“It dilutes an unambiguous message of saying that you need three doses,” said Jason Schwartz, a professor at the Yale School of Public Health whose research focuses on vaccine policy. “If you don’t have three doses, you shouldn’t have a sense of security that you’re adequately protected because, quite frankly, the evidence suggests you’re not.”

The CDC earlier this month announced that it would encourage all Americans over the age of 12 to get boosted for COVID-19 by using the term “up-to-date,” the same terminology used for immunizations on everything from the measles for children to rabies for pets. But the definition of what qualifies Americans as being fully vaccinated—two doses of an mRNA vaccine or one dose of the Johnson & Johnson vaccine—remained unchanged.

“On the one hand, the CDC is urging most Americans to get a booster dose, but on the other hand, it is not making boosters part of the definition of being ‘fully vaccinated,’” said Lawrence Gostin, director of the O’Neill Institute for National and Global Health Law at Georgetown Law Center. “So any given American can be both ‘fully vaccinated’ and also not ‘up-to-date’ on their COVID-19 vaccines. That makes little sense.”