Omicron boosters are unlikely to be very effective against mild Covid illness but are likely to prevent hospitalizations, experts say

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

The Centers for Disease Control and Prevention found in a real-world study published this week that the boosters are less than 50% effective against mild illnesses in nearly all adult age groups compared to people who have not been vaccinated.

For seniors, the booster was 19% effective in preventing mild disease when administered as their fourth dose, compared to the unvaccinated. It was 23% effective against mild disease when given as the fifth dose.

While the vaccine’s effectiveness against mild illnesses was low, people who received the boosters fared better than those who didn’t. The booster increased people’s protection against mild disease by 28% to 56% compared to those who received only the old injections, depending on age and when they received their last dose.

The Food and Drug Administration authorized the boosters in late August with the goal of restoring the high level of protection the vaccines demonstrated in late 2020 and early 2021. At that time, the injections were more than 90% effective against infection. But the CDC’s first real-world data indicates the boosters fall short of those lofty expectations.

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

Moore said people at higher risk of Covid have every reason to get a booster as it slightly increases protection. But he said commonsense measures such as masking and avoiding large crowds remain important tools for vulnerable groups as the boosters are not very effective against infection.

The CDC study looked at more than 360,000 adults with healthy immune systems who tested for Covid at retail pharmacies from September to November, when omicron BA.5 was dominant. The participants received the booster, received two or more doses of the old injections, or were not vaccinated. It then compared those who tested positive for Covid to those who didn’t.

The study didn’t evaluate how well the boosters performed against serious illness, so it’s still unclear whether they offer better protection against hospitalization than the old injections. The CDC said in a statement that it will provide data on more serious outcomes when they become available.

Andrew Pekosz, a virologist at Johns Hopkins University, said the fact that the injections provide some protection against infection in an era of highly immune-evasive omicron subvariants is a good sign that they will provide strong protection against hospitalization. The vaccines have always performed better against severe diseases than mild ones, he said.

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

Dr. Paul Offit, a member of the FDA’s vaccine advisory committee, said preventing mild disease is not a viable public health strategy because the antibodies that block infection simply decline over time.

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

Dr. Celine Gounder, a senior public health officer at the Kaiser Family Foundation, said she is not alarmed by the data. Reducing risk by even a modest amount at the individual level can have a significant positive effect on public health at the population level.

“If you can reduce the risk in the elderly by even 30%, even 20%, that is significant when 90% of COVID deaths occur in that group,” said Gounder. “For me, it’s really about whether you keep that 65-year-old out of the hospital.”

The boosters, called bivalent vaccines, target both omicron BA.5 and the original Covid strain that first emerged in Wuhan, China in 2019. The original injections, called monovalent vaccines, contain only the first Covid strain.

It is still unclear how the boosters will perform against more immune-evasive omicron subvariants, such as BQ.1 and BQ.1.1, which are now dominant in the US. Pfizer and Moderna said last week that early clinical trial data shows that the boosters stimulate an immune response against these subvariants.

According to CDC data, about 11% of those eligible for the new booster, or 35 million people, have received it to date. About 30% of seniors have received the injection.