The new omicron Covid boosters are unlikely to be very effective at preventing Covid infections and mild illness, but experts believe they will help keep the elderly and other vulnerable groups out of the hospital this winter.

In a real-world study published this week, the Centers for Disease Control and Prevention discovered that boosters are less than 50% effective against mild illness across almost all adult age groups when compared to unvaccinated people.

When administered as the fourth dose to seniors, the booster was 19% more effective than the unvaccinated at preventing mild illness. When given as their fifth dose, it was 23% effective against mild illness.

Despite the vaccine’s low effectiveness against mild illness, people who received boosters fared better than those who did not. Depending on age and when they received their last dose, the booster increased people’s protection against mild illness by 28% to 56% compared to those who only received the old shots.

The Food and Drug Administration approved the boosters in late August, with the goal of restoring the vaccines’ high levels of protection from late 2020 to early 2021. The shots were more than 90% effective against infection at the time. However, the CDC’s first real-world data show that the boosters aren’t living up to those lofty expectations.

“The boosters give you some additional protection but it’s not that strong, and you shouldn’t rely on it as your sole protective device against infection,” said John Moore, a professor of microbiology and immunology at Weill Cornell Medical College.

Moore believes that people who are at higher risk of Covid should get a booster because it only slightly increases protection. However, he stated that common sense precautions such as masking and avoiding large crowds are still important tools for vulnerable groups because the boosters aren’t very effective against infection.

The CDC study included over 360,000 adults with healthy immune systems who tested positive for Covid at retail pharmacies between September and November, when omicron BA.5 was dominant. Participants were either given the booster, two or more doses of the old shots, or were unvaccinated. The results were then compared between those who tested positive for Covid and those who did not.

Because the study did not assess how well the boosters performed against severe disease, it remains unclear whether they will provide better protection against hospitalization than the old shots. In a statement, the CDC stated that it will provide data on more severe outcomes as soon as it becomes available.

The fact that the shots are providing some protection against infection in an era of highly immune evasive omicron subvariants, according to Andrew Pekosz, a virologist at Johns Hopkins University, is a good sign that they will provide strong protection against hospitalization. He claims that vaccines have always performed better against severe disease than against mild illness.

“It’s better than nothing. Certainly, it doesn’t sort of show that the protection is incredibly high against infection,” Pekosz said. “I would expect that you would then see even greater protection from hospitalization or death.”

Dr. Paul Offit, a member of the FDA’s vaccine advisory committee, believes that attempting to prevent mild illness is not a viable public health strategy because the antibodies that prevent infection deteriorate over time.

“Protection against mild disease just isn’t that good in the omicron subvariant era. The goal is protecting against severe disease,” said Offit, an infectious disease expert at Children’s Hospital of Philadelphia who helped develop the rotavirus vaccine.

The data do not concern Dr. Celine Gounder, a senior public health fellow at the Kaiser Family Foundation. Even a small reduction in risk at the individual level can have a significant positive effect on population health.

“If you can reduce risk among the elderly by even 30%, even 20%, that is significant when 90% of the COVID deaths are occurring in that group,” Gounder said. “For me, what’s really gonna matter is are you keeping that 65 year old out of the hospital.”

The bivalent vaccines target both omicron BA.5 and the original Covid strain, which first appeared in Wuhan, China in 2019. The first shots, known as monovalent vaccines, contained only the first Covid strain.

It’s unclear how the boosters will fare against more immune evasive omicron subvariants like BQ.1 and BQ.1.1, which are now prevalent in the United States. Last week, Pfizer and Moderna announced that preliminary clinical trial data show that the boosters induce an immune response against these subvariants.

According to CDC data, approximately 11% of those eligible for the new booster, or 35 million people, have received it thus far. Around 30% of seniors have had the shot.

(Adapted from BeASmart.com)

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