasing vaccination coverage:
Length of time of social distancing:
|
McBryde (2021)Footnote 36 Predictive Model Australia Sep 2021* |
Model: A mathematical model which incorporates age-specific mixing, infectiousness, susceptibility and severity to assess the final size of the epidemic under different public health intervention scenarios in Australia. Outcomes:
Scenarios:
Assumptions:
|
Transmission:
Years of life lost, hospitalizations, and deaths:
|
Cuesta-Lazaro (2021)Footnote 35 Predictive UK Sep 2021* |
Model:
Outcomes:
Scenarios:
Assumptions:
|
The most effective interventions are a priori vaccination of children 12+ (at 80% coverage) followed by the largest reductions in school contact intensity (use of cohorting and mask policies). Number of infections:
Number of deaths:
PHMS:
|
Liu (2021)Footnote 37 Predictive Model China Sep 2021* |
Model: Outcomes:
Scenarios:
Assumptions:
|
Baseline scenario (epidemic on Sept 1, 2021):
Delaying the start of the epidemic:
PHMs during an outbreak:
Delaying the start of the epidemic and adopting PHMs:
Herd immunity:
|
*Publication date is used to estimate when the study was conducted. 89%HPDI= 89th percentile highest probability density interval (HPDI), considered more stable than 95%. |
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