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After weeks of speculation, the FDA amended the emergency use authorizations (EUAs) for and 's COVID-19 vaccines to include an additional booster dose for certain immunocompromised people, the .

These populations include solid-organ transplant recipients or others diagnosed with conditions "considered to have a similar level of immunocompromise" who previously received one of the two mRNA vaccines. The FDA kept the language purposely vague, as clinical considerations are set to be defined Friday during .

FDA merely added that individuals immunocompromised "in a manner similar" to those who underwent solid-organ transplantation have a reduced ability to fight disease and are vulnerable to infections, such as COVID-19.

The ACIP meeting agenda was updated to include a vote on additional doses of mRNA vaccines for immunocompromised people, and CDC staff are scheduled to present evidence for this practice on Friday morning.

, CMS said that booster doses will be available for immunocompromised Medicare recipients who qualify for an additional dose, and "they can receive it with no cost sharing."

"The FDA is especially cognizant that immunocompromised people are particularly at risk for severe disease," said acting FDA Commissioner Janet Woodcock, MD, in a statement. "After a thorough review of the available data, the FDA determined that this small, vulnerable group may benefit from a third dose of the Pfizer-BioNTech or Moderna vaccines."

According to the updated EUAs, support for the expanded authorizations came from two studies: a randomized trial of Moderna's vaccine in 120 solid-organ transplant recipients who failed to mount a sufficient antibody response after the standard two-shot series, and a single-arm study involving 101 consecutive transplant recipients treated in France with a third shot of Pfizer's vaccine after regulators there recommended a booster for these individuals.

In the randomized trial, a third dose of Moderna's shot 2 months after the second produced a serologic response shown to confer 50% virus neutralization or greater (anti-receptor-binding domain antibody level ≥100 U/mL) in 55% of patients after 4 months, as compared with 18% in a placebo group (P<0.001).

And in the French study, recipients demonstrating antibodies against SARS-CoV-2 increased from 40% before a third Pfizer vaccine dose to 68% a month after it. In individuals who were seronegative for antibodies before the booster, 44% became seropositive a month afterward.

Interestingly, there was no indication within FDA's statement that the two manufacturers applied for these amendments themselves. At a prior ACIP meeting in late July, FDA staff noted that the agency had yet to receive any data to support any regulatory mechanisms or actions regarding this population.

Pfizer has been vocal about needing vaccine boosters on a broader scale, but not for immunocompromised people specifically.

However, Woodcock was unequivocal in that this EUA only applied to immunocompromised people, and stated that other fully vaccinated individuals are "adequately protected" and do not need a booster dose at this time.

"The FDA is actively engaged in a science-based, rigorous process with our federal partners to consider whether an additional dose may be needed in the future," she said.

In addition to a booster dose of vaccine, FDA recommended that immunocompromised individuals discuss monoclonal antibody treatments with their providers, as this population would be at high risk of progressing to severe COVID-19.

They added that clinicians should counsel these patients to maintain "physical precautions" to protect themselves from the virus, and all close contacts should be fully vaccinated to provide this population further protection.

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