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Bivalent COVID boosters give some protection in CDC study, but not much

A healthcare worker prepares a dose of the COVID-19 bivalent booster at the start of a vaccination campaign.Esteban Felix/Associated Press

COVID-19 boosters from Moderna and the partnership of Pfizer and BioNTech that fight the latest Omicron variants provide only modest short-term protection against mild infections, and experts say it’s still unclear whether the updated shots are any better than earlier versions at preventing hospitalization and severe illness.

The bivalent boosters were just 43 percent effective at preventing mild illness compared to receiving no vaccine in adults 49 and under, according to a US Centers for Disease Control and Prevention study published Tuesday. In those ages 50 to 64 years, comparative protection against symptomatic COVID was 28 percent, while the booster was just 22 percent protective in adults 65 and older, the study showed.


Younger, vaccinated people benefited most from the shots that fight the BA.4 and BA.5 Omicron strains, according to the study that offers a glimpse at how the updated shots perform in the real world. For older people, protection was lower against mild illness, but still much better than being unvaccinated.

Americans so far have been reluctant to roll up their sleeves for a fourth, or even fifth, time ahead of holiday gatherings and colder weather. Experts have said testing results don’t answer key questions about the benefit of switching to formulas that the Biden administration asked companies to develop this summer to target the omicron subvariants.

The numbers “don’t show that the bivalent vaccine is any better than the monovalent vaccine,” said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “The government can help us by clearly outlining what is the goal of this vaccine.”

The bivalent boosters were authorized by the Food and Drug Administration on Aug. 31, before the completion of clinical testing of the new formulation. The findings support current COVID vaccination policies that recommend bivalent boosters for adults, wrote the authors led by researchers from CDC’s National Center for Immunization and Respiratory Diseases.


The Biden administration is announcing a six-week advertising campaign to convince people to get the shots. Bivalent boosters are needed because immunity wanes over time and variants continue to come out every several months Anthony Fauci, outgoing director of the National Institute of Allergy and Infectious Diseases, said in a White House briefing.

“You need to update the protection,” Fauci said, adding that the bivalent booster “clearly induces” a better immune response against the newer BA.4 and BA.5 variants.

Conflicting results from early studies on the bivalent booster shots spawned confusion over how well the updated vaccines work and whether they’re better than those based on the original vaccine formulations. In October, a small study from researchers at Columbia University and University of Michigan was the first to cast doubt over the benefit of bivalent boosters after the shots failed to raise neutralizing antibody levels significantly more against newer strains than four doses of the original vaccine formula. Another small, independent study from researchers at Harvard University seemed to further support those findings.

About a week later, Pfizer released more optimistic results from a company study showing their bivalent vaccine bolstered protective antibodies against omicron subvariants substantially more than its original booster in people older than 55. Moderna followed suit with similar data showing their BA.4 and BA.5-targeted shot raised antibody levels five-fold to six-fold higher than the older version.

The CDC study used data from pharmacy tests to show a short-term benefit from boosting with the new vaccines, according to Philadelphia Children’s Offit. But that was already known from both the independent and company studies, Offit said, and it doesn’t help clear up the question of whether the updated boosters provide additional, long-lasting protection against severe illness and hospitalization.


“If the goal of this vaccine is to prevent against mild disease,” he said, “you know that over time that’s going to fade.”

Messenger RNA COVID vaccines were introduced to great fanfare almost two years ago amid initial reports that they were more than 90 percent effective at staving off illness from the virus. Now, due to frequent viral mutations, those vaccines are starting to look less impressive.

“This is very difficult, nuanced, and rapidly changing science,” said Greg Poland, who heads Mayo Clinic’s Vaccine Research Group.

Factors such as age, gender, race, a person’s underlying immune system strength, and the time since last vaccination all play a role in how well shots work. The number of doses received and which variant is circulating in the area are also important to effectiveness, experts say.

At the White House press conference, Fauci addressed concerns about how well boosters work against another subvariant, XBB, that has rapidly in the Asia-Pacific region.

“You can expect some protection but not the optimal protection” against XBB, he said. He said he’s hoping that the combined immunity from vaccination and earlier infections will help keep transmission levels down this winter.

To assess booster effectiveness, the CDC study looked at data from patients with symptoms who tested positive for COVID between early September, when the new boosters rolled out, through Nov. 11. People with conditions that weaken their immune systems were excluded from the study published in CDC’s Morbidity and Mortality Weekly Report.


The majority of infections occurred during the period when BA.4 and BA.5 were still dominant. More recently, immune-evasive subvariants BQ.1 and BQ.1.1 have grown to account for more US cases.