A year ago, just two doses of a Covid-19 vaccine or even one, in the case of Johnson & Johnson's formulation " were thought to offer sufficient protection against the coronavirus. On Wednesday, the Centers for Disease Control and Prevention expanded eligibility for boosters to adolescents and backed away from describing anyone as "fully vaccinated" because two shots no longer seem adequate. Instead, one's vaccination status will now be "up to date" " or not. It's no surprise that many Americans are wondering: Where does this end? Are we to roll up our sleeves for booster shots every few months? Scientists are reluctant to predict the future. But in interviews this week, nearly a dozen said that whatever happens, trying to boost the entire population every few months is not realistic. Nor does it make much scientific sense. For starters, persuading people to line up for shots every few months is probably a losing proposition. Just as important, there are no data to support the effectiveness of a fourth dose of the current vaccines. If the coronavirus settles into a flulike seasonal pattern, as it seems possible, "you can imagine a scenario where we simply give boosters before the winter each year," Dr. Hensley said. Lessons from flu season also suggest that frequent vaccination is unlikely to be helpful. Giving the flu vaccine twice a year "has a diminishing return, and so it may not make sense to do vaccination so frequently," said [epidemiologist] Ben Cowling.