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Throughout the pandemic, mixed messaging has sometimes hindered the public’s understanding of how best to respond. On boosters, that seems to be happening again.

Pfizer and BioNTech, who’ve routinely signaled the likely need for follow-up COVID-19 shots, just unveiled plans to add another booster shot to their vaccine repertoire. Within hours of that news, though, the Centers for Disease Control and Prevention (CDC) and the FDA told Americans that they don’t need to worry about boosters right now.

Armed with promising lab data, Pfizer and BioNTech feel confident that a third dose of their original mRNA shot could “preserve the highest levels of protective efficacy” against the current rogue’s gallery of coronavirus variants, including delta. But in a bid to stay “vigilant,” the partners on Thursday unveiled (PDF) their contingency plan: an updated version of Comirnaty that targets the full spike protein of the delta variant.

They’ve already made the first clinical mRNA batch for that variant-specific booster, and the vaccine should be ready to enter human studies by August, the companies said in a release. 

But will the new formula even be necessary? The CDC and FDA urged a wait-and-see approach. In a joint statement issued late Thursday, the agencies made their position clear: “Americans who have been fully vaccinated do not need a booster shot at this time.”

For its part, the World Health Organization says more data are needed.

The delta formulation marks the third entry in Pfizer’s booster strategy. The company is also testing a third 30-mg dose of its current vaccine, plus an updated formula against the beta variant that scientists first detected in South Africa. 

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Comirnaty’s ability to prevent infection and symptomatic disease seems to wane six months after vaccination, Pfizer and BioNTech said in their Thursday booster program update, citing real-world data from the Israel Ministry of Health. The vaccine remains effective at preventing serious illness during that time frame, but it’s “likely” people will need a third dose some six to 12 months after their initial two, the companies said. 

The delta variant poses another concern. Seemingly able to spread faster than other variants, and potentially upping the risk of severe disease, the troubling mutation has already become the dominant version of the virus in places like India and the U.K. As of Wednesday, the U.S. joined that list. 

Even at two doses, the partners’ standard shot appears to work against delta, lab results from a recent paper in Nature showed. A third dose could “boost those antibody titers even higher, similar to how the third dose performs for the Beta variant,” Pfizer and BioNTech predict. The companies say they’re running preclinical and clinical studies to confirm that hypothesis. 

Pfizer and BioNTech also plan to file an application for a third shot with regulators in “the coming weeks.”

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While Pfizer has been relatively consistent on its fall booster forecast, the question of when people may need a follow-up shot is anything but certain. In fact, Pfizer and a key CDC advisory panel appear to be at odds on the issue, Bernstein analysts wrote in a recent note to clients. 

Pfizer has contended that boosters will be needed “as antibody blood concentration wanes,” the analysts said. That’s not the industry standard, nor the conclusion drawn by the CDC’s Advisory Committee on Immunization Practices (ACIP) at a meeting in late June, the Bernstein team pointed out.

Boosters will most likely be necessary once there’s a clear drop in vaccine efficacy, or as a countermeasure against a particularly evasive variant—not just a waning antibody response, the CDC experts said. If ACIP’s view is accepted, “frequent boosting would be recommended only for special risk groups … or if we learn symptomatic disease quickly follows antibody decline (which is less likely),” the analysts wrote. 

As for the FDA and CDC’s latest booster guidance, the agencies said they’re still looking into “whether or when” boosters could be needed. Those who’ve been vaccinated seem to be in the clear for now, with protection from severe disease and death, plus worrisome variants like delta, the FDA and CDC said. “Virtually all” COVID-19 hospitalizations and deaths, meanwhile, are among those yet to get the shot, they said.

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