EXPLAINER: Is it time to get a COVID-19 booster? Which one?

Millions more Americans just became eligible for COVID-19 boosters, but figuring out who's eligible and when can be confusing. And adding to the challenge is that this time around, people can choose a different brand of vaccine for that extra dose.

A number of factors, including the vaccine you started with and when your last dose was, help determine when you qualify. Just like the initial shots, boosters are free and will be available at pharmacies, doctors' offices and clinics.

Here are some things to know:

WHY ARE BOOSTERS NEEDED?

People who are fully vaccinated are still strong protected against hospitalization and death from COVID-19. But immunity against infection can wane over time, and the extra-contagious delta variant is spreading widely. U.S. health authorities want to shore up protection in at-risk people who were vaccinated months ago, though the priority remains getting the unvaccinated their first shots.

ARE BOOSTERS AVAILABLE FOR ALL THREE VACCINES AUTHORIZED IN THE U.S.?

Yes, Pfizer boosters began last month, and this week the government cleared extra doses of the Moderna and Johnson & Johnson vaccines too. But who’s eligible — and when — differs depending on which vaccine you got first.

CAN I GET A BOOSTER NOW?

If you got Pfizer or Moderna shots first, you’re eligible if your last dose was at least six months ago and you’re 65 or older, or are a younger adult who has health problems or a job or living conditions that put you at higher risk of severe illness or exposure to the coronavirus. Health care workers, for example, are included because they are regularly exposed to the virus and can’t come to work with even the mildest of infections.

WHAT IF I GOT THE J&J SHOT?

Anyone who got a J&J shot at least two months ago is eligible,...