Image of nurse administering vaccine to patient's arm

A third dose of the Pfizer or Moderna vaccines has been significantly more effective in preventing severe COVID-19 outcomes during the rise of the omicron variant than the primary doses alone, according to three large, real-world studies conducted by the Centers for Disease Control and Prevention.

One study examined cases and deaths across 25 states from April through December, spanning the rise of the delta and then the omicron variants. During this time, unvaccinated older adults were 49 times more likely to be hospitalized with COVID-19 than their peers who had received a primary series of shots plus a booster, according to an analysis of the data by the Washington Post. 

In addition, protection against COVID-19 infection and death was higher among all boosted adults when delta was dominant. When omicron came into prominence, the same was true for cases (data was not yet available for deaths in this later time period). 

What’s more, the added benefits of booster doses were “especially prominent” among people aged 50 years and older, CDC researchers reported.

Reduced acute-care encounters

Boosters also appear to reduce the risk of acute care encounters. Another CDC study among adults of all ages found that receipt of a third COVID-19 shot was more than 80% effective in preventing COVID-19-related emergency department and urgent care visits when either delta or omicron were dominant. It was also more than 90% effective in preventing COVID-19 hospitalizations during both time periods.

The third study, conducted among adults who sought COVID-19 testing in December, found that two doses of the Pfizer or Moderna vaccines were not significantly protective against infection with the omicron variant, but three doses provided 67% protection. 

“In the setting of omicron, higher booster coverage rates may be needed to achieve the same public health benefit as during delta predominance,” concluded Emma K. Accorsi, Ph.D., and colleagues from the CDC’s Epidemic Intelligence Service.