Protection from Omicron Infection in Residents of Nursing and Retirement Homes in Ontario, Canada.

This was an observational cohort, not a randomised trial, so it shows associations rather than proof that a vaccine caused protection. Infections were partly inferred from dried blood spot antibody levels, and many dates were estimated from blood collection rather than symptom testing. Genomic sequencing was not available for the cohort, so Omicron infection was assumed from the December 15, 2021 onward window. Analyses did not stratify by infection severity or clinical outcomes. The low antibody threshold was chosen to detect possible asymptomatic infections, but antibody levels may be lower after asymptomatic infection. Infections in the months before the Omicron wave were dominated by Delta, so the prior infection finding may not reflect Omicron-to-Omicron protection.

Do not read these associations as proof that a fourth dose, prior infection, or Moderna caused protection; this was an observational cohort, infections were partly inferred from antibodies, and prior infections were dominated by Delta rather than confirmed Omicron.