As more Americans receive Covid-19 vaccines, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, said Americans will likely need Covid-19 booster shots within "a year or so" of being fully vaccinated.

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When Americans might need booster shots

According to Fauci, the immune protection the human body generates after exposure to most coronaviruses "is generally not lifelong," which suggests that vaccine-induced immunity to Covid-19—which is caused by a novel coronavirus that emerged in 2019—likely will fade too.

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"We know that the vaccine durability of the efficacy lasts at least six months, and likely considerably more, but I think we will almost certainly require a booster sometime within a year or so after getting the primary," he said.

Fauci suggested, however, that it may not be necessary to target boosters at specific variants of the virus.

"Instead of having to play whack-a-mole with each individual variant and develop a booster that's variant-specific, it is likely that you could just keep boosting against the wild type, and wind up getting a good enough response that you wouldn't have to worry about the variants," he said.

Both Pfizer CEO Albert Bourla and Moderna CEO Stéphane Bancel have agreed that booster shots for Covid-19 vaccines will likely be necessary.

"The data that I see coming, they are supporting the notion that likely there will be a need for a booster somewhere between eight and 12 months," Bourla said.

Bourla added that Pfizer has yet to finish its trials on its booster vaccine. "I believe in one, two months we will have enough data to speak about it with much higher scientific certainty," he said. Any plan for administering booster shots would also depend on what FDA approves and believes is best for protecting Americans, Bourla added.

Bancel said he believes the United States should aim to distribute booster shots sooner rather than later. "I think as a country we should rather be two months too early, than two months too late with outbreaks in several places," he said.

"People at highest risks (elderly, health care workers) were vaccinated in December/January," he added. "So I would do (a) September start for those at highest risk."

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However, John Moore, a virologist at Cornell University, said it's important to consider drugmakers' statements in the context of their business goals. "It's not proven that we need boosters yet. Whereas it's appropriate to plan for boosters, you've got to look at whether there's a corporate agenda behind this," he said.

"As of now, we don't have any evidence that protective immunity has dropped to a troubling point, and certainly not for people immunized in December, January, February," Moore added. "It's hard to say where we will be in November because right now it's May."

Countries worldwide are preparing for the possibility that they may need to administer booster shots in the fall. The European Union this month said it had signed a contract with Pfizer and BioNTech for an additional 1.8 billion doses of their Covid-19 vaccine, which are earmarked for booster shots and donations but may be resold if they're not needed, according to the European Commission.

Meanwhile, the United Kingdom said in April it had ordered 60 million additional doses of Pfizer's vaccine in preparation for potential booster shots in the fall and winter.

And in the United States, David Kessler, chief science officer of the White House Covid-19 response team, said in April booster shots may be needed within a year and will be provided free of charge to all Americans.

Covid-19 cases continue to drop nationwide

As experts weigh the need for potential booster shots, the number of Covid-19 cases has continued to drop throughout the United States. According to Axios, the United States averaged around 30,000 cases a day over the past week, down 20% from the week prior.

In addition, 39 states saw improvements in Covid-19 case numbers over the past week.

The trends led CDC Director Rochelle Walensky on Wednesday to tell a Senate appropriations subcommittee she's "cautiously optimistic" about progress in the United States' fight against Covid-19.

"I think we would be remiss to say we are out of the woods," she said. "This pandemic has sent us too many curveballs," so it's "too early to declare victory."

She added that, as of now, it appears the Covid-19 vaccines authorized for use in the United States are working effectively against coronavirus variants. However, outbreaks elsewhere in the world "gives opportunity for more variants to emerge," she said (Langmaid/Vera, CNN, 5/19; Owens, Axios, 5/20; Douglas, Wall Street Journal, 5/20; King, Fierce Healthcare, 5/19; Baker, Axios, 5/20).