(AP) — Americans at high risk from COVID-19 because of severely weakened immune systems are now allowed to get a third vaccination in hopes of better protection as government advisers grappled with exactly who qualifies.

The Food and Drug Administration ruled that transplant recipients and other similarly immune-compromised patients can get a third dose of either the Pfizer or Moderna vaccine. But the decision offers an extra dose only to those high-risk groups — not the general public.

These patients have been clamoring for months for better protection, some of them traveling across state lines or lying to get another dose. About 1.1 million people already have gotten at least a third dose of the Pfizer or Moderna vaccines on their own, the Centers for Disease Control and Prevention revealed Friday — although it’s not clear how many did so because they had weakened immune systems.

Here are some things to know:

WHY DO SOME PEOPLE NEED AN EXTRA DOSE?

The Pfizer and Moderna vaccines offer powerful protection for otherwise healthy people, but many who take immune-suppressing medications or have diseases that tamp down their immune systems generally get less benefit from the standard two doses. The CDC cited one study suggesting about 40% to 44% of people hospitalized for a so-called breakthrough case — infection after vaccination — are among the immune-compromised.

Those hospitalized patients “did all the right things — they’re just suffering from a lack of good vaccine protection,” said Dr. Camille Kotton of Massachusetts General Hospital, one of CDC’s advisers.

WHO QUALIFIES?

Roughly 7 million American adults are classified as immune-compromised, but the FDA singled out transplant recipients and others with similar levels of immune suppression. The FDA didn’t spell out exactly who falls into those other categories. But according to the CDC, people with blood cancers, those taking certain chemotherapies, and those taking certain medications such as rituximab for rheumatoid arthritis tend to have especially poor responses to vaccination.

WILL I NEED A DOCTOR’S NOTE OR A BLOOD TEST?

The FDA didn’t require either. Some experts have said there should be no barriers to getting a third dose. “We would want to make that as easy as possible,” said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University.

WHAT WILL A THIRD DOSE COST?

Shots given under FDA’s emergency use authorization are free.

IS MIX-AND-MATCH ALLOWED?

The government encourages the third dose to be the same as the first two, but doesn’t mandate it.

HOW WELL DOES A THIRD DOSE WORK?

It helps at least some people. This week Canadian researchers reported 55% of transplant recipients given a third dose two months after standard vaccination had good antibody levels compared to 18% who were given a dummy third shot for comparison. Health experts urged these high-risk patients to continue masking and taking other precautions since there’s no guarantee a third dose will work.

WHAT IF A THIRD DOSE STILL DOESN’T WORK?

It’s not a substitute for vaccination, but the FDA has authorized an antibody treatment as a preventive treatment if high-risk patients are exposed to the virus. And it’s critical for family members and others close to fragile patients to be vaccinated to protect them from exposure.

More research is underway to better tease out whether some immune-compromised patients need still other options, such as carefully monitored changes to their medications.

WHAT IF I HAD THE SINGLE-DOSE JOHNSON & JOHNSON VACCINE?

There’s little data on how another dose works in high-risk people who received that vaccine, although it’s likely a small number considering fewer than 14 million Americans overall have received the J&J shot. Still, CDC counts at least 90,000 who have gotten another dose on their own.

FDA vaccines chief Dr. Peter Marks said the agency is working to get more information about immune-suppressed J&J patients but for now, the evidence only backs a recommendation of extra doses are for Pfizer and Moderna recipients with fragile immune systems.

IS THIS A BOOSTER?

Not really, because boosters are for people whose immunity wanes over time and these high-risk groups didn’t get enough protection to begin with. They now will qualify for a third dose at least 28 days after their second shot — making a third dose part of their initial prescription. That’s similar to how France has handled vaccinations for the immune-compromised since April.

WHAT ABOUT BOOSTERS FOR EVERYONE ELSE?

U.S. health authorities insist it’s not yet time for booster doses for the general population.

“We believe sooner or later you will need a booster for durability of protection” — but not yet, Dr. Anthony Fauci, the government’s top infectious diseases expert, told reporters this week.

The CDC is closely monitoring rates of COVID-19 hospitalizations and deaths, as well as long-running studies of how often vaccinated health workers experience so-called “breakthrough infections” especially with the extra-contagious delta variant. That evidence will drive any decision.

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