Faced with this dilemma, a COVID-conscious senior might be tempted to boost now, while their immunity might be low, and just grab another shot in a couple of months if cases start to swing back upward. But such a plan could backfire. John Wherry, an immunologist at the University of Pennsylvania, told me that boosting too often with the original-recipe vaccine—the only kind that’s available in the U.S. right now—could lead a person’s immune system to react to newer, less similar variants such as Omicron with reduced vigor. Plus, something better could be right around the corner. Pfizer and Moderna are testing Omicron-specific boosters—though the underwhelming preliminary results make Wherry think that those shots in particular are not worth holding out for. He’s more excited about the possibility of mucosal vaccines and the eventual arrival of Novavax’s protein-based shot, which has a promising efficacy and safety profile but is still awaiting the FDA’s green light. One of these alternative designs may end up being a better match for the mature immune system. (A protein-based shot, for example, is nearly twice as effective as a live-attenuated vaccine at preventing shingles in seniors.) The only way to know whether older people respond better to a different sort of vaccine will be to test it, Mark Slifka, an immunologist at Oregon Health and Science University, told me. “You need to cater to each particular bug.”

A fourth shot, then, might give seniors more protection for an unknown period of time against a disease that is currently in decline in the U.S., and it might jeopardize some of their protection against that same disease when cases begin to rise again. Lona Mody, a geriatrician at the University of Michigan, told me that she doesn’t think we have enough data yet to recommend second boosters to seniors across the board. But if one of her patients felt that they needed one—say they have a family event coming up where they’ll be inside with lots of people, some of whom might not be vaccinated—“I would definitely consider it,” she said. Slifka agreed that these decisions should be made in consultation with a doctor, and in consideration of whether a particular senior lives alone or in a congregate setting, their general health, and whether they have a high-exposure job.