The highly contagious delta variant of the novel coronavirus continues to surge across the United States. And while the unvaccinated are feeling the brunt of this wave of the pandemic, they aren’t the only ones testing positive. Data from the past several months indicate that the delta variant has weakened the vaccine’s ability to prevent infection from occurring entirely. 

“We are seeing an increased number of infections in vaccinated people,” acknowledges John Wherry, director of the University of Pennsylvania’s Institute for Immunology.

These breakthrough cases are thought to be relatively uncommon and are usually mild. The Centers for Disease Control and Prevention have emphasized that vaccines aren’t 100 percent effective at preventing illness, and some infections are expected to occur even in vaccinated people. 

None of this means the vaccines aren’t working. In fact, they still do a great job at preventing hospitalization and death. So far, data suggests that vaccines limit the spread of COVID-19, even if they can’t quash it completely, and even people who have already had COVID-19 benefit from their protection.

How are breakthrough infections in vaccinated people different from infections in people who are not vaccinated?

There are a few possible reasons for COVID-19 cases to be on the rise in vaccinated people. One is the wildfire spread of the delta variant. Another is that even though only a small fraction of vaccinated people will likely get a breakthrough infection, with over 171 million people in the U.S. now fully vaccinated, that percentage will add up to a lot of breakthrough infections. 

Antibody levels in vaccinated people may also wane over time, leaving them more vulnerable to infection. The good news, Wherry says, is that “there’s a backup plan built into the immune system.” In addition to antibodies, vaccinated people also produce white blood cells known as memory B cells and memory T cells. As their names suggest, these cells can recognize the virus if the person encounters it in the future. 

[Related: Will Pfizer’s FDA approval spell an uptick in COVID vaccination?]

“If the antibodies fail, or the antibodies have sort of a chink in the armor and a little bit gets through, those memory B cells and memory T cells…respond very quickly to detecting that new infection,” Wherry says. “They can limit that viral replication very quickly and actually prevent the virus from spreading around your body, which is what leads eventually to disease.” 

The vaccines seem to be better at protecting the lungs than the upper respiratory tract. “Somebody can acquire the virus, it can multiply in somebody’s upper airway, they can transmit it to somebody else theoretically,” says Martin Blaser, director of the Center for Advanced Biotechnology and Medicine at Rutgers University, “but their inner core, their lungs and other tissues, seem to be well-protected by the immunity given by the vaccine.”

The true number of infections in vaccinated people isn’t known. When they do become infected, though, vaccinated people are much likelier than the unvaccinated to have mild or no symptoms (for this reason, breakthrough infections are probably underreported).

Despite the rise of the delta variant, COVID-19 vaccines are still very effective in preventing serious illness and death. A recent report from California indicates that unvaccinated people in Los Angeles County were five times as likely to become infected and 29 times as likely to be hospitalized with COVID-19 as their fully vaccinated peers.

“People who do get the vaccine have much milder illness or no illness and are much less likely to end up in the hospital or to die, so the main reason to get the vaccine is to protect yourself,” Blaser says. “Anyone who doesn’t want to get a severe case of COVID should take the vaccine.” 

People with compromised immune systems are less likely to mount a robust immune response after vaccination than other people. The CDC recently recommended that immunocompromised people receive an extra shot.

“It just seems to be enough of a kick to get at least some of those people over the hump of making a decent antibody response,” Wherry says. “If people are worried about that, it’s a good time to ask your doctor, have that discussion, and see whether a booster may be in your future.”

Can vaccinated people spread the virus?

The three most important tools we have in stopping the spread of COVID-19 are masks, social distancing, and vaccination, Blaser says. 

“What we had hoped was that the vaccine would lower transmission,” he says. The delta variant seems to be making this task substantially more difficult. Researchers have found preliminary evidence that vaccinated people who catch this swiftly-replicating variant can accumulate similar levels of virus to unvaccinated people. Scientists have also grown live viruses from nasal swabs taken from vaccinated COVID-19 patients, and used it to infect and kill human cell cultures. This lends support to the idea that vaccinated people may still readily spread the virus if they become infected. 

However, another recent report suggests that vaccinated people who catch COVID-19 are less infectious than the unvaccinated. Researchers found that viruses sampled from vaccinated people replicated less efficiently than viruses from unvaccinated people. The findings, which haven’t yet undergone peer review, could mean that people who develop breakthrough cases don’t “shed” as much virus as unvaccinated people who catch COVID-19. 

[Related: Should we be mixing COVID-19 vaccines for better immunity?]

When the virus does invade the noses of vaccinated people, it will quickly be swarmed by antibodies, Wherry says. “That virus that you’re detecting or that you may expel from your nose is likely to have antibodies coating it that may limit its ability to infect another person.”

Vaccinated people who catch COVID-19 also seem to be contagious for a shorter window of time than those who weren’t vaccinated, which dramatically limits their chances to spread the virus to others, he adds.

“We have to accept the fact that setting a 100 percent threshold for success in this scenario is setting ourselves up for failure,” Wherry says. “Not 100 percent of people will be protected from disease, nor will we 100 percent prevent transmission in vaccinated people, but we sure are dropping it by a huge amount in vaccinated people.”

How similar is immunity in vaccinated people and COVID-19 survivors?

After catching COVID-19, Wherry says, “You do generate a pretty good immune response, and [there’s] good evidence that it is pretty durable.”

During a real infection, a person’s immune system is exposed to the entire virus, rather than just the parts targeted by the vaccine. This could lead to a broader immune response. On the other hand, people who survive mild infections have lower levels of antibodies than would develop after vaccination, Wherry says. According to data published this month by the CDC, COVID-19 survivors who stayed unvaccinated were more than twice as likely to be reinfected as survivors who got the shots. Furthermore, getting vaccinated seems to give COVID-19 survivors a powerful boost in immune cells and could even lead to an especially potent kind of “hybrid immunity.”  

“Altogether this points to the idea that the vaccine strongly boosts immunity,” Blaser says.

Researchers aren’t sure how long this immunity will last. Unlike the coronaviruses that cause the common cold that people are regularly exposed to, SARS-CoV-2 had never been seen in humans before late 2019. This means that we were completely vulnerable when it first swept across the globe, and there’s still a lot to learn about how the virus behaves and changes. 

“I can’t tell you that after the delta virus goes through and more and more people are vaccinated, everything’s going to quiet down, or [if] we will be susceptible to a new wave or a new variant that will come around,” Blaser says. “But at least we will have some level of preexisting immunity because of the vaccination.”

It’s possible that we’ll eventually get COVID-19 boosters every year, he says, similar to the seasonal flu shot. Still, Wherry notes, getting shots into unvaccinated people’s arms remains the top priority. 

“We do not end the pandemic by giving vaccinated people boosters. We end the pandemic by vaccinating unvaccinated people,” he says. “That’s still the giant source of fuel in the fire; vaccinated people are barely a wet log.”