EXPLAINER: Who's eligible for Pfizer booster shots in US?

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...