In 2 high-risk populations (residents and staff of California prisons), mRNA vaccinations and previous infections were effective against the omicron variant of SARS-CoV-2, researchers report, with lower estimates of effectiveness among those infected before the period of delta predominance. Those who had received 3 vaccine doses had significantly more protection than those who had received 2 doses, including among previously infected persons, the authors report.
The retrospective cohort study analyzed the risk of infection during the omicron wave (Dec. 24, 2021, through Apr. 14, 2022). Weighted Cox models compared the effectiveness (measured as 1 minus the hazard ratio) of vaccination and previous infection across combinations of vaccination and infection history.
“Among 59,794 residents and 16,572 staff, the estimated effectiveness of previous infection against omicron infection among unvaccinated persons who had been infected before or during the period of delta predominance ranged from 16.3% (95% confidence interval [CI], 8.1 to 23.7) to 48.9% (95% CI, 41.6 to 55.3),” write the researchers. “Depending on previous infection status, the estimated effectiveness of vaccination (relative to being unvaccinated and without previous documented infection) ranged from 18.6% (95% CI, 7.7 to 28.1) to 83.2% (95% CI, 77.7 to 87.4) with two vaccine doses and from 40.9% (95% CI, 31.9 to 48.7) to 87.9% (95% CI, 76.0 to 93.9) with three vaccine doses. Incremental effectiveness estimates of a third (booster) dose (relative to two doses) ranged from 25.0% (95% CI, 16.6 to 32.5) to 57.9% (95% CI, 48.4 to 65.7) among persons who either had not had previous documented infection or had been infected before the period of delta predominance.”