Pfizer CEO Albert Bourla on Sunday said he believes a fourth dose of the company's Covid-19 vaccine will be "necessary," however some health experts believe another booster shot will only be needed for certain vulnerable populations.

Pfizer's CEO says a fourth dose will be needed

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."

"We are going to submit to FDA a significant package of data about the need for a fourth dose," Bourla added, saying that ultimately, Pfizer hopes to "make not only a vaccine that will protect against all variants, including omicron, but also something that can protect for at least a year."

Kit Longley, a spokesperson for Pfizer, said the company will submit data to FDA when it's "ready," noting that the results "are early and preliminary."

"We will continue to collect and assess all available data and remain in open dialogue with regulators and health authorities to help inform a Covid-19 vaccine strategy as the virus evolves," Longley said.

Early data indicates waning immunity

Whether a fourth dose of a Covid-19 vaccine will be necessary has caused debate among health experts. Research from CDC has found that protection against Covid-19 hospitalization with three doses of a vaccine drops from about 90% to around 80% after four months—a finding that Sara Tartof, an epidemiologist at Kaiser Permanente, said is concerning.

"Overall, protection against severe disease remains higher than against infection, but evidence is emerging that protection against severe disease may decline with time against omicron," Tartof said.

"[I]f you have an overall higher risk of hospitalization, this waning might cause a bigger impact than for somebody who has an overall very low risk of severe disease to begin with," she added.

Otto Yang, an infectious disease expert at the University of California, Los Angeles, said he believes that a fourth dose might be a good idea.

"Based on the fact that immunity against this virus seems intrinsically short-lived, which is why immunity from the vaccines is short-lived, this is probably a good idea," he said. "The fact that the virus has evolved into omicron is what has sped up the need for boosters—the antibodies against the vaccine are targeted against the original strain, and so much higher levers are needed against omicron."

Will a fourth dose be necessary?

Some health experts argue that a fourth dose isn't necessary at this time. Peter Marks, head of FDA's vaccines division, said there isn't "enough data to know that [recommending a fourth dose is] a good thing to do."

A preprint study published online last month from Israel found that an additional booster shot may not be needed for the general population. In the study, researchers compared infection rates among around 300 health care workers who received a fourth dose of a Covid-19 vaccine and roughly 400 health care workers who had received just three shots.

They found that, while the fourth shot did increase antibody levels to the initial levels following the third dose, the fourth dose only reduced infection risk by around 10% to 30%.

"Not a third dose, not a fourth dose, not a fifth dose will do anything to stop infections [long term]," said Gili Regev-Yochay, an infectious disease specialist at Sheba Medical Center and lead author of the study.

At a press briefing in January, experts form the European Medicines Agency (EMA) argued against giving Covid-19 booster shots too close together.

"We are rather concerned about a strategy that entangles repeated vaccination within a short term. We cannot really continuously give a booster dose every three or four months," said Marco Cavaleri, head of biological health threats and strategy at EMA.

"If we have a strategy in which we give boosters, let's say every 4 months approximately, we will end up potentially having a problem with the immune response, and the immune response may end up not being as good as we would like it to be," he added. "So we should be careful in not overloading the immune system with repeated immunization."

William Schaffner, an infectious disease expert at Vanderbilt University, said it's important the immune system be given time to process vaccines.

"It's just well-established that the immune system needs some time to process the information that it gets: the stimulus from a vaccine or a natural infection," Schaffner said. "And then if you want to boost it, you have to give it a certain amount of time to 'digest' that information so that it can respond optimally."

However, a fourth dose may be useful for certain populations that are at an increased risk of developing severe Covid-19, like the elderly and the immunocompromised.

"These people will likely be the first group to require another booster," said Akiko Iwasaki, an immunologist at Yale University. "Their immune response is not as robust as a healthy young person."

However, researchers don't yet know when they'll need that shot, Iwasaki added. "It depends on how fast the immunity is waning. So we'll have to wait and see."

"There may be the need for yet again another boost—in this case, a fourth-dose boost for an individual receiving the mRNA [vaccine]—that could be based on age, as well as underlying conditions," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. (Lovelace, NBC News, 3/11; Saric, Axios, 3/13; Doucleff, "Goats and Soda," NPR, 2/22; Krstic, Good Housekeeping, 2/19; Pratt, Healthline, 1/17)