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 strongly 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, regardless of age or other factors.

WHY ARE THERE DIFFERENT RECOMMENDATIONS FOR THE DIFFERENT VACCINES?

A single shot of the J&J vaccine is less effective than two doses of the Moderna or Pfizer formulas, and health authorities decided it was important for the J&J recipients to achieve a similar level of protection. As for the timing, J&J simply tested more people with a two-month booster than one at six months. For recipients of Moderna or Pfizer vaccinations, there's no clear data that everybody needs another dose, but immunity against infection in at least some people appeared to wane around six months.

WHAT IF I DON'T WANT TO WAIT SIX MONTHS?

Experts agree that getting a booster too soon can reduce the benefit. Timing matters because the immune system gradually builds layers of defenses over months, and letting that response mature improves the chances another, later dose will provide even stronger protection.

WHAT DOES 'MIXING AND MATCHING' BOOOSTER DOSES MEAN?

It means a booster of a different brand from your original vaccination. That gives flexibility in situations such as nursing homes where only one type of booster might be brought in. It also gives people at risk of a rare side effect linked to one kind of vaccine the option of switching to a different shot.

SHOULD I SEEK OUT A DIFFERENT VACCINE?

The Centers for Disease Control and Prevention and Food and Drug Administration didn't recommend that people switch but left open the option. Preliminary results of a government study found an extra dose of any vaccine triggered a boost of virus-fighting antibodies regardless of what shots people got to begin with. For people who originally got a J&J vaccination, the Moderna and Pfizer shots appeared to offer a stronger boost. But researchers cautioned the study was too small to say one combination is better than another.

DO I NEED A BOOSTER TO STILL BE CONSIDERED FULLY VACCINATED?

No, the CDC says people still are considered fully vaccinated starting two weeks after the second dose of the Moderna or Pfizer vaccines, or the single-dose J&J shot.

WILL THIS BE MY LAST BOOSTER?

Nobody knows. Some scientists think eventually people may get regular COVID-19 shots like annual flu vaccinations. But researchers will need to study how long protection from the current boosters lasts.

___

The Associated Press Health & Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content.