Higher SARS-CoV-2 Delta variant transmission among K-12 schoolchildren can be reduced by interventions, says study

The coronavirus disease 2019 (COVID-19) pandemic has taken more than four million human lives, among hundreds of millions of documented cases of infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The advent of vaccines at the start of 2020 was a sign of hope that the pandemic might end within the next year or so.

The emergence of the delta variant threw cold water on these hopes, with its 60% hike in transmissibility and ability to resist neutralization by immune antibodies raised by natural infection with earlier variants or by the vaccines.

A new study reports the results of a model framed to explore the transmission of the delta variant among schoolchildren and the most effective intervention methods to minimize such spread.

A preprint version of the study is available on the medRxiv* server, while the article undergoes peer review.


Researchers previously thought that children below 10 might be spared the infection relative to older children and adults. Children below 18 also typically have mild or asymptomatic infection, with very low fatality rates of 2 per million. This picture may change with the delta variant, however, especially with some states opening their schools, despite low vaccination coverage and refusal to mandate mask-wearing in some areas.

One example is California, where schools have been asked to open for the fall semester of 2021, under conditions such as three-foot spacing between students, or else symptom screening, testing of symptomatic cases, and hand washing. The current study modeled parameters such as changing vaccination coverage within the school and/or community, universal mask use, and delta vs. alpha variant.

How was the study done?

An agent-based model was used by the researchers, with rates of community contact based on a survey of school-age children in nine counties of the Bay Area of California.

They explored three levels of vaccination coverage were examined, at 50%, 60% and 70%, including children aged 12 and over as well as teachers, each level matching that in the community at large, with an 85% vaccine efficacy in preventing infection.

They looked at the use of masks at varying rates of effectiveness against viral transmission, from 15% to 35% for elementary to high school students and 50% for teachers and staff. This was then combined with universal weekly testing at 85% sensitivity, with the quarantine of contacts for 14 days.

Another scenario involved bubbles of 20 students who could mix freely with each other, but reduced outside interactions by 75%.

With a vaccination coverage of 70% in the community, they varied within-school coverage between 70-95% to obtain the estimated effect. Finally, the practice of universal masking vs. masking only for the unvaccinated was modeled, without any other precautions, with universal vaccination coverage of 70% and vaccine efficacy from 45-85%.

What were the findings?

The model showed that the increase in cases over the semester, of 128 days, was least for high school students compared to other students at all combinations of interventions. The same difference was observable but less marked for teachers.

The increase was inversely proportional to universal vaccination coverage, especially among those eligible for the vaccine. With the highest coverage, 70%, but no other non-pharmaceutical interventions (NPIs), they estimated that cases would increase by 4-9 symptomatic infections/100 students.

That means that in high schools, middle schools and elementary schools, there would be 27, 37 and 25 more cases per day. With universal mask use, this would reduce to eight, 13 and 3, respectively. Approximately 2-3% more of teachers in these groups would be infected.

Transmission from adults to children is important at schools, as shown by the estimated reduction of transmission by a quarter as community and teacher vaccination rates went up, from 50% to 70%, transmission at school went down by 2%, from 8.7% to 6.6%.

Interventions within schools reduced symptomatic cases at all levels of community vaccination coverage for elementary and middle schools, but only with low coverage for high schools. This means that, for instance, at 50% community coverage, the increase in cases was 8.7 to 12.5 per 100 students in the former setting, vs. ~10 in high schools.

When masks were also used, the estimated incidence fell to 3 and 5.6 per 100 for elementary and middle school students, vs. 2 per 100.

Hospitalizations were rare under 50% coverage conditions in the absence of other precautions, at 1.3 and <5 per million students, while with masks, the rates dropped to <3 per 10 million students.

For staff and teachers, however, hospitalization rates were higher, at approximately 49 per 100,000 without NPIs, dropping to 12.6 with universal masks, but still less to 1.4 per 100,000 per semester.

Increased vaccination coverage, at 95%, was estimated to cut down the increase of cases among elementary students by over 40%, but among teachers by over 70%. Among high school students and teachers, vaccination without NPIs reduced excess cases due only to school spread to <10 per 1,000. In fact, at >90% within-school coverage, the estimated median increase was zero.

The researchers concluded that tolerable risk levels in schools could be achieved by combining mask use with at least 70% vaccination coverage in the school and community, resulting in <50 excess cases per 1,000 students and teachers overall. For high schoolers, >70% coverage was sufficient without NPIs, which became necessary among elementary students and teachers.

Should younger children be as susceptible to SARS-CoV-2 as older children and adults, masking alone may not be sufficient to achieve low rates of transmission among elementary school populations,” they commented.    

For less than two excess cases per school per semester, masks, cohorts and testing were found to be necessary, depending on the grade level and vaccination coverage.

With a high vaccine efficacy of 85% or more, universal masking vs. unvaccinated person masking made little difference to the risk of transmission. At lower efficacies, of 45%, for example, universal masking with vaccination coverage of 70% would reduce symptomatic infections by approximately 4% and 6% of middle and high school students, vs. 13% and 19% of teachers at these grade levels, respectively.  

The dominance of the delta variant led to almost ten times higher excess viral spread at 70% vaccination coverage with universal masking, relative to the alpha variant, with more hospitalizations and deaths.

What are the implications?

Vaccination is recognized by the CDC [Centers for Disease Control and Prevention] and CDPH as the leading public health strategy for reducing within-school transmission, and our results highlight that increased vaccination coverage—both among the general community and among the eligible school population—plays an essential role in limiting symptomatic illness attributable to school transmission.”

Universal masks are also important, including in high schools with <90% vaccination coverage. Cohorting reduces interactions between students by 75%, further decreasing excess cases to less than 10 per 1,000 school population. The lowest rates among the low-risk category of high school students may be achievable only with the use of both masks and weekly testing, even with vaccination rates comparable to that of the surrounding community.

Elementary school children are not yet eligible for vaccination, making it more important than community and teacher vaccination coverage be optimal to protect these children from the virus.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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