Since the beginning of October, the U.S. is administering more COVID-19 booster shots than first doses.

According to the Centers for Disease Control and Prevention, boosters are an important tool in providing continued protection for certain vulnerable populations – like the immunocompromised, Americans 65 and older, and people who work or live in settings where’s there’s an increased risk of infection, like schools or homeless shelters.

But health officials agree that administering first doses is still the top priority. In the last two weeks, the number of Americans who’ve gotten jabbed for the first time has risen slightly, but it’s still well short of initial doses given in late August and early September, when several widespread vaccine mandates were announced. As of last week, 22% of the adult population have not received a single dose.

According to a CDC study published last month, the unvaccinated are 10 times more likely to be hospitalized, and 11 times more likely to die of COVID-19, than those who were fully vaccinated.

Booster shots are only available for qualifying recipients of the Pfizer vaccine. Moderna and Johnson & Johnson have both submitted evidence to the Food and Drug Administration which they say demonstrates the benefits of a booster shot of those vaccines.

On Tuesday, the FDA released a preliminary review of Moderna’s evidence. The agency found that a Moderna booster – which is half its initial dose – increased the level of virus fighting antibodies.

FDA advisors will meet on Thursday to further review the evidence. If they approve boosters for Moderna and Johnson & Johnson, the CDC will also have to sign off.