Table 1. Cumulative Incidence of Covid-19 and Estimated Effect of Lifting Masking Requirements during the 15 Weeks after the Statewide Masking Policy Was Rescinded.*.
| Population† | Cumulative Covid-19 Cases during the 15-Week Period | Difference-in-Differences Estimates of Covid-19 Cases Associated with the Lifting of Masking Requirements during the 15-Week Period | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| All Districts | Districts That Lifted Masking Requirements | Districts That Sustained Masking Requirements | No. of Additional Cases per 1000 (95% CI)‡ | No. of Additional Cases (95% CI)§ | Percentage of Cases in Districts That Lifted Masking Requirements (95% CI)¶ | Percentage of Cases in All Districts (95% CI)‖ | ||||
| no. of cases | no. of cases per 1000 | no. of cases | no. of cases per 1000 | no. of cases | no. of cases per 1000 | |||||
| Students and staff | 40,416 | 119.8 | 35,651 | 134.4 | 4,766 | 66.1 | 44.9 (32.6–57.1) | 11,901 (8651–15,151) | 33.4 (24.3–42.5) | 29.4 (21.4–37.5) |
| Students | 32,198 | 110.6 | 28,524 | 124.1 | 3,674 | 60.0 | 39.9 (24.3–55.4) | 9,168 (5594–12,743) | 32.1 (19.6–44.7) | 28.5 (17.4–39.6) |
| Staff | 8,218 | 178.4 | 7,127 | 202.1 | 1,091 | 101.0 | 81.7 (59.3–104.1) | 2,882 (2092–3673) | 40.4 (29.4–51.5) | 35.1 (25.5–44.7) |
The Massachusetts Department of Elementary and Secondary Education rescinded the statewide masking policy on February 28, 2022. Details regarding the difference-in-differences analysis are provided in the Supplementary Appendix. Covid-19 denotes coronavirus disease 2019.
The study included a total of 340,614 students and staff (294,084 students and 46,530 staff) across 72 school districts. The number of districts that lifted masking requirements and the number of districts that sustained masking requirements varied according to reporting week; the mean population per week during the 15-week period was 265,173 students and staff (229,899 students and 35,274 staff) in districts that lifted masking requirements and was 72,053 students and staff (61,250 students and 10,803 staff) in districts that sustained masking requirements. The sum of these mean populations per week does not equal the total study population because of the exclusion of less than 1% of person-weeks (see the Supplementary Appendix).
A difference-in-differences model was used to estimate the cumulative average treatment effect among the treated during the 15-week period. The number of additional Covid-19 cases per 1000 population that were associated with the lifting of masking requirements was estimated by calculating the difference between the observed number of Covid-19 cases per 1000 population in districts that lifted masking requirements and the expected number of Covid-19 cases per 1000 population if masking requirements had been sustained during the 15 weeks after the statewide masking policy was rescinded.
The number of additional Covid-19 cases that were associated with the lifting of masking requirements was estimated by multiplying the number of additional Covid-19 cases per 1000 population that were associated with the lifting of masking requirements (from the difference-in-differences model) by the mean population per 1000 in school districts that lifted masking requirements.
The percentage of Covid-19 cases in school districts that lifted masking requirements that were associated with the lifting of masking requirements was estimated by dividing the number of additional Covid-19 cases per 1000 population that were associated with the lifting of masking requirements (from the difference-in-differences model) by the observed number of Covid-19 cases per 1000 population in school districts that lifted masking requirements.
The percentage of Covid-19 cases in all school districts that were associated with the lifting of masking requirements was estimated by dividing the number of additional Covid-19 cases that were associated with the lifting of masking requirements by the observed number of Covid-19 cases in all school districts.