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"Mixing" or "matching" booster doses of COVID-19 vaccine generated the most discussion Wednesday at a CDC Advisory Committee on Immunization Practices (ACIP) meeting.

While ACIP could not make any formal interim recommendations about boosters -- they are awaiting FDA regulatory action on third doses of Pfizer's vaccine (Comirnaty) -- CDC staff asked the committee which issues were most important to address regarding booster doses.

There was much talk but little data on "heterologous" boosting. Liaison member John Beigel, MD, of the National Institute of Allergy and Infectious Diseases, said the full dataset for a study examining Moderna boosters, Johnson & Johnson boosters, and Pfizer boosters will be available later in the fall. They plan to assess immunogenicity on day 15 and day 29, as well as T cells, he said.

Sarah Long, MD, of Drexel University College of Medicine in Philadelphia, described a "big public health panic" if those immunized with a primary series other than Pfizer are unable to get a booster dose, given there is no formal data on mixing vaccines.

"That's going to leave half of people immunized ... having received a vaccine and being told that they're now at risk for waning immunity and hospitalization and unable to get a booster dose," she said, adding they need to settle the question of mixing and matching and "whether we're willing to panic the recipients of Moderna."

Wilbur Chen, MD, of the University of Maryland in Baltimore, agreed that ACIP's recommendations should not "lock a person into whatever primary series they received for that booster dose." He said he hoped the committee could revise these recommendations in some way to be "agnostic of the primary series."

However, liaison member Doran Fink, MD, PhD, of the FDA, said that from the agency's perspective, "data are not available to inform the interchangeability of" different boosters "with a primary series."

Beth Bell, MD, of the University of Washington in Seattle, also requested clarity about whether ACIP's recommendations would be only directed at those who received the Pfizer primary series.

If emergency use authorization (EUA) is going to be issued by the FDA, "then ACIP policy will have to be in line with that language of those conditions of use," said Amanda Cohn, MD, of the CDC.

While ACIP has previously made all their recommendations on individual COVID-19 vaccine products, ACIP chair Grace Lee, MD, of Stanford University in California, said she thought that, eventually, COVID vaccine recommendations should be handled like flu, where they are generalized to a population versus a particular product, which would "make sense from a public health perspective," she said.

Committee members also felt it was important to clarify the point of a vaccination program is to prevent serious illness and death, given that it is unlikely that the vaccine prevents transmission.

Helen Keipp Talbot, MD, MPH, of Vanderbilt University in Nashville, added that the goals for vaccinating groups such as healthcare workers, would be to "reduce mild disease so healthcare workers can go back to work."

CDC staff also presented data on individuals who received a third dose of vaccine after the FDA's EUA, which found there was no increased risk of adverse events following the third dose. While there was one preliminary case of myocarditis, it was in a 73-year-old man, 22 days following vaccination, so CDC staff added it was unlikely to be related to the vaccine.

A third dose of Pfizer vaccine was recommended for EUA in older adults and high-risk groups on Friday by the FDA Vaccine and Related Biological Products Advisory Committee, which rejected approval of a third dose for all individuals ages 16 and up, like manufacturer Pfizer was seeking.

ACIP is scheduled to meet Thursday in anticipation of a decision from the FDA.

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