Pfizer and BioNTech have submitted an application for FDA authorization of a second Covid-19 booster shot for seniors—a move that comes days after Pfizer CEO Albert Bourla and Moderna president Stephen Hoge offered different views on whether a second booster shot will be necessary for most Americans.

Your top resources on the Covid-19 vaccines

Pfizer and BioNTech seek FDA authorization of a second booster shot

In a statement, Pfizer and BioNTech noted that an additional booster shot at least four months after an adult age 60 and over receives their third shot could improve waning levels of antibodies and improve protection against severe Covid-19. The companies added that "people who are vaccinated, particularly those who have received a booster, maintain a high level of protection, particularly against severe disease and hospitalization."

In its application sent to FDA, Pfizer cited two preprint studies from Israel, where second booster shots are already being offered to adults ages 60 and older.

In one of the studies, 1.1 million health records of seniors in Israel were analyzed, and researchers determined that Covid-19 infection rates were two times lower and rates of severe Covid-19 were four times lower among those who received a second booster shot compared with those who just received one.

"Giving the fourth dose to individuals who were at risk to develop severe disease has been instrumental in limiting the burden on hospitals in Israel during the fast and wide-spreading omicron surge," the researchers wrote.

The second study looked at health care workers in Israel who had received a second booster shot, and found the shot increased the amount of virus-fighting antibodies in the person's body, but was not particularly effective at preventing less severe Covid-19 or asymptomatic infection.

"The protection that you are getting from the third [shot], it is good enough, actually quite good for hospitalizations and deaths. It's not that good against infections," Bourla said. "But we are just submitting [that] data to the FDA and then we will see what the experts also will say outside Pfizer."

Pfizer and Moderna leaders offer differing views on fourth doses

In an interview on Sunday, Bourla said Pfizer's research suggests protection from the first booster shot may start to wane after three or four months. When asked whether a fourth dose would be required, Bourla said, "It is necessary."

However, Hoge in an interview on Monday stopped short of saying a fourth shot would be necessary for everyone.

"Is it necessary? I think that's a strong word," Hoge said. "I think it will provide a benefit to anyone who gets it."

Hoge said Moderna would "strongly recommend and encourage" a fourth shot "[f]or those who are immune-compromised, those who are older adults, over the age of 50 or at least 65," but said he doesn't expect "any kind of recommendations that would be across the board for everyone."

"I personally will get a booster every year because I don't want to ever lose my sense of smell," Hoge said. "Long Covid sounds nasty."

Is a second booster necessary?

According to Jesse Erasmus, from the University of Washington School of Medicine in Seattle, "[i]n the context of the general population [individuals who are relatively healthy], I think waning immunity is not something we all need to be concerned about."

Even if antibody levels decline over time, protection against severe disease and hospitalization is still strong, he added.

When talking about vaccine efficacy, Alessandro Sette, from the La Jolla Institute for Immunology, said it's important to distinguish "efficacy against what?" If the goal of vaccines is to prevent infection, then high antibody levels are necessary, but if they're to prevent severe disease and hospitalization, cellular immunity is important, Sette said.

"What we have been seeing is that the vaccines have been losing capacity to reduce infection, but in general, breakthrough infections have been causing mild disease," he said. "That has been a sign that the vaccines still [provide] protection against severe disease, which is what is most important."

In regard to whether a second booster shot is necessary for everyone, "I think we just don't have enough data yet," Erasmus said.

"There are certainly scientists and physicians who are addressing this question actively," he said. "We may start to see that data emerge over the next couple of months."

Lona Mody, a geriatrician at the University of Michigan, said there isn't enough data to determine whether second boosters are necessary for seniors, but added that, if one of her patients felt they needed one, "I would definitely consider it." (Miller/Neergaard, Associated Press, 3/15; LaFraniere, New York Times, 3/16; McGinley et al., Washington Post, 3/15; Saric, Axios, 3/14; Henderson, MedPage Today, 3/15; Gutman, The Atlantic, 3/15)