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Effectiveness of three versus six feet of physical distancing for controlling spread of COVID-19 among primary and secondary students and staff: A retrospective, state-wide cohort study

Our take —

The US CDC recently updated its guidelines for schools to recommend at least three feet of spacing between students in classroom settings, half of the six feet it had previously recommended. This study of 251 Massachusetts school districts provides some of the only evidence from a head-to-head comparison of different distancing requirements. The authors found no statistically significant difference in the incidence rates of SARS-CoV-2 infection comparing districts requiring at least 6 feet of distance to districts requiring only 3 feet of distance. Actual classroom spacing was not observed or measured; district-level requirements were used as a proxy for spacing. The study was conducted during a time when most schools had lower attendance than normal and required near-universal masking, and could not determine whether infections were acquired in school or elsewhere.

Study design

Retrospective Cohort

Study population and setting

This study compared COVID-19 case counts in 251 Massachusetts school districts with different requirements for physical distancing between students (at least three feet vs. at least six feet) from September 24, 2020 to January 27, 2021. Districts were classified by the minimum distance permitted across grades (e.g., a district with a minimum requirement of at least 3 feet for some grades, even if other grades required at least 6 feet or greater distances were described as “preferred,” would be classified as at least 3 feet). Districts with intermediate distance requirements were excluded from the primary analysis. Data on distance requirements in district-level infection control plans were taken from publicly available sources, and were abstracted and classified by three investigators. District-level case counts were taken from the website of the Massachusetts Department of Elementary and Secondary Education, to which districts were required to report cases on a weekly basis. Cases were defined by a positive laboratory test for SARS-CoV-2 in a student or staff member who had been physically present in a school building during the seven days preceding the positive test. Weekly incidence was calculated by district for students and staff separately, and tested for association with distancing requirements (at least 6 feet vs. at least 3 feet) with negative binomial regression. Analyses were adjusted for community SARS-CoV-2 incidence (at the county level weighted by zip code), proportion of children living in poverty in the district, and the racial/ethnic distribution of enrolled students in the district.

Summary of Main Findings

In the 251 school districts during the study period, 537,336 students (6,400,175 person-weeks) and 99,390 staff members (1,342,574 person-weeks) were present in school buildings. There were 4,226 COVID-19 cases among students and 2,382 cases among staff members. Incidence rates among students and staff exhibited a high correlation with community incidence rates but were generally lower than those in the community. Most (77%) districts required at least 6 feet of distance between students, while 19% required at least 3 feet, and 4% had intermediate requirements. Most (64%) districts opened with less than 80% of students on campus during the study period. All districts required universal masking for all staff and for students in grade 2 and above, while 70% of districts required masking for younger grades. During the study period, most districts (>90%) implemented student cohorting, enhanced disinfection, and various ventilation interventions. The unadjusted incidence rate ratio (IRR) comparing districts requiring at least 6 feet of distancing to those requiring at least 3 feet of distancing was 0.89 (95% CI: 0.59 to 1.34) for students and 1.02 (95% CI: 0.75 to 1.37) for staff. After adjustment for community incidence and district-level demographic variables, the IRR was 0.79 (95% CI: 0.53 to 1.18) for students and 0.92 (95% CI: 0.67 to 1.25) for staff.

Study Strengths

Classification of distance requirements was performed carefully by three reviewers with several reliability checks. There appears to have been near-uniformity in masking requirements, which minimizes the possibility for confounding by this intervention.


The study examined distance requirements at the district level, not actual distancing behavior in classrooms. There may have been considerable variation in the actual distance between students within a given classification, and this misclassification may have biased results toward the null. Most districts were operating with significantly lower student attendance than normal, which likely permitted greater distancing than would be possible otherwise. Thus, it may be the case that some districts requiring only 3 feet of distance were able in practice to achieve greater than 3 feet of distance. There were several differences in other interventions (e.g., ventilation, cohorting) between the two types of districts that the authors did not adjust for, though it appears from Table 2 that the interventions were more common in districts requiring 6 feet of distance. Although the authors adjusted for district-level demographics and county-level infection rates, there may have been heterogeneity of transmission determinants within districts that led to residual confounding. No information was available about whether new cases were acquired in schools or elsewhere. Data were not disaggregated by grade level; any effects of distancing may differ across age groups. Because of likely effect modification by mask use, results may not apply to environments in which masks are not universally mandated.

Value added

This is the first published comparison of different distance requirements in public schools in the context of SARS-CoV-2 transmission.

This review was posted on: 26 March 2021