With over 50% of the adult population in the U.S. fully vaccinated against the coronavirus, attention for much of this age group has shifted from getting their first dose to questions about the potential need for a booster shot.
Pfizer CEO Albert Bourla recently told Axios that a COVID-19 booster shot will likely be necessary “somewhere between eight and 12 months.” He said that booster shots could be needed as soon as September for the earliest vaccine recipients who got their first dose in December.
His comments stirred questions over whether another vaccine shot would be necessary and, if so, when it would be recommended.
Will a booster shot be necessary?
Leading infectious disease expert Anthony Fauci recently told Congress that he believes recipients of the coronavirus vaccine will require a booster shot, though he added that he was “not exactly sure when” the shot would be needed.
But not all experts are ready to commit to another dose of the coronavirus vaccine just yet.
Monica Gandhi, a professor at the University of California San Francisco, detailed in an opinion piece in The San Francisco Chronicle several reasons – including high vaccine efficacy and protection against variants – for “why you shouldn’t have to worry about getting a booster.”
“The best way to keep people safe now is to put the discussion of boosters aside and work hard on global vaccine distribution,” Gandhi wrote.
What could trigger a call for booster shots?
To know whether or not another dose will be needed, Anna Durbin of Johns Hopkins Bloomberg School of Public Health says that public health officials will have to keep a close eye on the number and severity of infections that come from vaccinated individuals, also known as “breakthrough” cases.
“When we’re evaluating the need for boosters, what we want to see is … are people no longer being protected from disease, particularly severe disease,” Durbin says. “If the immunity induced by vaccines is waning to the point that you’re no longer getting protection against significant disease, then that would be a driver for boosters.”
An increase in COVID-19 hospitalizations would be a “red flag” that the booster shots are needed, she says.
“If every breakthrough case is asymptomatic, and we’re not seeing the emergence of more virulent variants, we wouldn’t care. We would know we have enough immunity to protect ourselves from significant illness,” Durbin says. “But if we start to see that the protection is not there, and we’re starting to see more hospitalized cases in vaccinated people … then we’re going to need those extra doses.”
What if the number of severe breakthrough infections doesn’t increase?
As Gandhi wrote, coronavirus vaccines authorized in the U.S. offer significant protection. A new study from the Centers for Disease Control and Prevention found that the vaccines from Pfizer and Moderna reduce the risk of infection by 91% for fully vaccinated people. It also found that fully or partially vaccinated individuals might be less likely to spread the virus to others.
Some data has shown that vaccines have been effective at preventing severe disease and hospitalizations caused by circulating coronavirus variants and that’s “really the key public health impact,” according to Durbin.
So if the vaccines are highly effective against the original strain and offer enough protection from circulating variants, an extra dose might not be necessary yet.
“Certainly, a third dose is not going to hurt you. It’s going to help your immune response. The big question is, is it really needed,” she says.
If in the near future the U.S. is only seeing mild or asymptomatic breakthrough infections in vaccinated people, Durbin says additional doses would likely be put to better use in other countries experiencing outbreaks that have little access to vaccines.
“If we start to see disease in this country again, then yes, we need to think about vaccinating people here,” Durbin says.
How long does immunity last?
The term of protection – whether from vaccination or from natural infection – is a big unknown factor. Fauci has said that the interval of protection from the vaccines lasts at least six months and likely a year, and recent studies suggest that protection following natural infection is also likely to be long-lived.
Ali Ellebedy, an author of one of the studies on natural infection, says that the research should not be interpreted to mean that those who previously got sick with COVID-19 don’t need to get vaccinated.
“When we look at people who get vaccinated after being affected, we see a beautiful response, which is obviously expected because they have these memory cells that have been formed, and those memory cells now engage and produce a really robust response,” Ellebedy of Washington University School of Medicine in St. Louis says.
He also underscored that not everyone responds to the virus in the same way. Some will gain strong protection from COVID-19 after natural infection while others won’t. And even if someone produces antibodies, that doesn’t necessarily mean they have enough immunity to fight off the virus.
So who might need a potential booster shot?
The groups who would probably be targeted first for another dose could be travelers to a country that has high transmission of a variant, health care workers if the U.S. starts seeing an increase in hospitalizations and the elderly who are usually more susceptible to disease, according to Durbin.
On the other hand, someone who was infected with coronavirus and also received the vaccine might not immediately need a booster shot because the natural infection likely acted like a first vaccine dose would, priming the body for a second dose.
Answers to questions about booster shots are on the way. Some studies could produce preliminary results in the coming months. The National Institutes of Health recently announced a trial to see whether people fully vaccinated with any authorized coronavirus vaccine could see improved immune responses with a booster shot of Moderna’s vaccine.