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The Biden administration’s plans to make covid-19 booster shots available next month has drawn a collective scream of protest from the scientific community.

As some scientists see it, the announcement is rash and based on weak evidence, and they worry it could undercut confidence in vaccines with no clear benefit of controlling the pandemic. Meanwhile, more information is needed on potential side effects or adverse effects from a booster shot, they say.

Perhaps even worse, the announcement has fueled deeper confusion about what Americans need to do to protect themselves from covid.

“I think we’ve scared people,” said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and an adviser to the National Institutes of Health and the Food and Drug Administration.

“We sent a terrible message,” he said. “We just sent a message out there that people who consider themselves fully vaccinated were not fully vaccinated. And that’s the wrong message, because you are protected against serious illness.”

As of Thursday, 51% of the U.S. population was fully vaccinated, Centers for Disease Control and Prevention data shows. Biden administration officials ― citing data from Israel, a study from the Mayo Clinic that is not yet peer-reviewed and new CDC studies ― say it’s necessary to plan for boosters to prevent a worsening of the pandemic as the delta variant powers a surge in cases and overwhelms hospital intensive care units.

In essence, officials are caught between a rock and a hard place ― trying to be prepared while simultaneously not undermining messaging about how well the existing vaccines work.

Officials must weigh two unknowns: the risks of moving ahead aggressively with booster shots versus the risks of waiting to learn much more about the virus and the power of the vaccines. The government’s normal path to regulatory approval is, by design, slow and deliberate. The virus has its own schedule, fast and unpredictable.

“Arguably, I think that the federal government is simply trying to stay ahead of the curve,” said Dr. Joshua Barocas, associate professor of medicine at the University of Colorado. But, he said, “I have not seen robust data yet to suggest that it is better to boost Americans who have gotten two vaccines than invest resources and time in getting unvaccinated people across the world vaccinated.”

Beginning in late September, boosters would be made available to adults (age 18 and up) eight months after they received the second dose of a Pfizer-BioNTech or Moderna covid vaccine, President Joe Biden said. But his plan comes with big caveats: It does not yet have the blessing of a CDC advisory panel, and the FDA has not authorized boosters for all adults.

The urgent question is whether the vaccines are losing their power against covid.

“We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalization and death,” Surgeon General Vivek Murthy said.

But many scientists and public health experts say the data doesn’t demonstrate a clear benefit to the public in making booster shots widely available, and the Biden administration’s message confuses people about what the covid vaccines were designed to do.

“They’re not a force field. They don’t repel the virus from your body. They train your immune system to respond when you become infected … with the goal of keeping you out of the hospital,” said Jennifer Nuzzo, an epidemiologist and associate professor at the Johns Hopkins Bloomberg School of Public Health.

Meanwhile, questions abound. Will boosters for fully vaccinated adults make the virus less transmissible ― that is, slower or less likely to spread to others?

“I certainly hope that’s the case … but the bottom line, with full transparency, we don’t know that right now,” Dr. Anthony Fauci, Biden’s chief medical adviser, said Wednesday.

What about side effects? “It would be nice to understand what side effects people have after their third dose,” Nuzzo said.

“We don’t have any reason to believe, based on the safety profile of the vaccine itself, that we’re going to see significant adverse events with booster shots,” Barocas said. However, those things are “just now being studied.”

The concerns are real. While serious side effects from covid vaccines have been rare, some have caused alarm ― including mRNA vaccines being linked to cases of myocarditis, or inflammation of the heart.

“At the individual level, we need to know the side effect profile of a 3rd dose, especially in younger people. Until now, the benefits of vaccination have far outweighed the potential side effects,” Dr. Jeremy Faust, an emergency medicine physician at Brigham and Women’s Hospital in Boston, wrote in a blog post outlining why he was skeptical about a plan to give boosters to everyone.

Even in light of the new CDC studies published Wednesday, experts say one thing is clear: The vaccines still work very well at what they were meant to do, which is to protect people against the worst outcomes of getting infected with the virus.

One study, relying on data from 21 hospitals in 18 states, found no significant change in the vaccines’ effectiveness against hospitalization between March and July, which coincides with delta becoming the prevalent covid strain. Another, using data from New York, also found the vaccines highly effective in preventing hospitalization, even as there was a decline in effectiveness against new infections. The third, evaluating the Pfizer and Moderna vaccines in nursing home residents, saw a drop in how effective they were at preventing infection ― but the research didn’t distinguish between symptomatic and asymptomatic cases.

“It’s like we’re engaged in friendly fire against these vaccines,” Nuzzo said. “What are we trying to do here? Are we just trying to reduce overall transmission? Because there’s no evidence that this is going to do it.”

Fauci, in outlining the case for boosters, highlighted data showing that antibody levels decline over time and higher levels of antibodies are associated with higher vaccine efficacy. But antibodies are only one component of the body’s defense mechanisms against a covid infection.

When the antibodies decrease, the body compensates with a cellular immune response. “A person who has lost antibodies isn’t necessarily completely susceptible to infection, because that person has T-cell immunity that we can’t measure easily,” said Dr. Cody Meissner, a specialist in pediatric infectious diseases who sits on the FDA’s vaccine advisory panel.

John Wherry, director of the Penn Institute of Immunology at the University of Pennsylvania, recently published a study finding that the mRNA vaccines provoked a strong response by the immune system’s T cells, which researchers said could be a more durable source of protection. Wherry is working on a second study based on six months of data.

“We’re seeing very good durability for at least some components of the non-antibody responses generated by the vaccines,” he said.

For protection against serious disease, “really all you need is immunological memory, and these vaccines induce immunological memory and immunological memory tends to be longer-lived,” Offit said. Federal scientists also are studying T-cell response, Fauci said.

Pfizer and Moderna have said they think boosters for covid will be necessary. But it’s up to the government to authorize them. Federal officials say they are sifting through new data from the companies and elsewhere as it becomes available.

There’s not a deep playbook for this: Emergency use authorization, or EUA, of vaccines has been sparingly used. The FDA has already amended Pfizer’s prior EUA clearance twice, first in May to expand the vaccines to adolescents 12 to 15 years old and, again, this month to allow immunocompromised people to obtain a third dose. The FDA did not respond to questions about the process for authorizing widespread booster shots.

Pfizer announced in July that it expects $33.5 billion in covid vaccine revenue this year. Its stock has risen 33% this year, closing at $48.80 Thursday. Moderna reported sales of $5.9 billion through June 30 for 302 million doses of its vaccine. The company’s stock has skyrocketed 236% year-to-date, closing at $375.53 Thursday.

In applying for emergency authorization, the FDA requires vaccine manufacturers to submit clinical efficacy data and all safety data from phase 1 and phase 2 clinical trials as well as two months of safety data from phase 3 studies. For full approval, the FDA requires manufacturers to submit six months of data.

Pfizer this week announced it has submitted phase 1 clinical trial data to the FDA as part of an evaluation for future approval of a third dose. The company said phase 3 results are “expected shortly.”

Pfizer said its preliminary trial results showed a third dose was safe and increased antibody levels against the original virus and the delta variant. Moderna found a third dose had safety results similar to a second dose and produced a strong antibody response.

Typically, any distribution of shots would occur after the CDC’s Advisory Committee on Immunization Practices also developed recommendations. But with the Biden administration’s announcement about boosters, public health experts worry the message suggests the outcome is preordained.

“They have completely and unfairly jammed FDA and ACIP. They’ve left them no choice. If there’s no booster program, FDA gets blamed and that’s not appropriate,” said Dr. Nicole Lurie, a former senior Health and Human Services official in the Obama administration and U.S. director of the Coalition for Epidemic Preparedness Innovations, the global epidemic vaccines partnership.

KHN senior correspondent Sarah Jane Tribble and editor Arthur Allen contributed to this report.

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