10. 1056/NEJMoa2211029-t1
Table 1. Cumulative occurrence of Covid-19 and estimated effect of mask in the 15 weeks following cancellation of state mask policy. *
* The Massachusetts Department of Elementary and Secondary Education rescinded the state’s mask policy on February 28, 2022. Details related to the investigation of differences in variances are provided in the supplementary annex. Covid-19 refers to coronavirus disease 2019.
†The study covered a total of 340,614 students and staff (294,084 students and 46,530 employees) in 72 school districts. The average population per week during the 15 weeks was 265,173 students and staff (229,899 students and 35,274 staff) in districts that eliminated mask needs and 72,053 students and staff (61,250 students and 10,803 staff) in districts that maintained mask needs. the sum of these average populations consistent with the week is not equivalent to the total population examined due to the exclusion of less than 1% of consistent conson-weeks (see Additional Appendix).
‡ A difference-in-differences style was used to estimate the mean cumulative effect of treatment among other people treated over the constant 15-week period. calculating the difference between the observed number of Covid-19 cases consistent with 1000 inhabitants in districts that eliminated mask needs and the expected number of Covid-19 cases consistent with 1000 inhabitants if mask needs had been maintained for 15 weeks after the cancellation of the masking policy. Statewide masking.
§ The number of additional Covid-19 cases related to the lifting of mask needs estimated by multiplying the number of additional Covid-19 cases consistent with 1,000 citizens related to the lifting of mask needs (from the difference in style differences) across the average population consistent with 1,000 in school districts that eliminated masking needs.
¶ The consistent percentage of Covid-19 cases in school districts that eliminated mask needs related to the lifting of mask needs was estimated by dividing the number of additional Covid-19 cases consistent with 1,000 population that were related to the lifting of mask needs (from the difference-in-differences model) through the observed number of Covid-19 cases consistent with 1,000 population in school districts that have eliminated masking needs.
‖ The percentage of COVID-19 cases across all school districts related to the lifting of mask use needs estimated by dividing the number of additional Covid-19 cases related to lifting mask wearing needs by the observed number of Covid-19 cases across school districts.