One recent study evaluating the bivalent booster in people over the age of 12 showed that it worked equally well in individuals of all ages. The researchers compared how people fared during the three months after they received a monovalent booster (May to August 2022) with the three months after people received a bivalent booster (September to December 2022). They found that the monovalent booster was 25 percent effective at preventing hospitalization or death, while the bivalent booster was 62 percent effective.
Although the booster worked for everyone, experts say because older adults are much more likely to be hospitalized for Covid-19, they will experience a greater benefit. “Even if this effectiveness is the same, it’s still more important for older people to get boosted because their absolute risk is higher,” said Danyu Lin, a professor of biostatistics at the University of North Carolina at Chapel Hill who led the research.
A C.D.C. study looking at whether the bivalent booster protects against infection in people aged 18 to 49 was also encouraging. Compared with people who received between two and four doses of the original vaccine, people who got the bivalent booster were roughly 50 percent less likely to have a symptomatic infection from either BA.5 or XBB/XBB.1.5.
However, as with the original vaccine, the bivalent booster slightly increases the risk of myocarditis, inflammation of the heart muscle, in people aged 18 to 35. As a result, some experts are hesitant to recommend more booster doses to this group.
“If you’re young, say you’re 35, 40, you’re otherwise healthy, you’ve already gotten vaccinated and boosted and probably had an infection or two in the past, I think that person is pretty well protected for quite some time,” said Dr. David Ho, a professor of medicine at Columbia University who led one of the antibody studies. “Until more data is available, I would not compel such a person to get an annual vaccination.”
What will boosters look like in the coming months and years?
The F.D.A. has suggested that, for most Americans, the booster could be given annually in the fall, like the flu vaccine, and high-risk individuals could still receive multiple doses a year. It’s not clear when, or if, it will formally recommend this approach.
Dr. Lin has unpublished research comparing outcomes by the number of booster doses people receive per year. His data shows that people who average less than one booster a year have higher hospitalization and death rates than people who get one or more doses. There is a much smaller difference between one and more than one booster dose per year. He said this suggested that an annual booster was sufficient for most people; however, for older adults, even the small benefit derived from multiple boosters a year is probably worthwhile.