Millions of Americans are now eligible to receive a Pfizer booster shot to help increase their protection against the worst effects of the coronavirus.

A look at the nuts and bolts of this new phase of the vaccination campaign:

WHO SHOULD GET THE PFIZER BOOSTER?

People who got two Pfizer shots at least six months ago and who fall into one of these groups should get the booster:

— People 65 and older, nursing home residents and assisted living residents.

— Others ages 50 to 64 with a long list of risky health problems including cancer, diabetes, asthma, HIV infection and heart disease. Being overweight or obese is a category that qualifies roughly 70% of people in this age group.

WHO ELSE CAN CONSIDER GETTING IT?

The Centers for Disease Control and Prevention says these people may get a booster, but stopped short of a full recommendation:

— People 18 to 49 who got their Pfizer shots at least six months ago with risky health problems can consider the booster based on their individual benefits and risks.

— Anyone 18 to 64 with a risky job, such as health care, can consider boosters. Prisoners and people living in homeless shelters are also in this group.

WHAT ARE THE SIDE EFFECTS?

Serious side effects from the first two Pfizer doses are exceedingly rare, including heart inflammation that sometimes occurs in younger men.

WEREN’T SOME PEOPLE ALREADY ELIGIBLE FOR A THIRD DOSE?

Yes, people with severely weakened immune systems were already eligible to get a third dose of Pfizer or Moderna. This group includes people taking immune-suppressing medications and those with diseases that tamp down their immune systems. They didn’t have to wait six months to get a third dose.

WHAT IF I GOT MODERNA? CAN I GET A PFIZER BOOSTER?

Not yet. Health officials say they don’t have enough data on mix-and-match vaccinations. Moderna has applied to U.S. health regulators for its own booster, one that would be half the dose of the original shots. The Food and Drug Administration is considering that application.

WHAT IF I GOT J&J?

People who originally got the single-dose Johnson & Johnson also must wait. The government doesn’t recommend mixing-and-matching. J&J hasn’t yet filed a booster application. But earlier this week, the company released data showing two doses of its vaccine provided stronger immunity than one — whether the extra dose was given either two months or six months after the first.

WHERE CAN I GET MY BOOSTER?

Health departments, clinics and drugstores are offering boosters, and many people have already gotten them ahead of the official green light. You may have to show your vaccine card. Proving how you qualify is on the honor system. Your word about your risky job or health condition is likely to be enough.

ARE BOOSTERS FREE?

Yes, shots given under FDA’s emergency use authorization are free. And there should be enough supplies.

AM I ‘FULLY VACCINATED’ WITHOUT A BOOSTER?

Yes, two doses of Pfizer or Moderna, or one of J&J, is still considered fully vaccinated.

WHY WERE BOOSTERS SO HOTLY DEBATED?

The need is not crystal clear. Studies show the vaccines are still offering strong protection against serious illness for all ages. And many experts want to focus attention on getting shots to the unvaccinated, the group most in danger of infection, hospitalization and death.

On the other hand, there is a slight drop in the vaccine’s effectiveness among the oldest adults. And immunity against milder infection appears to wane months after people’s initial shots. Protecting health care workers from even mild illness may help some hospitals now struggling to care for unvaccinated COVID-19 patients.

ARE OTHER COUNTRIES OFFERING BOOSTERS?

Britain and Israel are already giving boosters over strong objections from the World Health Organization that poor countries don’t have enough for their initial doses.