Your Questions About COVID-19 Breakthrough Infections, Answered

Here's what to know about infections that occur among people who've been fully vaccinated

Person laying down looking at a thermometer reading. Photo: Guido Mieth/Getty Images

When Paul Adamson, MD, first noticed he had a runny nose one day in July 2021, he figured it was an allergic reaction to the dust he’d kicked up doing yardwork. But when the runny nose didn’t clear up, he decided to get tested for COVID-19. Even though he was fully vaccinated, he had plans to fly to San Francisco to attend a baby shower, and he didn’t want to risk exposing the attendees—or the other passengers on the airplane.

“I didn’t actually think I had it,” says Adamson, an infectious disease physician at the David Geffen School of Medicine at UCLA in Los Angeles. But just to be safe, he bought a rapid antigen test at a grocery store. “It came back positive 15 minutes later.” He canceled the trip.

A follow-up PCR (polymerase chain reaction) test confirmed his COVID-19 diagnosis: Adamson had joined the growing ranks of fully vaccinated people with “breakthrough infections.” These cases have become more common since the fall, with evidence showing that the vaccine protection against infection wanes somewhat over time. And while scientists are still working to understand the new omicron variant, vaccinated people appear to be more susceptible to breakthrough infections from it than they were from previous variants, such as delta.

Still, if you’re fully vaccinated against COVID-19, the chances you’ll contract the disease are much lower than they were before you got your shots. And getting a booster shot, if you’re eligible, can help increase protection.

Here, answers to questions you may have about breakthrough infections.

Do Breakthrough Infections Mean the Vaccines Are No Longer Working?

No. Although the delta and omicron variants are more transmissible than the original strain of the virus, research so far has found that the existing vaccines are still highly effective at preventing hospitalization and death.

“What we really wanted was a vaccine to prevent severe disease,” says Kawsar Talaat, MD, an associate professor in the department of international health at the Johns Hopkins Bloomberg School of Public Health in Baltimore. “The fact that we got a vaccine that prevents most infections is totally a bonus.” Still, like all vaccines, the COVID-19 vaccines don’t provide 100 percent protection, especially when a disease is spreading widely. 

It may be confusing to see ongoing reports of outbreaks among vaccinated people. But think about seat belts, suggests Josh Petrie, PhD, an epidemiologist with the University of Michigan School of Public Health in Ann Arbor. These days, almost everyone wears one, which means that most traffic fatalities occur among people wearing seat belts. But that doesn’t mean that seat belts don’t significantly reduce traffic fatalities, just as COVID-19 cases among the vaccinated don’t indicate that the vaccines aren’t significantly reducing the number of infections. 

Real-world vaccine effectiveness can be lower than in the clinical trials, which were conducted with a healthier group of people than the average population, says Gregory Poland, MD, director of the Vaccine Research Group at the Mayo Clinic in Rochester, Minn. Even so, studies suggest that vaccine effectiveness remains high against serious disease. A CDC study published in September found that between March and August 2021, among people without immunocompromising health conditions, the Moderna vaccine was 93 percent effective against hospitalization, the Pfizer vaccine was 88 percent effective, and the Johnson & Johnson vaccine was 71 percent effective. How efficacy may change now that omicron is dominant is still uncertain, although the evidence so far suggests that the vaccine protection against severe disease from omicron remains high, particularly when people receive a booster shot.

“I still have extreme confidence in the vaccine,” Adamson says. “Never did I feel scared that I was going to end up in the hospital or die from this. I think that I felt that way because of the vaccine.”

How Common Are Breakthrough Infections?

We don’t have an exact count, although federal data does offer a look at the relative risk of a breakthrough COVID-19 case compared with a COVID-19 case in someone who is unvaccinated. The latest data from the CDC, which aggregates information from 27 health departments across the U.S., shows that compared with vaccinated people, unvaccinated people are five times more likely to test positive for COVID-19 and about 14 times more likely to die from the disease.

Some state-level data is also available. An August New York Times analysis of breakthrough infections that result in hospitalization or death found that in 40 states and the District of Columbia, vaccinated people represented between a fraction of a percent to 6 percent of hospitalizations and deaths from COVID-19. (At the time of the analysis, fully vaccinated people represented about half of all Americans.) 

Still, good data on the overall rate of breakthroughs is hard to come by. Vaccinated people, who are less likely to have symptoms, may in turn be less likely to seek out testing. And the prevalence of at-home rapid antigen testing means some cases may not be publicly recorded at all. What’s clear is that with cases at all-time highs and more disease circulating in general, breakthroughs will be more common, too.

Does the Omicron Variant Make Breakthroughs More Likely?

The omicron variant seem to be the most contagious form of the SARS-CoV-2 virus we’ve encountered so far. That pushes up your chances of getting infected, even if you’re protected by vaccination.

Some evidence also suggests that omicron may be better at causing breakthrough infections. The vaccines were designed based on the original strain of the SARS-CoV-2 virus, and the omicron variant’s many mutations make it harder for the antibodies that arise in response to vaccination to block it, says Alejandro Balazs, PhD, an assistant professor of medicine at the Harvard Medical School. In a study he co-authored, the research team found that the immune response to omicron was muted in blood samples from people who had been vaccinated more than six months earlier. But a booster dose seemed to provide an effective jolt of protection. “That third dose interestingly did produce antibodies that were able to combat omicron, even though the vaccine is based on the original strain,” says Wilfredo Garcia-Beltran, MD, PhD, the study’s lead author and a clinical pathology resident at Massachusetts General Hospital. 

Also, out in the world, full vaccination is still providing protection against severe disease from omicron—and getting a booster increases the protection against hospitalization even more. “I hope people don’t get discouraged from the fact that they may hear a lot about family and friends getting breakthroughs despite boosters,” Garcia-Beltran says. “What they probably can count on is that despite the breakthrough, it’s not sending them to the hospital as much as if they hadn’t gotten the booster.”

While omicron’s extreme transmissibility may make it feel like catching a case is all but inevitable, Adamson urges people not to give up trying to avoid it, because limiting viral transmission can help reduce the burden on hospitals, emergency rooms, and the overall health system. “No one is going to escape being exposed to COVID-19 or omicron,” he says. “But being exposed to it doesn’t mean you’re just going to get an infection.” Getting your booster shot and wearing a high-filtration mask, such as an N95 or KN95, can go a long way to keeping you healthy.

Are Certain Vaccines More Protective Than Others?

So far (and again, with the caveat that more data is needed on the omicron variant), all three of the vaccines currently approved in the U.S. offer strong protection against severe disease.

More on COVID-19 Vaccines

That’s “the really, really important thing that I try to stress to my family and my patients and my friends that aren’t in the medical field,” says Crystal North, MD, a physician-researcher in the pulmonary and critical care division at Massachusetts General Hospital in Boston.

Still, some evidence suggests that the protection from the Johnson & Johnson vaccine may not be as good as that of the Moderna or Pfizer shots. A study published in November in Science confirmed that the protection against infection from the vaccines waned over time but fell the most among people who received the single-dose Johnson & Johnson vaccine.

Whatever vaccine you originally received, however, protection rises significantly when you get a booster.

If I'm Vaccinated and I Get COVID-19, How Sick Will I Be?

Most breakthrough infections are considered mild, and some are even asymptomatic. Still, that doesn’t rule out the possibility that you might run a fever and feel quite lousy for a few days, says William Schaffner, MD, a professor of infectious diseases at Vanderbilt University Medical Center in Nashville, Tenn.

That was Adamson’s experience. The next night after he developed a runny nose, he started to run a fever and had shaking chills and night sweats, followed the next day by muscle aches. The day after that, he was starting to feel better, but he lost his sense of smell. His wife had made him butter chicken, and as he was eating he realized it tasted “just like hot mush.” His smell and taste gradually started to return after about two weeks. Adamson says that while his experience would be considered a mild COVID-19 infection, it was still “the sickest I’ve been in over a decade.”

In a study published in July in The New England Journal of Medicine, among 1,497 fully vaccinated healthcare workers tested for COVID-19 at one Israeli medical center, 39 people tested positive for the disease. Of those, 13 had no symptoms, while the others experienced symptoms such as congestion, aches and pains, loss of smell or taste, or fever. None were sick enough to be hospitalized.

Little research exists yet on long-term effects of a breakthrough infection, though some evidence suggests that they may be able to cause what’s known as “long COVID,” or chronic symptoms that last for weeks or months after the initial infection. In the NEJM study, 19 percent of those who had a breakthrough infection were still experiencing symptoms after six weeks. 

A small proportion of breakthrough infections become severe. This is particularly a risk for people who are immunocompromised, who might not mount a robust immune response to the vaccine. An overwhelming majority of hospitalizations and deaths from COVID-19 are among the unvaccinated.

If I’m Vaccinated and I Get a Minor Cough or a Stuffy Nose, Should I Get a COVID-19 Test?

Yes. If things were different, and your community had low levels of COVID-19 circulating and more elevated levels of other viruses such as flu and colds, testing wouldn’t be as important. This was the situation in many places early in the summer, Talaat says. But now that so much COVID-19 is still being transmitted in so many communities, it’s best to err on the side of caution and get tested even if you’re vaccinated, she says, and “even if it’s just the sniffles.” For more on COVID-19 tests, see our guide here.

Can Vaccinated People Spread COVID-19 to Unvaccinated Friends and Family Members, Such as Young Kids?

Yes. The extent to which this can occur is still unclear, Petrie says, but it can and has happened. The current thinking, he says, is that vaccinated people are probably less likely to transmit the virus than unvaccinated people, but that it’s still a real possibility.

And COVID-19 cases in children remain high, according to the American Academy of Pediatrics, accounting for about 21 percent of all cases reported during the week of Jan. 6. And when there are higher levels of disease all-around, children—many of whom are still not yet eligible for vaccination—are more likely to be infected. However, “the overall risk of severe illness is still much, much, much lower in children,” Petrie says.

Editor’s Note: This article has been updated to reflect recent news and data. It was originally published on Aug. 19, 2021.


Catherine Roberts

As a science journalist, my goal is to empower consumers to make informed decisions about health products, practices, and treatments. I aim to investigate what works, what doesn't, and what may be causing actual harm when it comes to people's health. As a civilian, my passions include science fiction, running, Queens, and my cat. Follow me on Twitter: @catharob