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The Food and Drug Administration on Thursday authorized the administration of an additional dose of one of the messenger RNA Covid-19 vaccines for people who have certain immunocompromising conditions amid growing evidence that they do not get adequate protection from the normal two-dose regimen of the Pfizer-BioNTech or Moderna vaccines.

The decision was formally an amendment to the emergency authorization under which the mRNA vaccines are already being used. It does not apply to the one-dose Johnson and Johnson vaccine, which has been used far less in this country than the mRNA products. It is unclear how many immunocompromised Americans would have received the J&J vaccine.

“Today’s action allows doctors to boost immunity in certain immunocompromised individuals who need extra protection from Covid-19,” Acting FDA Commissioner Janet Woodcock said in a statement released shortly before midnight. “As we’ve previously stated, other individuals who are fully vaccinated are adequately protected and do not need an additional dose of COVID-19 vaccine at this time.”

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Patient groups and the physicians who treat them have been advocating for immunocompromised people — for instance, solid organ transplant recipients who take immunosuppressant drugs — to be given an additional dose, especially in the face of the spread of the more transmissible Delta variant. Several countries — France, Germany, Britain, and Israel among them — are or will soon be giving third doses to immunocompromised people.

The amended emergency use authorizations say the third doses of vaccine should be given at least 28 days after the second shot in the series. The Pfizer vaccine, which can be given to people 12 years of age and older, is administered in two doses, 21 days apart. The Moderna vaccine, which can be given to people 18 and older, is given in two doses 28 days apart.

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The documents do not enumerate in detail who is eligible for an additional vaccine dose. They say people who have had a solid organ transplant or who have been diagnosed with conditions that are “considered to have an equivalent level of immunocompromise” can get a third dose of vaccine.

The vaccine advisory panel that makes recommendations to the Centers for Disease Control and Prevention on use of vaccines indicated late last month that it was amenable to recommending a different dosing schedule for immunocompromised patients, but could not do so while the emergency use authorizations under which the vaccines are deployed stipulate only a two-dose or one-dose regimen.

“Our hands are really tied with the current regulatory situation,” Camille Kotton, director of the transplant and immunocompromised host infectious diseases clinic at Massachusetts General Hospital, said during the July 22 meeting of the Advisory Committee on Immunization Practices.

The group is scheduled to meet on Friday to recommend the changes the FDA’s revision allows. It is expected that CDC Director Rochelle Walensky will sign off on the recommendation later on Friday.

If the Covid vaccines were fully licensed, doctors could decide on their own to give an additional dose to some of their patients. So-called off-label use is common medical practice. But the unusual status of these vaccines — they are the first ever to be deployed under emergency use authorizations — precludes that possibility.

The confusion about how to proceed has created a bit of a free-for-all, with some immunocompromised people — and some people without immune system deficiencies but who are worried by the Delta variant — crossing state lines and using subterfuge to obtain an additional dose of vaccine.

At least one state, Mississippi, decided not to wait for federal action, telling vaccine providers and physicians to assess whether their immunocompromised patients should be given an additional vaccine dose.

In a presentation during ACIP’s meeting in July, CDC staff said that immunocompromised people make up about 2.7% of the American population. They include people with cancer who are undergoing some forms of therapy, people with advanced or untreated HIV infection, people born with severe immunodeficiencies, and organ or stem cell transplant patients.

People who are immunocompromised are more likely to get severely ill from Covid-19 and are more likely to have breakthrough infections, the CDC experts said. They are also more likely to have prolonged infections, creating the possibility that they will give rise to troublesome mutations of SARS-CoV-2, the virus that causes the disease.

Last week the World Health Organization called for a moratorium on giving additional doses of vaccine to people who are fully vaccinated in a bid to speed vaccination in lower-income countries where few doses have yet been available. But agency officials made clear they make an exception for immunocompromised people. “There is increasing evidence that there are some populations … who don’t respond to the vaccine as well as the general population does,” said Kate O’Brien, WHO’s director of immunizations, vaccines, and biologics. “We don’t want that to be confused with the policy … of whether booster doses should be given.”

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