Six Important Things to Know About Breakthrough Infections

As the Delta variant likely drives more cases of Covid-19 in vaccinated individuals, experts weigh in with helpful information

Individuals Wearing Masks Into Grocery Store
Individuals wear masks while shopping at a grocery store in Los Angeles. Masks help prevent breakthrough infections. Chris Delmas / AFP via Getty Images

Your vaccinated family member or friend got their shots months or even weeks ago, but they just tested positive for Covid-19. These familiar happenstances are becoming more frequent as the highly transmissible Delta variant surges. You’re probably wondering what these reports mean about the risk of your own vaccinated body getting infected.

A case of Covid-19 that arises in someone who’s been fully immunized—that is, 14 days after their final dose of the vaccine—is known as a breakthrough infection. The term implies that the virus “broke through a protective barrier provided by the vaccine.”

It’s not clear yet how common breakthrough infections are. The Centers for Disease Control and Prevention said it would no longer track all breakthrough infections in vaccinated individuals, only ones that led to hospitalization or death. A July 30 estimate published by the Kaiser Family Foundation found that among 25 states that regularly report Covid-19 breakthrough events, infections among fully vaccinated individuals were well below 1 percent. But the data is a total tally beginning in January and likely doesn't accurately reflect the odds of a breakthrough infection due to Delta.

Our understanding of breakthrough Covid infections is still evolving, but here’s what we currently know.

Breakthrough cases don’t mean that the vaccines aren’t working.

Public health officials, government leaders and scientists alike all expected breakthrough infections to happen. They are known to occur after vaccination against other diseases, such as influenza and measles. Why? Because no vaccine is 100 percent effective.

“What we need to keep in mind is that all of the vaccines, although very effective, are not perfect,” says Edward Jones-Lopez, an infectious disease doctor at Keck Medicine at the University of Southern California, of the Covid vaccines available in the United States and Europe. “There are still cases of Covid occurring despite the vaccinations.”

Even the measles vaccine, which is incredibly effective, fails to protect about 3 percent of vaccinated individuals who are exposed to the virus. Jonas Salk's polio vaccine—hailed a medical miracle—was 80 percent to 90 percent effective at preventing paralysis caused by the polio virus. Breakthrough infections of flu are even more common. While the exact effectiveness of the flu vaccine fluctuates year-to-year, it ranges between 40 percent and 60 percent.

Measles and polio breakthrough infections aren't just rare because the vaccines are so effective but also because those who are vaccinated rarely interact with infected people. Even with highly effective vaccines for Covid-19, breakthrough infections are likely to keep happening because the virus is so widespread.

The Delta variant is probably driving more breakthrough infections.

While our vaccines are still effective, they’re not quite as protective against the Delta variant. A July 21 study published in the New England Journal of Medicine found that two doses of the Pfizer vaccine were nearly 94 percent effective in individuals with the alpha variant, commonly known as the “U.K. variant,” and 88 percent effective among those with Delta. Meanwhile, two doses of the AstraZeneca vaccine, which is being used outside the United States, were about 75 percent effective against alpha and 67 percent effective against Delta. Early data suggest the Moderna and Johnson & Johnson vaccines are also less effective against Delta but remain very good at preventing serious illness.

Robert Darnell, a physician and biochemist at Rockefeller University in New York who’s been studying the coronavirus, explains that the Delta variant harbors a unique set of viral mutations that make it much more contagious than other variants. “It has evolved in ways that make it more efficient at getting into cells and more efficient at replicating in cells,” Darnell says. “So there's just more of it, likely a lot more of it, per person who gets infected.”

On top of the fact that it can infect cells more readily, scientists have also detected significantly more viral particles in the respiratory tracts of individuals infected with Delta. If more virus exists in someone’s nose and throat, that person can expel more virus into the air and thus spread it more easily. A study posted online in July by Chinese researchers reported that viral loads in individuals infected with Delta were around 1,000 times higher than in those infected with other variants. The paper has not yet been peer-reviewed or published in a scientific journal.

Delta’s increased efficiency, low vaccination rates in many areas, and relaxed restrictions on masking and social distancing are likely all contributing to the rise in breakthrough infections.

Breakthrough infections typically cause mild to moderate symptoms.

In the United States, more than 164 million people are fully inoculated against Covid-19 as of August 2. According to CDC data, there have been 7,101 hospitalizations and 1,507 deaths due to breakthrough infections.

“The preponderance of evidence suggests that the vaccines are highly effective against hospitalization and death for all variants,” says Kate Ellingson, an epidemiologist at the University of Arizona. Those who are fully vaccinated who develop breakthrough infections are likely to have mild to moderate illness, if they develop symptoms at all.

Six Important Things to Know About Breakthrough Infections
A nurse administers a vaccine in Springfield, Missouri. Vaccines are highly effective against hospitalization and death for all variants. Spencer Platt / Getty Images

Another benefit of the vaccines is that they likely shorten the length of illness for many individuals who do become infected. In a study published in June, Ellingson and her colleagues found that vaccination reduced disease severity and duration of illness in healthcare workers and other first responders who reported breakthrough infections. Vaccinated participants had a 58 percent lower risk of fever and also reported two fewer days sick in bed, and an overall length of illness that was six days shorter than that of unvaccinated individuals on average.

Individuals with breakthrough infections can possibly spread the virus.

Previously, scientists believed that vaccinated individuals rarely transmitted the virus. But the Delta variant has changed the game. New data collected by the CDC led the agency to once again recommend that both vaccinated and unvaccinated individuals living in areas with high Covid-19 infection rates wear masks in public indoor settings. The CDC announced its revised recommendation on July 27. The agency updated its guidance based on emerging evidence that vaccinated individuals can spread the Delta variant to some degree.

“The information we are seeing now indicates that we could all potentially spread this virus to our susceptible families and community members,” says Ellingson. “That’s worth paying attention to as we wait for more definitive data.” However, experts think that unvaccinated individuals are still transmitting the virus at a higher rate than vaccinated individuals.

Some individuals are more at risk for breakthrough infections.

Individuals with weakened immune systems are at higher risk of getting a breakthrough infection because the vaccines work less well for them. Immunocompromised individuals number in the millions in the United States. and include organ transplant recipients, cancer patients and those with advanced HIV or uncontrolled diabetes. In one peer-reviewed study published in May, 46 percent of 658 patients who had received solid organ transplants did not mount an antibody response after two doses of an mRNA vaccine.

“Everybody responds to the vaccine by making antibodies against the coronavirus spike protein,” explains Darnell. “Some have fewer neutralizing antibodies and other people have more. It just comes down to individual variability.” Neutralizing antibodies are a special type of protective protein made by the immune system that inactivates the virus. Individuals who make more of these neutralizing antibodies in response to the vaccine are more likely to have mild symptoms by a significant factor.

The Food and Drug Administration is expected to recommend a booster shot for immunocompromised individuals soon. If you’re immunocompromised and wondering if your vaccine worked, talk to your healthcare provider. Rapid antibody tests only provide a yes-no result, but your doctor may be able to order you a more comprehensive antibody test.

Older adults are also more at risk of developing severe complications from breakthrough infections. According to the CDC, around three-quarters of breakthrough infections that caused hospitalizations or deaths occurred in those aged 65 and older.

Aside from being immunocompromised, your odds of getting a breakthrough infection increase the longer you stay indoors without a mask in a crowded, poorly ventilated space.

Masks provide another layer of protection against breakthrough infection.

A year-and-a-half into the pandemic, it’s understandable to feel frustrated by continued mask-wearing. But if you want to boost your protection against the Delta variant, masking up is a good idea.

“It's not that the vaccines don't work, it's that the two things in combination work better,” says Catharine Paules, an infectious disease physician at Penn State Health Milton S. Hershey Medical Center. “If you get the vaccine, you're very highly protected against hospitalization and death, you're pretty well protected against infection, and to further protect you against infection and potentially spread to others, you put on a mask to give that extra layer of protection.”

Former FDA commissioner Scott Gottlieb recommends N95 or KN95 masks for the best protection. While experts agree these masks are the most protective, cloth masks can also work if they fit tightly and have a dedicated filter layer.

Paules says if your city or region has a low vaccination rate, you’re at higher risk of getting a breakthrough infection. Close contact and the duration of that contact are still major risk factors for infection. She recommends masking up if you plan to be indoors for longer than 15 minutes and don't know the vaccination status of those around you.

Gathering outside remains less risky than spending a prolonged period of time indoors. However, it’s still not a good idea to congregate in crowds outside, as evidenced by recent outdoor music festivals linked to Covid-19 outbreaks. Breakthrough infections sound alarming, but it’s important to keep in mind that they’re extremely unlikely to result in hospitalization or death.

Getting infected with the coronavirus comes down to your odds of exposure. “Exposure is a probabilistic issue,” says Jones-Lopez. “What is the probability of you encountering someone who has the virus in their respiratory secretions? The more people you have around you, the higher the probability is.”