Risk factors associated with in-school transmission of SARS-CoV-2

Children's Health

Recently, the Centers for Disease Control and Prevention (CDC) highlighted the need to reopen the schools during the fall of 2021 to benefit from in-person learning. In addition, to prevent coronavirus disease 2019 (COVID-19) at K-12 schools, CDC recommended universal indoor masking by all students, staff, teachers, and visitors, regardless of their vaccination status.

As schools resume in the United States and COVID-19 cases amongst children begin to rise, a new study published on the medRxiv* preprint server has highlighted the risk factors for in-school transmission of COVID-19. They also provide evidential support for the layered prevention strategies in schools settings.

The likelihood of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission in schools is low when appropriate preventive measures are implemented. However, data on the relative impact of such strategies on transmission behavior is limited.

The measure of secondary attack rate (SAR) can provide an indication of how social interactions in schools would relate to transmission risk. The team of researchers from the United States has investigated in-school SARS-CoV-2 SAR and the factors associated with transmission risk in Massachusetts schools (during the year 2020-21).

Secondary attack rate (SAR) and Risk ratio (RR)

SAR is the proportion of the population exposed to the primary case that contracts the disease (COVID-19 in the current study) as a result of SARS-CoV-2 exposure. It focuses on the fate of a single infected index case (first documented case) that comes into contact with many susceptible individuals in the population and is defined as the ratio of total secondarily infected population to the total susceptible people exposed to the index case.

Risk ratio or relative risk (RR) is another important measure employed in this study that compares the risk of disease among one group with the risk among another group. It is derived by dividing the SAR in group 1 by the SAR in group 2.

The study

Data resulting from 70 schools of 8 Massachusetts K-12 school districts with >33,000 enrolled students were evaluated. De-identified information on SARS-CoV-2 index cases and their in-school contacts were reported using a standardized contact-tracing tool. Exposure-specific SAR was compared using unadjusted risk ratios (RR) with 95% confidence intervals (CI).

Study findings

Drawing on the data from 70 Massachusetts schools, the team identified 435 index cases and 1,771 school-based contacts. Most contacts (1327/1771 [75 %]) underwent SARS-CoV-2 testing and 39/1,327 contacts tested positive for the virus. Of the 39 secondary cases, 10 contracted the disease very clearly from sources outside of school and, therefore, were excluded from the study. Based on 29 possible in-school transmissions, an overall SAR of 2.2 % (29/1,327*100) was observed.

Of the 29 secondary cases, 24.1 % were staff-to-student transmissions, 20.7 % were staff-to-staff transmissions, 44.8 % were student-to-student and 10.3 % were student-to-staff transmissions. On breaking down the figures of 435 index cases, the team found that 286 were student and 149 were staff index cases with mean contacts/case 4.06 and 3.30 for each, respectively.

Despite having lower mean contacts/case compared to students, staff had a significantly higher SAR of 3.6% (students’ SAR – 1.7%) (RR 2.18, 95% CI 1.06-4.49; p=0.030). Also, the SAR was significantly higher if the exposure occurred at lunch versus elsewhere (RR 5.74, 95% CI 2.11-15.63; p<0.001; all lunch transmissions were staff-to-staff), and if both groups were unmasked versus both masked (RR 6.98, 95% CI 3.09-15.77; p<0.001). Thus, staff-to-staff dining and lack of masking were associated with increased risk of SARS-CoV-2 transmission.

Interestingly, there was no significant difference by grade level in students’ SAR.

Study implications

The team identified low SARS-CoV-2 SAR in public school settings where thorough mitigation measures were followed.

“This study provides in-depth analysis of transmission context, extending existing literature by highlighting the potential benefit of masking in school settings, the risk of lunchtime adult-to-adult transmission, and the similarity of SARs across ages/grade levels,” say Nelson and colleagues.


The team warns that while schools are now resuming full in-person education, the emergence of more transmissible delta variants and reduced preventive measures such as higher classroom density, reduced physical distancing, and variable approaches to masking might affect the applicability of this data which was derived from the year 2020-21.

*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:

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