COVID-19 vaccine & booster effectiveness against omicron: 12 CDC findings

Two separate studies published by the CDC Jan. 21 explored vaccine and booster dose effectiveness against the periods in which delta and omicron were emerging and predominant.

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. 


  • 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). 


  • 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. 


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