Booster shots of Covid-19 vaccines significantly increased protection against hospitalization from both the omicron and delta variants, according to three new studies released by CDC that offer the most comprehensive look yet at booster shot effectiveness in the United States.

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How booster shots perform against omicron

In one of the studies, published in CDC's Morbidity and Mortality Weekly Report (MMWR), researchers examined data on hospitalizations, ED visits, and visits to urgent care clinics in 10 states from Aug. 26, 2021, to Jan. 5. The delta variant drove most infections nationwide during the first part of that period, while the omicron variant dominated during the latter part—so by looking at how risks evolved over time, researchers could estimate how effectively vaccines protected against each variant.

The researchers found that, for individuals who had received their second shot of an mRNA vaccine more than six months earlier, protection against hospitalization with the omicron variant was about 57%, while protection against ED and urgent care visits was about 38%. A booster shot increased those figures to 90% and 82%, respectively.

Another study, also published in MMWR, examined almost 10 million Covid-19 cases and more than 117,000 Covid-19 deaths in 25 states between April 4, 2021, and Dec. 25, 2021. The study found that vaccinated people had significantly lower death rates than unvaccinated people, and that boosted individuals were significantly better-protected than unboosted individuals. Booster shots were especially protective among people ages 65 and older, the researchers found.

Separately, CDC released hospitalization data from Nov. 6, 2021, to Dec. 25, 2021, that showed similar results, finding that unvaccinated adults ages 65 and older were almost 49 times as likely to go to the hospital for Covid-19 as those who had received a booster shots.

In a third study, published in JAMA, researchers examined data from more than 70,000 people who received a Covid-19 test. They found that boosted individuals had better protection against symptomatic Covid-19 than those who had not received a booster or were unvaccinated, although the protection was estimated to be lower against the omicron variant.

What experts are saying about boosters and omicron

About the recent data, CDC Director Rochelle Walensky said, "Protection against infection and hospitalization with the omicron variant is higher for those who are up to date with their vaccination."

"There are still millions of people who are eligible for a booster dose and have not yet received one," Walensky added. "As we continue to face the omicron variant … I urge all who are eligible to get their booster shot to get it as soon as possible."

While the studies suggest booster shots provide significant protection against the omicron variant, it's too soon to know whether that protection may wane over time, said Natalie Dean, a biostatistician at Emory University.

"We just have to recognize that all these estimates of omicron third-dose protection are going to be people who are pretty recently boosted," she said. (Mandavilli, New York Times, 1/21; Weixel, The Hill, 1/21; Doherty, Axios, 1/21; Walker, MedPage Today, 1/21; Stobbe, Associated Press, 1/21; Lonas, The Hill, 1/21)