Here’s what you should know about COVID-19 vaccine booster shots

Roughly six months ago, on December 11, the Food and Drug Administration authorized the first COVID-19 vaccine for emergency use in the United States. What followed was a push to get that shot, developed by Pfizer and BioNTech, to those at high risk (SN: 12/1/20). Moderna’s jab wasn’t far behind, securing emergency use authorization just a week after Pfizer’s (SN: 12/17/20; SN: 12/11/20). And then in February 2021, there were three COVID-19 vaccines when the FDA authorized Johnson & Johnson’s shot (SN: 2/27/21). 

Now, around 40 percent of the U.S. population is fully vaccinated. Just over half of residents have gotten at least one dose. Meanwhile, U.S. cases of COVID-19 and deaths have plunged to their lowest levels since March 2020.  

Amid the ongoing effort to vaccinate people, two big questions loom: Will immune protection against the coronavirus be long-lived? Or will people soon need booster shots?

Right now, “no one knows” if boosters will be necessary, says Kirsten Lyke, a vaccinologist at the University of Maryland School of Medicine in Baltimore. But researchers are working on figuring that out.

Here’s what we know so far about coronavirus immunity and potential booster shots. 

Immunity lasts at least six months and possibly much longer.

Whether people need COVID-19 booster shots or not largely hinges on how long the body’s immune response protects against getting severely ill. So far, this protection lasts at least six months and possibly much longer, researchers say.

Much of what scientists know right now about long-term immunity comes from what they have gleaned from people infected with the coronavirus. And it appears that immune memory to the virus largely follows the rules, at least for most people, says Ali Ellebedy, an immunologist at Washington University School of Medicine in St. Louis.

That means that after the virus gains a foothold, the body unleashes a wave of immune proteins called antibodies and immune cells dubbed T cells to fight off the virus. Antibodies typically attack the virus itself while T cells raise additional alarm bells or kill infected cells. Together, antibodies and T cells defeat the virus and then help the immune system form a memory of the pathogen, Ellebedy says. That immune memory is crucial for protection if a person gets exposed to the virus again.

Studies are uncovering evidence that most people develop immune memory to the coronavirus. Ellebedy has found signs of antibody memory, for instance, in people who recovered from an infection. People who had mild COVID-19 had long-lived antibody-producing immune cells in the bone marrow 11 months after infection, he and colleagues reported May 24 in Nature. These cells continue to make antibodies against the virus long after it has left the body, providing protection if a person is exposed again.

Evidence is building that the vaccines offer similar, if not better, protection. In that case, boosters might not be needed for some time. In the last update from vaccine developers, “things looked pretty good,” Lyke says. People who received Moderna’s shot still have high levels of antibodies six months after getting the second dose, researchers reported in April. And Pfizer’s jab has an efficacy of 91.3 percent against COVID-19 symptoms after six months, the pharmaceutical company announced April 1 in a news release.  

Still, “we don’t know how any of these COVID-19 vaccines perform past the one-year mark,” Lyke says. The earliest trials that tested whether the vaccines prompt an immune response are just now reaching that point, and researchers are following up with participants (SN: 7/21/20).

Coronavirus variants could make booster shots more likely.

Even if the protection provided by the immune system is long-lasting, viruses like the coronavirus are adept at evading those responses. Case in point: the emergence of viral variants that can make COVID-19 vaccines less effective than they are against the original version of the virus (SN: 5/11/21).

“I don’t think we would be talking about potentially boosting” if it weren’t for the variants, Ellebedy says. “What we are seeing so far is that the vaccine is really robust, so why would we even need a booster if the virus doesn’t change?”

Available vaccines still protect people from the worst of COVID-19, even if they’re infected by one of the circulating variants. But that might not always be the case. “There may be a future variant that we’re unaware of that may come down and surprise us,” Lyke says. Still, multiple COVID-19 vaccines are flexible in design and can easily be adapted to tackle new variants (SN: 1/27/21). Then, it becomes a matter of manufacturing the doses.   

Some companies, including Pfizer and Moderna, are already testing booster shots to fight emerging variants, in particular, the beta variant that first emerged in South Africa. Early results from Moderna hint that people who received a booster shot that uses the version of a viral protein from the beta variant had antibodies that were better at stopping the variant from infecting cells compared with people who got a third dose of the original vaccine.  

Still, one question is what the best variant booster might look like, says Jerome Kim, a vaccinologist and director-general of the International Vaccine Institute in Seoul, South Korea. Researchers around the globe meticulously monitor circulating influenza strains, for example, to figure out which strain or strains should be included in flu vaccines. In the future, experts may need to keep an eye on the coronavirus in a similar way.

Low vaccination rates worldwide may also make booster shots more likely.

As countries like the United States begin to emerge from the worst of the pandemic, there are many others lagging behind in vaccinations (SN: 2/26/21). That’s in part because wealthy countries have bought the bulk of available doses to vaccinate their populations, leaving lower-income countries struggling to get shots.   

To date, more than 2 billion doses have made it into arms around the world. Some countries, including Canada, the United Kingdom, Chile and Israel, have given at least one dose to around 60 percent of their populations. But around only 1 percent of people in places like Nigeria and Sierra Leone have gotten at least one vaccine dose.

Low vaccination rates in many places pose a problem for efforts to quell transmission and bring the pandemic to an end. And the more the coronavirus spreads, the more opportunities there will be for new variants to emerge, increasing the likelihood of booster shots. The coronavirus “has managed to find the gaps, like any virus,” Kim says. “It’s in the nature of viruses to find the weak points. And before you know it, there’s another mutant.”

Some countries are flying blind due to a lack of genetic surveillance. Bolstering that ability in regions like Africa, Latin America and South Asia would help get a handle on coronavirus diversity in those places and capture emerging variants before they become a global problem.   

It’s also important to get vaccines to the places that need them through efforts like COVAX, an international initiative to help distribute COVID-19 to low-income countries. “We have to start going to the places that have really bad outbreaks because we know mutants will be generated,” Kim says.

“Mix-and-match” COVID boosters might offer even more protection.

To prepare for a future where people need COVID-19 boosters, the U.S. National Institute of Allergy and Infectious Diseases launched a clinical trial on June 1 to test mixing and matching COVID-19 vaccines.

The big question is whether mix-and-match vaccines strengthen the immune response against the coronavirus, says Lyke, one of the researchers leading the trial. If someone is given an mRNA vaccine, like Moderna or Pfizer’s, and then is given a Johnson & Johnson booster, “can we increase [the immune response] by switching up the platform?” Lyke says.

Mixing different types of Ebola vaccines or HIV vaccines, for example, can trigger stronger immune responses than getting multiple doses of the same vaccine (SN: 6/4/21). The idea is that each shot will activate multiple parts of the immune system, Lyke says. An mRNA vaccine might prompt the body to make lots of antibodies that attack the virus. Then a dose of a vaccine like Johnson & Johnson’s — which uses a common cold virus that has been modified so it can’t cause disease — might trigger more T cells. “If you combine [the shots], we hope to prove that you get the best of both responses that work synergistically,” Lyke says.

Early results from a similar trial being conducted in the United Kingdom hint that the answer for COVID-19 shots is yes. People given AstraZeneca’s COVID-19 shot followed by a dose of Pfizer’s eight weeks later developed potent immune responses, researchers reported in a preliminary study posted June 1 at medRxiv.org. Antibodies from people who received the two different vaccines were better at recognizing variants like beta compared with those in people who got two doses of Pfizer’s vaccine.

A separate study in Spain also found that people given a dose of AstraZeneca’s COVID-19 shot followed by a dose of Pfizer’s had high levels of antibodies compared with people who got only one dose of AstraZeneca’s vaccine, researchers reported in May. But it’s unclear how those levels compare with people who got two doses of the same shot.   

One benefit of the U.S. trial is that it has a flexible design, Lyke says. That means that if new variants arise, researchers can add new groups to the trial to test newly developed boosters. “As we get data [about variants] that comes in from other countries, we can really start to hone in on holes in our data and what we need to answer.”