Another round of shots will be needed to provide more long-lasting protection against COVID-19, vaccine-maker Pfizer’s chief executive said in a weekend interview, but opinions vary on who really needs that fourth dose.

“Right now, the way that we have seen, it is necessary, a fourth booster,” said Albert Bourla, Pfizer’s CEO, in an interview Sunday on CBS’s Face the Nation, explaining that another dose could protect against future variants and waning immunity, which is why people who are fully vaccinated and boosted have been getting mild cases of COVID.

Some health experts have questioned whether it is realistic or necessary to have a vaccine that prevents even mild illness — when from the start the main goal of the vaccine has been to prevent serious cases and hospitalizations.

Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, and a member of the FDA Vaccine Advisory Committee, has said people who have had the existing vaccine series likely won’t have to worry about serious illness and death from COVID for years, even if they skip additional shots. Preventing serious illness and death should be the goal of the country’s vaccination program, he said, not staving off COVID entirely.

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“If the goal is to prevent mild infections, then frequent boosting would be required,” Offit said Monday in response to Bourla’s comments. “But that is not a sound public health policy.”

Offit has even questioned whether boosters were necessary for healthy Americans who had received a two-dose series of Pfizer or Moderna vaccines. He thought the additional shots were valuable mostly to older people and those with health conditions that made them more likely to develop serious complications from COVID infections. Fourth shots are a worthwhile precaution only for people with immune deficiencies, he has said.

Also in his Face the Nation appearance, Bourla anticipated children under 5 may be approved for vaccination in May, and talked about the medical possibilities MRNA vaccines have opened up beyond COVID.

What could a fourth COVID-19 vaccine dose do?

The existing COVID vaccines, Bourla said, aren’t providing durable protection, even though he acknowledged a two-dose series, along with a booster shot, effectively prevents hospitalization and death from COVID.

“They are not getting very durable immune protection,” he said in the interview with Margaret Brennan. “If you get sick, you can get sick again next year with the same.”

People who have been vaccinated can still catch COVID-19 because the antibody response spurred by vaccination is short-lived, lasting only about four months. Vaccinated people rarely get seriously ill from COVID, though, because vaccines also boost other immune responses in a more lasting way. Those protections may not prevent an initial infection, but they do keep it from becoming severe.

Bourla said Pfizer, which created the first effective vaccine against COVID in 2020, is working on an updated version.

“So what we are trying to do, and we are working very diligently right now,” he said, “it is 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.”

COVID shots would follow the model of flu vaccinations, he said, with people getting annual doses to protect against the virus.

Getting some Americans to accept any vaccine doses has been a challenge. About a quarter of the nation has not yet received a single dose. About two-thirds of Americans are fully vaccinated.

When will you be eligible for a second booster shot?

Bourla didn’t say when an updated vaccine would be ready, but it would need the approval of the Food and Drug Administration and Centers for Disease Control and Prevention before doses go into people’s arms. Last year, the FDA and CDC both had ambivalent responses to requests to make booster shots available to all Americans, with the FDA advisory panel initially not granting permission for boosters beyond people at particular risk from the virus and the elderly. They then approved boosters for all adults in November.

“And I think right now we need to be very well-coordinated, CDC, FDA and the industry so that we are all providing to the American people and to the world a cohesive picture rather than confusion,” Bourla said.

The CDC has recommended fourth doses for people with compromised immune systems. Those should be available to the immunocompromised wherever shots are administered, including pharmacies and clinics.

COVID vaccines for children

Bourla predicted data on a three-dose vaccine series for children under 5 years old would be available next month, and federal approval for children to receive the doses could come as soon as May. While children typically experience mild cases of COVID-19, it is possible for them to develop serious illness, and more than 1,000 Americans 17 and younger have died of the virus since January 2020. Children also can play a role in spreading the virus to adults.

“I think they need to be protected,” Bourla said.

As research into MRNA technology continues, he said, he anticipated the possibility of a more effective flu vaccine, and revolutionary changes in cancer treatments. In the same way the MRNA vaccines train the immune system to attack COVID, he said, a similar approach could teach the body how to eradicate cancer cells.

“We will know if we are successful, I think, in the next two or three years,” Bourla said. “It could be that in some forms of cancer, we could have so strong responses that they will almost be like a cure for certain parts of the population.”