So 11 of the districts included in the “control” group already had mask waivers prior to Feb 28, 2022 (when MA DESE mask mandate was lifted for all districts regardless of vaccination status).
As of October 15, 2021, districts in MA that had a vaccination rate of at least 80% could apply for a “mask waiver”. The districts highlighted are those who applied for and were granted such waiver between Oct 15, 2021 and Feb 8, 2022. Guidance from MA DESE