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Pilot Investigation of SARS-CoV-2 Secondary Transmission in Kindergarten Through Grade 12 Schools Implementing Mitigation Strategies – St. Louis County and City of Springfield, Missouri, December 2020

Our take —

This small pilot study aimed to assess the effectiveness of a modified quarantine procedure for in-school SARS-CoV-2 exposure, whereby fully masked persons at time of exposure to a confirmed case were exempt from the typical 14-day home quarantine. However, only two incident infections were observed among contacts of confirmed SARS-CoV-2 cases, neither of whom were eligible for the modified quarantine procedure, which limited study inferences. Despite a small sample size and other key limitations, this study provides a good blueprint for how modified quarantine procedures within schools and school-like settings (e.g., daycares) could be studied in larger populations.

Study design

Prospective Cohort

Study population and setting

Fifty-five schools in the city of Springfield, Missouri and in St. Louis County, Missouri conducting in-person school from December 8-18, 2020 were monitored for SARS-CoV-2 cases as part of a two-week pilot study. Schools in both locations employed a number of mitigation strategies to limit the spread of SARS-CoV-2 in schools during this time period, including masking, ventilation, increased spacing between desks, symptom screening, and quarantine procedures for close contacts of individuals who tested positive. However, one key difference between them was that Springfield’s quarantine procedures for close contacts allowed contacts to remain in school if they complied with masking policies during time of exposure to a confirmed case (i.e. “modified quarantine”).

School administrators were informed about persons who tested positive and who had been present at the school or a school-related event. School officials then traced that person’s close contacts and offered SARS-COV-2 testing. Contacts were subjected to either at-home or modified quarantine procedures, depending on the school district and whether criteria for the modified quarantine procedure were met (i.e., mask compliance at time of exposure)

Students, teachers, and staff who tested positive for SARS-CoV-2 and their contacts were considered eligible for the pilot study and offered enrollment. A thorough contact tracing procedure, including a process to determine where the infection most likely occurred (i.e., within or outside of school), and SARS-COv-2 testing were administered to study participants.

Summary of Main Findings

There were 56 SARS-CoV-2 index cases identified in 22 out of 55 schools. Among these cases, 270 contacts were enumerated and offered enrollment into the study. Of the 326 identified cases and contacts, 193 (59%) agreed to participate (37 cases and 156 contacts), of whom the majority (>65%) were students. Among participating contacts, 54 (35%) declined SARS-CoV-2 testing. Of the 102 contacts who were tested, only two tested positive for SARS-CoV-2, and both of these individuals were within schools in Springfield, where the modified quarantine procedure was in place. However, these two cases were subjected to at-home quarantine because they did not meet criteria for the modified quarantine procedure.

Study Strengths

The study employed intensive, well-defined contact tracing efforts that determined the likely sources of SARS-CoV-2 exposure and infection within schools.


There were several limitations to this study. First, participation and testing rates among eligible cases and their contacts were low (e.g., 38% of eligible contacts were tested for SARS-CoV-2). This raises the possibility of selection bias, whereby people who chose to participate may have had different SARS-CoV-2 positivity rates than those who did not. Second, the sample size of this study was small with very few outcomes (only two positive cases among contacts), which limits conclusions that can be drawn about differences in test positivity by quarantine procedure. Third, there may have been significant differences between schools and local epidemics dynamics that could have impacted study inferences.

Value added

This pilot study provides important preliminary data on the potential utility of modified quarantine procedures for in-school SARS-CoV-2 exposures, which should be evaluated in future studies.

This review was posted on: 7 May 2021