The new US Covid booster campaign needs a dose of clarity about its goals and limitations. The latest “bivalent” vaccine — retooled to protect against the currently circulating BA.5 variant — will benefit some more than others. The oldest and most vulnerable citizens are likely to benefit most. Public health officials should aim to protect them through a targeted messaging campaign convincing them to get the shot. Younger people should only be encouraged to get it if they’re more than six months out from their last shot.
Right now, however, the Centers for Disease Control and Prevention has a broader focus — recommending that everyone over 12 get the booster if they’re more than two months out from their last shot or three months out from an infection. That diffuse message is less likely to reach those who need it most.
The situation was simpler during the initial vaccine rollout in 2021. The clinical trial data suggested that vaccinations would go a long way toward preventing infection in the first place, and so getting vaccinated was considered not just a personal health choice but a civic duty for everyone. There was broad scientific consensus that widespread vaccination would minimize cases and maybe even end the pandemic.
That hope was crushed by the discovery of immunity-evading new variants. But there was still a pretty wide consensus that people should get a first booster, thanks to growing evidence that an extra shot, given months later, would help reduce cases and prevent severe illness.
But expert opinion had splintered by the spring of 2022. Some wanted to keep boosting everyone every six months or so, either with the original vaccines or with updated boosters. The problem was a lack of evidence that repeated boosting would make a substantial dent in cases. The new bivalent BA.5 boosters could plausibly reduce the odds of infection, but we don’t know by how much.
“At the end of the day, probably what counts most is the time from the last immunization or infection,” says Alessandro Sette, a professor at the La Jolla Institute of Immunology. He says right now there’s too much emphasis on the number of boosters people are getting, rather than their timing. For four or six months after infection or a previous booster, your immune system probably isn’t very boost-able.
Sette reiterated what Harvard University immunologist Duane Wesemann told me for a previous column: Over the months following an infection or vaccine dose, your immune system is slowly improving the quality of your antibody-making B-cells and generating slight diversity that increases the odds of effectiveness against a new variant. The number of antibodies circulating in your bloodstream can decline, but these B-cells continue to retain the ability to make new ones pretty quickly for about six months.
That’s why University of California, San Francisco infectious disease doctor Monica Gandhi told me she’s been arguing that the recommended interval be six months for healthy people — not the two currently recommended (and in some places, mandated).
In principle, pushing forward a massive fall booster campaign could blunt a winter wave, but Covid waves can’t yet be predictably tied to seasonal changes, and nobody knows whether BA.5 or something else will be behind the next surge.