Should Covid-19 booster shots be used only to prevent severe illnesses that lead to hospitalization or death—or should they also seek to stop milder cases that could drive coronavirus transmission?

Top health experts, including National Institute of Allergy and Infectious Diseases Director Anthony Fauci, are sharply split on the question.

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The case for focusing on severe illnesses

At a meeting of CDC's Advisory Committee on Immunization Practices (ACIP) last week, several members of the committee said they believed booster shots should be used to prevent severe illness—but not necessarily other infections.

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"I would just encourage us not to lump infection and hospitalization," Sarah Long, an ACIP member and a pediatric infectious disease expert at Drexel University College of Medicine, said.  "I don't think there's any hope that vaccines such as the ones we have will prevent infection after the first, maybe couple weeks that you have those extraordinary immediate responses."

Rachel Slayton, a CDC scientist who presented a modeling study of booster doses in nursing home residents, said Covid-19 cases will still occur in nursing homes where community transmission is high even after boosters are administered.

"It's unclear that everyone needs to be boosted, other than a subset of the population that clearly would be at high risk for serious disease,” Michael G. Kurilla, an ACIP member and official at NIH, said.

Separately, Vinay Prasad, a hematologist-oncologist and associate professor of medicine at the University of California, San Francisco, said the coronavirus is likely to become an endemic virus—meaning that infection will be inevitable even with vaccination.

Writing for MedPage Today, Prasad said even with a booster dose, protection could wane, and people may still eventually be infected by the coronavirus. Instead of aiming to prevent infection, vaccination should be used to greatly reduce people's risk of hospitalization and death, he said.

And other public health experts have argued that it's inappropriate—and potentially counterproductive—for the United States to give boosters to its own residents at a time when many people worldwide haven't yet gotten a first dose.

Craig Spencer, an emergency medicine doctor at New York University, said, "We're sitting on hundreds of millions of doses that could immediately be used in places to stop the spread and stop people from dying from a largely vaccine-preventable illness."

The case for seeking to prevent mild infections

But in a recent interview with The Atlantic, Fauci rejected the argument that boosters should not be used to prevent infection as well as severe illness and death.

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"I think it is not entirely correct to make this very strong dichotomy between waning protection against hospitalization and death and waning immunity against infection and mild-to-moderate disease," Fauci said. "It is an assumption that it's okay to get infected and to get mild-to-moderate disease as long as you don’t wind up in the hospital and die."

According to Fauci, booster shots should also be used to "[prevent] people from getting sick from Covid even if they don't wind up in the hospital."

"If the vaccine prevents you from getting sick, prevents you from losing work time, prevents you from getting to the hospital and prevents you from dying, that’s a really, really successful vaccine, even if you have a breakthrough infection," he said.

Another consideration, Fauci added, is that boosters could help restrain the spread of the coronavirus. "[I]f you do have a breakthrough infection, you are still capable of transmitting that infection to someone else," he said.

Fauci's sentiments align with official booster recommendations from FDA and CDC released last week. FDA on Wednesday authorized third doses of the Pfizer-BioNTech Covid-19 vaccine for people over the age of 65, those with underlying medical conditions, and those who are at high risk of infection due to their institutional or occupational settings.

CDC on Friday followed suit, with CDC Director Rochelle Walensky expanding upon ACIP's initial recommendation that the additional doses vaccine be offered only to people ages 65 and older, nursing home residents, and those ages 50 to 64 with underlying medical conditions.

Are three doses 'the optimal regimen'?

In his interview with The Atlantic, Fauci added that he believes "the optimal regimen for the mRNA [vaccines] is going to include [the] booster shot." This means full vaccination could include three doses of an mRNA Covid-19 vaccine instead of just two, The Atlantic reports.

Because the mRNA vaccines were rolled out so quickly amid the pandemic, public health officials couldn't know at the time whether a third dose would eventually be necessary, Fauci said. "[W]e didn't have the luxury to say, 'Wait a minute, we're going to try multiple different doses and make sure we get it just right,'" he said.

But Fauci argued that current data has shown waning protection over time—suggesting that an additional dose is necessary to offer complete protection.

"I believe that a third-shot booster for a two-dose mRNA [vaccine] should ultimately and will ultimately be the proper, complete regimen," Fauci said. "The vaccine is very successful. The durability of it is something that's a subject of considerable discussion and sometimes debate." (Banco, Politico, 9/30; Gutman, The Atlantic, 9/28; Prasad, MedPage Today, 9/22; Mandavilli, New York Times, 9/23)