“It’s not unheard-of to give vaccines periodically, but I think there are better ways than doing boosters every six months,” said Akiko Iwasaki, an immunologist at Yale University. Other strategies, she said, could “get us out of this forever-boosting kind of a situation.”

For starters, persuading people to line up for shots every few months is probably a losing proposition. About 73 percent of American adults are fully vaccinated, but so far just over a third have opted for a booster.

“This doesn’t seem to be a sustainable long-term strategy, for sure,” said Deepta Bhattacharya, an immunologist at the University of Arizona.

Just as important, there are no data to support the effectiveness of a fourth dose of the current vaccines.