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

  1. Sandra B. Nelson
  2. Caitlin M. Dugdale
  3. Alyssa Bilinski
  4. Duru Cosar
  5. Nira R. Pollock
  6. Andrea Ciaranello

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Abstract

Introduction

The SARS-CoV-2 secondary attack rate (SAR) in schools is low when mitigation measures are adopted, Data on the relative impact of such strategies are limited. We evaluated the SARS-CoV-2 SAR in Massachusetts schools during 2020-21 and factors associated with transmission risk.

Methods

In a convenience sample of 25 Massachusetts public K-12 school districts, de-identified information about SARS-CoV-2 cases and their school-based contacts was reported using a standardized contact-tracing tool. Index cases were included if they were in school while infectious. SAR was defined as the proportion of in-school contacts acquiring SARS-CoV-2 infection and designated as possible or probable in-school transmission by school-based teams. We compared exposure-specific SAR using unadjusted risk ratios (RR) with 95% confidence intervals (CI); p-values were calculated using Fishers exact tests.

Results

Eight districts (70 schools with >33,000 enrolled students) participated. There were 435 index cases and 1,771 school-based contacts (Table 1). Most contacts (1327/1771 [75%]) underwent SARS-CoV-2 testing and 39/1327 (2.9%) contacts tested positive. Of 39 positive contacts, 10 (25.6%) had clear out-of-school exposures and were deemed not in-school transmissions, so were excluded from further calculations. Twenty-nine (74.4%) contacts were deemed possible or probable in-school transmissions, resulting in an in-school SAR of 2.2%. Of the 29 in-school transmissions, 6 (20.7%) were staff-to-staff, 7 (24.1%) were staff-to-student, 3 (10.3%) were student-to-staff, and 13 (44.8%) were student-to-student; 6 (20.7%) occurred from index cases attending work/school while symptomatic. The unadjusted SAR (Table 2) was significantly higher if the index case was a staff member versus a student (RR 2.18, 95% CI 1.06-4.49; p=0.030), if the index case was identified via in-school contact tracing versus via school-based asymptomatic testing (RR 8.44, 95% CI 1.98-36.06; p=0.001), 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 parties were unmasked versus both masked (RR 6.98, 95% CI 3.09-15.77; p<0.001). For students, SAR did not differ by grade level.

Conclusions

Secondary attack rates for SARS-CoV-2 were low in public school settings with comprehensive mitigation measures in place before the emergence of the delta variant; lack of masking and staff-to-staff dining were associated with increased risk.

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  1. NCRC

    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. 

    Limitations

    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.

  2. ScreenIT

    SciScore for 10.1101/2021.09.22.21263900: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    EthicsIRB: Full methodologic details are included in Supplemental Material The study was approved by the Mass General Brigham Institutional Review Board.
    Sex as a biological variablenot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.

    Table 2: Resources

    No key resources detected.

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    Our study has important limitations. Although contact-tracing information was recorded in real time, districts may have inferred missing data inaccurately, with potential for recall bias. These data are derived from the 2020-21 school year, before the emergence of the more-transmissible delta variant. We lacked sufficient data to assess other potentially confounding factors, such as duration of exposure, classroom distancing, ventilation, and classroom density. Notably, all reported classroom exposures were masked, so these results do not directly inform the impact of masking within classrooms. The generalizability of these data for the 2021-22 year is uncertain. Most schools are now resuming full in-person learning with greater classroom density, reduced distancing, and variable approaches to masking. While community and student/staff vaccination should reduce the number of people entering school buildings with SARS-CoV-2 infection, this impact may be offset by reduced mitigation measures and more transmissible variants.4 Ongoing surveillance of in-school SARS-CoV-2 transmission is critical to inform decisions about school-based mitigation measures as the pandemic evolves.

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    Results from scite Reference Check: We found no unreliable references.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.

  3. Version published to 10.1101/2021.09.22.21263900v1 on medRxiv