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Prevalence and risk factors for in-school transmission of SARS-CoV-2 in Massachusetts K-12 public schools, 2020-2021

Our take —

In this preprint study, which has not yet been peer-reviewed, researchers found a relatively low number of in-school transmissions and low secondary attack rate (2.9%) among 70 public K-12 schools in Massachusetts during 2020-2021, prior to the emergence of the delta variant. By using standardized and detailed contact tracing methods in a large number of schools, the study provided important information about in-school exposures. However, there are significant concerns regarding the generalizability of the study, including which interventions were used in the school studied and if the results would hold in the context of potentially more transmissible variants.

Study design

Prospective Cohort

Study population and setting

The study objective was to characterize the secondary attack rate of SARS-CoV-2 in Massachusetts schools and identify risk factors of in-school transmission during the 2020-2021 academic school year. A convenience sample of 25 public K-12 school districts in Massachusetts were invited to participate, of which 8 districts participated and contributed data on 70 schools and >33,000 enrolled students. Each school received a standardized spreadsheet to report de-identified information on SARS-CoV-2 cases and their in-school contacts, including their role in school (e.g., student/staff), case identification method (e.g., asymptomatic or symptomatic testing), location of exposure, mask use during exposure, and number of in-school close contacts. In the primary analysis, the in-school secondary attack rate was calculated as the proportion of in-school contacts that tested positive for SARS-CoV-2 among those that were tested. 

Summary of Main Findings

There were 436 index cases identified with a total of 1,771 school-based contacts. Most school-based contacts were tested for SARS-CoV-2 (n=1327 [75%]), of which 2.9% (39/1327) were positive for SARS-CoV-2. Twenty-nine of the 39 school-based contacts that tested positive for SARS-CoV-2 were considered to reflect possible or probable in-school transmissions, resulting in an in-school secondary attack rate of 2.2% (29/1327). There was evidence of staff-to-staff (n=6), staff-to-student (n=7), student-to-staff (n=3), and student-to-student (n=13) transmissions. A greater in-school secondary attack rate was associated with the index case being a staff member (vs. student), the index case being identified via in-school contact tracing (vs. asymptomatic screening), and when the exposure occurred during lunch (vs. elsewhere). All of the in-school transmission events identified during lunch were among staff. In addition, the in-school secondary attack rate was 7 times higher if both the index case and secondary contact were unmasked as compared to when both individuals were masked. The in-school secondary attack rate did not vary by grade level of the index case or contact.

Study Strengths

A key strength of this study was the use of a standardized contact-tracing tool that collected very detailed information in real-time across a relatively large number of schools. It is also a strength that there was high SARS-CoV-2 testing coverage among in-school contacts. 


The schools included were a convenience sample and it is unclear from the study what mitigation policies were implemented during the study period, so it is difficult to know to which settings the results can be generalized.

There may have also been imperfect recall of in-school contacts and the nature of those interactions. Finally, all data were collected prior to the emergence of the delta variant, so these results may not hold for this more transmissible variant.

Value added

This large study provides critical information regarding the risk of SARS-CoV-2 transmission among >33,000 students and staff within 70 K-12 schools.

This review was posted on: 5 January 2022