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The Food and Drug Administration on Thursday night gave formal approval for severely immunocompromised Americans to get a third shot of the Pfizer-BioNTech and Moderna COVID-19 vaccines. Studies have shown that transplant recipients and others with seriously weakened immune systems don't get the same level of protection against the coronavirus from the first two doses, but many mount a stronger immune response after a third dose.

"Today's action allows doctors to boost immunity in certain immunocompromised individuals who need extra protection from COVID-19," said Dr. Janet Woodcock, the FDA's acting commissioner. Less than 3 percent of the U.S. population will be approved for the booster shot, and the FDA's statement did not mention immunocompromised people who received the one-dose Johnson & Johnson vaccine. And it isn't clear which high-risk groups would benefit from a booster shot. 

"This is all going to be very personalized," said Dr. Dorry Segev, a transplant surgeon at Johns Hopkins University who is conducting a National Institutes of Health study on booster shots for organ transplant recipients. But it's clear the risks are greater for the immunocompromised. One recent study led by Sergev found that vaccinated transplant recipients are 82 times more likely to get a breakthrough COVID-19 infection and 485 times more likely to be hospitalized or die from the coronavirus. 

U.S. health officials are closely monitoring how long the vaccines protect people without suppressed immune systems, with an eye to whether booster shots will eventually be necessary for the general population. Everyone will likely need a booster shot at some point, White House coronavirus adviser Dr. Anthony Fauci said at a briefing Thursday, but "apart from the immunocompromised, we do not believe that others, elderly or non-elderly, need a vaccine at this moment."