Masks Work -Here’s the Data

Open Access: MMWR: Effectiveness of Face Mask or Respirator Use in Indoor Public Settings for Prevention of SARS-CoV-2 Infection — California, February–December 2021

Methods: This interview-questionnaire study used a test-negative case-control design, enrolling persons who received a positive (case-participants) or negative (control-participants) SARS-CoV-2 test result, from among all California residents, without age restriction, who received a molecular test result for SARS-CoV-2 during February 18–December 1, 2021. A total of 652 case- and 1,176 control-participants were enrolled in the study equally across nine multi-county regions in California.

Limitations included the following:

  • This study did not account for other preventive behaviors that could influence risk (eg distancing)
  • This analysis relied on an aggregate estimate of self-reported face mask or respirator use across, for some participants, multiple indoor public locations
  • Estimates do not account for face mask or respirator fit
  • Data collection occurred before the expansion of the SARS-CoV-2 B.1.1.529 (Omicron) variant
  • Face mask or respirator use was self-reported
  • Variability of exposures