Between 4 May 2020 and 12 January 2022, a total of 1191 health care workers were assessed for eligibility, and 1009 were enrolled. There were 500 randomly assigned to medical masks and 509 to the N95 respirator (Figure 1). There were 268 participants from Canada, 34 from Israel, 187 from Pakistan, and 520 from Egypt. The baseline characteristics were well balanced overall and were similar within each country (Table). However, seropositivity at baseline varied by country, with few seropositive participants in Canada (2%) and a majority (81%) seropositive in Egypt (Table). Overall, there were 185 (37.5%) participants in the medical group versus 185 (37.2%) in the N95 respirator group who were seronegative at baseline—that is, had no SARS-CoV-2 spike IgG or nucleocapsid IgG antibodies at baseline.

Follow-up began on 4 May 2020 and ended on 29 March 2022. Participants were enrolled from 4 May 2020 to 22 May 2021 in Canada, from 11 November 2020 to 27 January 2021 in Israel, from 24 June 2021 to 18 December 2021 in Pakistan, and from 19 December 2021 to 29 March 2022 in Egypt. The mean duration of follow-up was similar between the 2 study groups—9.06 weeks in the medical mask group and 9.03 weeks in the N95 respirator group. Five participants who were randomly assigned but never followed were excluded from analysis—3 in the medical mask group (1 was previously positive for COVID-19 on RT-PCR and 2 withdrew) and 2 in the N95 respirator group (1 was previously positive for COVID-19 on RT-PCR and 1 withdrew) (Figure 1). Of the resulting 1004, follow-up was complete (that is, full 10 weeks or 14 days after first vaccination) in 483 (97.1%) in the medical mask group and 489 (96.4%) in the N95 respirator group.

The primary outcome in the intention-to-treat analysis, RT-PCR–confirmed COVID-19, occurred in 52 of 497 (10.46%) in the medial mask group versus 47 of 507 (9.27%) in the N95 respirator group (HR, 1.14 [95% CI, 0.77 to 1.69]).

(via Brownstone Institute)

[This is one study, of course, but it’s still a very notable result. — Ed]