(Bloomberg) -- U.S. regulators cleared giving a third Covid-19 vaccine dose to people with weakened immune systems in an effort to improve protection for those with impaired responses to standard shots.

The authorization applies to both Moderna Inc. and Pfizer Inc., with organ transplant recipients or those whose immune systems are similarly compromised covered, the U.S. Food and Drug Administration’s said in a statement late Thursday. Other fully vaccinated individuals don’t need an additional dose right now, the FDA said.

Bolstering the agency’s move was a study published in the New England Journal of Medicine on Wednesday showing that an extra dose of Moderna’s vaccine significantly raised antibody levels against Covid in transplant patients. The findings showed that among patients who got a placebo shot, rather than the third dose, just 18% showed the desired antibody levels.

“I feel very relieved that they are doing this, because it will save lives,” said Janet Handal, a kidney transplant recipient. “I wish that they had done it sooner.”

While the U.S.-authorized Covid-19 vaccines are highly effective at preventing severe disease, hospitalization and death in most people, booster shots are especially important for transplant patients and others with weak immune systems, who often don’t get adequate responses to their first vaccine course. Immune compromised people compose about 3% of adults, Rochelle Walensky, director of the Centers for Disease Control and Prevention, said on a press call.

Many patients sought out extra doses on their own as findings emerged on their responses to vaccines. Dorry Segev, a professor of surgery and epidemiology at Johns Hopkins University, said his research team has observed more than 200 transplant recipients who have gotten third shots, and he suspects many more immunocompromised patients have done so as well.

Advisers for the CDC raised concerns at a meeting last month about patients getting additional doses in an unsupervised manner and the inequities that could arise.

“The people who’ve gotten it now are the people who navigate the healthcare system well, who are willing to go to great lengths,” Segev said. FDA approval “would then expand access of these boosters in a more equitable way to people who might not be as good as navigating the healthcare system and might have been too shy or too worried about pursuing something.”

Follows Other Countries

Only those with weakened immune systems will be offered boosters, White House chief medical adviser Anthony Fauci said on NBC’s ”Today” show. For other vaccinated groups, such as the elderly, data is being collected to determine if or when their protection goes “below a critical level”.

“Right now at this moment, other than the immune compromised, we’re not going to be giving boosters to people, but we will be following them very carefully, and if they do need it, we’ll be ready to give it to them,” he said.

The U.S. now joins a number of other countries, such as Israel, Germany and France, that are offering boosters to vulnerable groups. Other countries, including Russia, Hungary and the United Arab Emirates, are offering a third shot more broadly.

About 60% of eligible Americans, age 12 and up, are fully vaccinated, according to the CDC. And while the pace of shots is rising after falling sharply from a high set in mid-April, the Biden administration is urging people to get shots as infections surge in the U.S., fueled by the delta variant.

An advisory committee to the CDC is also set to discuss booster doses of Covid–19 vaccine for immunocompromised patients on Friday.

Handal and other transplant recipients formed Transplant Recipients and Immunocompromised Patient Advocacy Group during the pandemic that has urged the FDA to streamline approval of additional doses for those with weakened immune systems.

“People that are immunocompromised are continuing to live with the fear that everyone felt in the early days of Covid,” Handal said in an interview. They’re carrying on with their lives as the world opens up, but “it’s just an undercurrent of fear, and it’s exhausting to live with.”

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