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This post has been updated. It was last published on August 12, 2021.

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As the highly infectious delta variant of the novel coronavirus spreads, COVID-19 cases in the United States have continued to tick upwards. The number of new cases reported daily has increased fourfold over the last month, prompting the New York Times to comment that “the national outlook is worsening quickly.” 

Several states with unvaccinated hotspots, such as Florida, have been hit particularly hard. Cases are also on the rise elsewhere in the world, with an overall increase of 10 percent or more every week in July, according to the World Health Organization

All of this has prompted discussion among scientists, vaccine manufacturers, and government agencies about whether booster shots will be needed to protect us from current and future variants, and if so, when they’ll be necessary. Here’s what you need to know about the situation. 

Which variants are circulating right now?

It’s normal for viruses to make mistakes when they copy themselves, and most of these mutations aren’t cause for concern. However, the SARS-CoV-2 virus has caused nearly 200 million confirmed infections since its emergence in late 2019, so it’s not surprising that over time it has mutated in ways that make it more difficult to deal with. 

Many newer lineages of SARS-CoV-2 have various mutations affecting the spike-shaped protein on the surface of the virus, perhaps boosting its ability to invade host cells or evade immune defenses. Right now the Centers for Disease Control and Prevention recognize four noteworthy variants in the U.S. 

The alpha variant was first detected in the United Kingdom last fall and was reported in the U.S. in December. It’s about 50 percent more contagious than the original version of SARS-CoV-2 and may make people sicker. 

The beta variant, first detected in South Africa, reached the U.S. this past winter and is similarly contagious. The gamma variant originated in Brazil and was detected in the U.S. in January. Some monoclonal antibody treatments are less effective against these variants.

The delta variant, first discovered in India in late 2020, has become the dominant variant in the U.S. It’s currently causing 83.2 percent of COVID-19 infections in the country and is estimated to be twice as transmissible as the original virus. People infected with this variant produce far more viral particles than did people who caught the original strain. 

There’s some preliminary evidence that the delta variant may also put people at greater risk of hospitalization, but further research is needed to determine whether it causes more severe illness. 

“We need more data to understand the relative risk of these different variants,” says ​​Philip Santangelo, a biomedical engineer with expertise in virology and infectious diseases at Emory University and Georgia Tech. “Unfortunately the data is kind of mixed about all of them.”

Finally, another version of the virus known as the lambda variant was discovered in Peru last August and has been identified in several states. However, while it has mutations suspected to increase its transmissibility, lambda isn’t “anywhere near as concerning as the delta variant,” S. Wesley Long, medical director of diagnostic microbiology at Houston Methodist Hospital, where the lambda variant was recently detected, told The Washington Post.

Are vaccines effective against the new variants?

The currently available COVID-19 vaccines may be somewhat less effective in preventing infection and mild symptoms from the new variants, but still appear to offer strong protection against serious illness and death. 

The Pfizer-BioNTech and Moderna vaccines received emergency authorization in the U.S. last December, followed by the Johnson & Johnson vaccine in late February. When they were approved, the vaccines had demonstrated, respectively, 95 percent, 94.1 percent, and 72 percent efficacy in preventing symptomatic COVID-19. 

“The vaccines still look promising, but certainly as [the virus mutates] we don’t expect that percentage to go back up,” Santangelo says. “It’s going to continue to go down.”

Early estimates for the Pfizer vaccine’s prowess against the delta variant vary. Reports from Israel’s Ministry of Health have suggested that the vaccine was 64 percent effective at preventing infections during the period from June 6 to July 3, and 39 percent effective between June 20 and July 17, while remaining more than 90 percent effective at averting serious illness and hospitalization. 

The delta variant has been gathering momentum in Israel. The data could indicate that vaccines aren’t as powerful against this variant, or that their protection is waning over time. However, the latter report comes with a number of caveats, the New York Times noted, including its small size and the fact that the initial wave of vaccines last winter went to older people, so it’s hard to use that data set as a proxy for all vaccinated people.

Meanwhile, a study published on July 21 in the New England Journal of Medicine found that Pfizer’s vaccine was 93.7 percent effective in preventing symptomatic disease caused by the alpha variant and 88 percent effective against the delta variant. The study drew on records of everyone who’d received the vaccine in England up until May 16.

Moderna reported on June 29 that antibodies sampled from the blood of eight vaccinated people were able to neutralize “all variants tested,” including the beta and delta ones. And, most recently, a study analyzing 50,000 people from the Mayo Clinic Health System found that Moderna’s effectiveness against COVID-19 had fallen from 86 percent in early 2021 to 76 percent in July, which is when the Delta variant started to peak. For Pfizer’s vaccine, the same study found that its effectiveness had fallen from 76 percent to 42 percent.

The J&J vaccine is given as a single shot and uses a modified adenovirus to ferry genetic material from SARS-CoV-2. The company reported earlier this month that its vaccine provokes an immune response that remains robust eight months post-vaccination

On July 19, though, another team of scientists reported that antibody samples from people who’d received a single dose of the J&J vaccine were significantly less effective at neutralizing the delta and lambda variants compared with samples from people who’d received both doses of the Pfizer or Moderna vaccines. 

“The message that we wanted to give was not that people shouldn’t get the J&J vaccine, but we hope that in the future, it will be boosted with either another dose of J&J or a boost with Pfizer or Moderna,” Nathaniel Landau, a virologist at NYU’s Grossman School of Medicine who led the study, told the New York Times.

However, these preliminary findings haven’t yet undergone peer review, and may not be reflective of how the vaccine performs in the real world, outside the lab. 

Do I need a booster shot?

On August 18, the CDC announced that all Americans would be eligible for booster shots, starting eight months after they received their second dose of either the Moderna or Pfizer vaccines. Its likely those who received the J&J vaccine will need booster shots as well, but the logistics and timing are still being sorted out, and researchers are still waiting on some key data to make that decision. This news came only a week after the CDC announced that those who are immunocompromised would benefit from a booster shot, and should get them as well.

One of the key questions scientists have been focusing on when deciding whether a booster shot is necessary is whether the immunity bestowed by the vaccine wanes over time. 

“With the tetanus vaccine we need to get boosters every few years because we have to keep the antibodies at a certain level to have that protection,” says Deborah Fuller, a vaccinologist at the University of Washington School of Medicine. “We don’t know where that is yet with coronaviruses, whether or not we’re going to need a certain threshold of antibodies in our bodies to protect us.” 

One encouraging sign: Most people who received the Pfizer and Moderna vaccines mount an immune response that seems to last at least 15 weeks after they received their first shot, scientists reported in June. The findings raise hopes that the vaccines may grant long-term protection against the novel coronavirus. Pfizer reported today that its vaccine does seem to become slightly less effective at preventing symptomatic COVID-19 over time (the decline starts at about two months after the second dose), but the vaccine continues to ward off severe illness months after immunization.

“That would suggest we will not need [a booster] for the first year,” says Edward Jones-Lopez, an infectious disease specialist at Keck Medicine of USC. However, he acknowledges, “That may change over time as more data accumulates and more time passes.” 

Additionally, the vaccines are likely to be more effective in some groups than others. People who are immunosuppressed seem to generate weaker antibody responses than others who have been vaccinated, which may be boosted by a third shot.

“For the immunosuppressed patients, it’s increasingly clear that they will need a booster,” Jones-Lopez says.

Another consideration is how quickly the virus mutates. The novel coronavirus isn’t changing as rapidly as the seasonal flu virus.

“The vaccines seem to be holding out against what we care about most, which is protection against severe disease and hospitalization,” Fuller says. “It’s just a matter of time to see whether a new variant emerges that really knocks down the efficacy of vaccines.”

There’s evidence that receiving a single dose of both an adenovirus-based vaccine and an mRNA vaccine leads to a stronger immune response than receiving two doses of the adenovirus vaccine. It’s possible that, when people return for a COVID-19 booster at some point in the future, Fuller says, they will have the option of receiving a vaccine developed by a different manufacturer than their original injection.

As long as large pockets of people remain unvaccinated, she points out, we’ll continue to see surges in COVID-19 cases and new variants. Fuller suspects that over the next several years there may be regular boosters tailored to whichever variant has become dominant, similarly to the annual flu vaccines.

Vaccine developers will have to determine how well the booster shots actually target the mutated versions of the virus, Santangelo says. Still, he says, getting a booster when it becomes available probably “wouldn’t be a bad idea.”

“I think that virus is having a party and doesn’t really want to stop,” Santangelo says. “I don’t think it’s going away; it’s having too much fun.”

When will booster shots be available? 

The FDA has authorized booster shots to all Americans. In its announcement earlier this week, the CDC recommends getting a booster shot eight months after your second dose of either the Moderna or the Pfizer shots. Again, the CDC is still mulling over the data, and waiting on some more key information, before making a recommendation for those who received the J&J vaccine.

Around the world, other countries have already started offering booster shots. Since April, France has been offering a third shot to people with compromised immune systems. Moderna is already testing boosters with updated formulas, and Pfizer announced last month that it expected a third dose of its vaccine will be necessary six months to a year after people have been immunized. That’s in line with what the CDC just recommended.

Right now, though, the vast majority of people who are being hospitalized or dying from COVID-19 in the United States are unvaccinated. Additionally, most vaccine doses have gone to higher-income countries, leaving much of the world more vulnerable than ever to the virus. Children younger than 12 years also remain unprotected, with clinical trials of the Pfizer and Moderna vaccines still underway. 

It will be crucial to ensure that vaccines are available to the populations most at risk. 

“It’s a little bit like a battle that we’re having, and the unvaccinated are coming into the battle barefoot, with no shield or weapon,” Fuller says. “Viruses find the vulnerable people, they infect them, they create new variants, and that could potentially in time make our vaccines less effective, making everybody vulnerable again.” 

Many people don’t return for their second dose of the mRNA vaccines, she adds. Recent CDC data indicates that more than one in 10 eligible U.S. residents have missed their second dose of the coronavirus vaccine. A single shot may convey less protection against the delta variant than it would against previous strains, Fuller says, so making sure that people are fully immunized will be another priority.

“It’s almost like a nonstop race against the virus, and that is why the critical strategy here is to try and reduce the emergence of new strains by vaccinating as many people as quickly as possible,” Jones-Lopez says. “How quickly and efficiently we do that will determine how many more variants we will encounter as a global community.”

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