Review 1 "Model-Based Assessment of SARS-CoV-2 Delta Variant Transmission Dynamics Within Partially Vaccinated K-12 School Populations"

Jinzhi Lei

doi:doi:10.1162/2e3983f5.bf9d67e7

Model-based assessment of SARS-CoV-2 Delta variant transmission dynamics within partially vaccinated K-12 school populations

by Jennifer R. Head, Kristin L. Andrejko, and Justin V. Remais

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dx.doi.org

Description

AbstractBackgroundWe examined school reopening policies amidst rising transmission of the highly transmissible Delta variant, accounting for vaccination among individuals aged 12 years and older, with the goal of characterizing risk to students and teachers under various within-school non-pharmaceutical interventions (NPIs) combined with specific vaccination coverage levels.MethodsWe developed an individual-based transmission model to simulate transmission of the Delta variant of SARS-CoV-2 among a synthetic population, representative of Bay Area cities. We parameterized the model using community contact rates from vaccinated households ascertained from a household survey of Bay Area families with children conducted between February – April, 2021.Interventions and outcomesWe evaluated the additional infections in students and teachers/staff resulting over a 128-day semester from in-school instruction compared to remote instruction when various NPIs (mask use, cohorts, and weekly testing of students/teachers) were implemented in schools, across various community-wide vaccination coverages (50%, 60%, 70%), and student (≥12 years) and teacher/staff vaccination coverages (50% - 95%). We quantified the added benefit of universal masking over masking among unvaccinated students and teachers, across varying levels of vaccine effectiveness (45%, 65%, 85%), and compared results between Delta and Alpha variant circulation.ResultsThe Delta variant sharply increases the risk of within-school COVID-transmission when compared to the Alpha variant. In our highest risk scenario (50% community and within-school vaccine coverage, no within-school NPIs, and predominant circulation of the Delta variant), we estimated that an elementary school could see 33-65 additional symptomatic cases of COVID-19 over a four-month semester (depending on the relative susceptibility of children <10 years). In contrast, under the Bay Area reopening plan (universal mask use, community and school vaccination coverage of 70%), we estimated excess symptomatic infection attributable to school reopening among 2.0-9.7% of elementary students (8-36 excess symptomatic cases per school over the semester), 3.0% of middle school students (13 cases per school) and 0.4% of high school students (3 cases per school). Excess rates among teachers attributable to reopening were similar. Achievement of lower risk tolerances, such as <5 excess infections per 1,000 students or teachers, required a cohort approach in elementary and middle school populations. In the absence of NPIs, increasing the vaccination coverage of community members from 50% to 70% or elementary teachers from 70% to 95% reduced the estimated excess rate of infection among elementary school students attributable to school transmission by 24% and 41%, respectively. We estimated that with 70% coverage of the eligible community and school population with a vaccine that is ≤65% effective, universal masking can avert more cases than masking of unvaccinated persons alone.ConclusionsAmidst circulation of the Delta variant, our findings demonstrated that schools are not inherently low risk, yet can be made so with high community vaccination coverages and universal masking. Vaccination of adult community members and teachers protects unvaccinated elementary and middle school children. Elementary and middle schools that can support additional interventions, such as cohorts and testing, should consider doing so, particularly if additional studies find that younger children are equally as susceptible as adults to the Delta variant of SARS-CoV-2.LimitationsWe did not consider the effect of social distancing in classrooms, or variation in testing frequency, and considerable uncertainty remains in key transmission parameters.

RR:C19 Evidence Scale rating by reviewer:

Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.

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Review:

This article developed an individual-based transmission model to simulate transmission of the Delta variant of SARS-Cov-2 among a synthetic population, representative of school-aged population and their adult family members in Bay area cities. The model separated schools into elementary, middle and high schools, and assigned individual grades and classroom within each school, based on age. All individuals interacted with all other individuals in one of six ways, according to hierarchy of highest shared membership: household > classroom or workplace > grade > school > community.

Stochastic simulation used to simulate SARS-CoV-2 transmission. At each time increment (one day), each individual was associated with an epidemiological state. Transmission was implemented probabilistically for contacts between susceptible and infectious individuals in the asymptomatic or symptomatic and non-hospitalized states. Movement of an individual on one day from a susceptible to exposed class was determined by a Bernoulli random draw with probability of success given by the infection rate and contact relations. The model included various contact relationship through the contact rate functions of different cohort individuals. Effects of various non-pharmaceutical interventions (NPIs) were also considered in the model through variances in model coefficients.

This study evaluated the additional infections in students and teachers/staff resulting over a 128-day semester from in-school compared to remote instruction when various NPIs implemented in schools, across various community-wide vaccination coverages and student and teacher/staff vaccination coverages. This study also quantified the added benefit of universal masking over masking among unvaccinated students and teachers, across varying levels of vaccine effectiveness, and compared results between Delta and Alpha variant circulation.

Findings in this study demonstrated that schools are not inherently low risk under Delta variant circulation, yet can be made so with high community vaccination coverages and universal masking. This conclusion is in general supported by the proposed model.