Study: A multistate analysis of 222,772 emergency department and urgent care encounters and 87,904 hospitalizations among adults with COVID-19-like illness during Aug. 26, 2021 to Jan. 5, 2022.
Findings:
- Estimates of vaccine effectiveness against COVID-19 declined during the omicron-predominant period compared to the delta-predominant period.
- During both periods, vaccine effectiveness was significantly lower among patients who received their second mRNA COVID-19 vaccine dose more than 180 days before the medical encounters compared to those vaccinated more recently.
- During both delta and omicron periods, receipt of a third vaccine dose was highly effective (94 percent and 82 percent, respectively) at preventing COVID-19 emergency department and urgent care encounters and preventing hospitalizations (94 percent and 90 percent).
- The effectiveness of two mRNA vaccine doses were higher against COVID-19 hospitalizations than against ED or urgent care encounters, especially amid omicron.
- Compared to the general public, immunocompromised adults had lower third-dose vaccine effectiveness against COVID-19 ED and urgent care encounters and hospitalization.
Study: Case and death rates and incidence rate ratios were estimated among unvaccinated and fully vaccinated Americans by receipt of booster doses amid different time periods: pre-delta (April-May 2021), delta emergence (June 2021), delta predominance (July-November 2021) and omicron emergence (December 2021).
Findings:
- Average weekly, age-standardized rates of cases and deaths per 100,000 people were higher during periods of delta predominance and omicron emergence than during pre-delta and delta emergence periods.
- Case and death rates were consistently higher in all periods among unvaccinated Americans.
- The added benefits of booster doses against infection and death were especially prominent among Americans ages 50 and up.
- Rates of COVID-19 cases were lowest among fully vaccinated people with a booster dose.
- Differences in case rates between fully vaccinated people with and without a booster dose decreased over time; however, more protection was afforded from booster doses, even during omicron emergence.
- Case rates increased substantially among unvaccinated and vaccinated people when omicron became predominant in December, resulting in declining vaccine effectiveness estimates.
- Because of reporting lags, the influence of the omicron variant on COVID-19 deaths by vaccination status in December could not be evaluated.
