The information included in this article was current as of publication, however, information changes rapidly regarding Covid-19 and may be out of date. 

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Once Sharone Peikes, 46, got her COVID-19 vaccine in March, she felt more comfortable going to her office, eating out and seeing friends and family. Excited to travel again, she planned a summer birthday trip to Miami. Then, days after returning home to New York, she woke up with a fever and a sore throat. Test results showed she was positive for COVID-19. 

Now, about a week later, her symptoms are improving, but her sense of taste is still off. “I’m not sure if I got COVID on the plane or someplace else,” says Peikes. “But I wish I never took off my mask and I doubled up.”

Some breakthrough infections, meaning those in fully vaccinated people, were always expected. But, according to a new internal CDC report, the hyper-transmissible Delta variant, currently responsible for the vast majority of new COVID cases, is more likely to cause breakthrough infections than other versions of the virus.

In the groundbreaking report, circulated by news outlets on July 29th, the CDC also concluded that those who are vaccinated can actually transmit the virus to others. This finding prompted the agency to revise its face-mask guidance earlier this week, recommending that fully vaccinated people wear masks indoors in areas with “substantial” or “high” transmission.

“You have to adapt to what the enemy is doing, and the enemy, in this case COVID-19, has thrown us a curveball,” says Dr. William Schaffner, a professor of preventive medicine at the Vanderbilt University Medical Center. “That curveball is Delta. We didn’t know that in the summer. Now that Delta is among us, not changing [guidance] would be silly.” 

While experts say there’s no need to panic about breakthrough cases, it’s worth understanding the basics of these post-vaccination infections, and what to do if you suspect you have one.

How well do COVID vaccines protect against the Delta variant?

The vaccines appear to offer less protection against asymptomatic and minor infections from the Delta variant than other strains of the virus. But if you’re vaccinated, getting very sick from COVID remains unlikely.

“If you have a breakthrough case, your risk of progressing to severe disease or even death is very low,” says Dr. Ed Septimus, an infectious disease specialist and professor of medicine at Texas A&M. “The vaccine is doing what it’s supposed to do.”

In a study from Israel released earlier this month, researchers found the Pfizer vaccine to be 91 percent effective against severe COVID infections from the more contagious Delta variant, but only 39 percent effective overall and 41 percent effective against symptomatic infections. The study was based on cases reported between June 20 and July 17. A UK study painted a more optimistic picture, finding two doses of Pfizer to be at least 88 percent effective against symptomatic Delta infections.

“All this boils down to two things,” says Schaffner. “If you’re unvaccinated, get vaccinated, please. If you’re vaccinated already, and you go indoors to a group event, wear a mask.”

If you’re fully vaccinated, when should you get tested for a COVID breakthrough infection?

For starters, it’s important to understand what “fully vaccinated” means, says Dr. Amy Mullins, director for quality and science at the American Academy for Family Physicians. It takes two weeks for currently available COVID vaccines to take full effect. (For two-dose vaccines, like Pfizer and Moderna, that means two weeks after the second shot.) Until that period is up, you’re vulnerable to contracting a more serious case of COVID. 

Most people test positive for COVID around five days after exposure, Septimus says. If you’ve knowingly been exposed to COVID-19, you should get tested anywhere from three to five days later, whether or not you have symptoms, according to the CDC. Septimus recommends getting a PCR test, because it’s the most reliable type.

If you develop cold or flu-like symptoms, you should get a COVID test no matter what. Now that people are out and about in the world again, the minor respiratory viruses that died down during quarantine are circulating again. You may very well feel under the weather from a cold, not COVID. But it’s worth knowing. “It’s important to figure out what your illness is for both yourself and to protect others,” he says. 

Why get tested for a breakthrough infection?

Even if you don’t have any symptoms, it’s important to get tested if you’ve been exposed to COVID so you don’t spread it. 

Should you assume you’ve contracted a certain variant? Does it matter?

More than 80 percent of new COVID-19 cases in the US are linked to the Delta variant, so it’s more likely than not that a breakthrough infection is Delta. Knowing the variant can be useful epidemiologically, says Septimus, but it shouldn’t affect how long you isolate or monitor your symptoms. Also, identifying the variant could help inform treatment if you need medical attention for a breakthrough infection. But determining whether or not you have the Delta variant requires additional screening, which is only available at some testing facilities. 

If you test positive, what’s the next step?

First, put on a mask if you haven’t already, especially if you’re around unvaccinated people, including young children. Then, “stay home so you don’t expose anyone else,” Septimus says. If your result is positive, CDC guidance is to isolate for 10 days and wear a mask indoors in public for 14 days after exposure or until you get a negative test result. “We assume the amount of virus in people who are fully immunized is relatively low,” Septimus says. “The question is, is it high enough to cause transmission to others? We have to take the right precautions until we know for sure.”

Do breakthrough infections have the same symptoms as cases in unvaccinated people?

“It’s unusual for the vaccinated population to be sick enough to go to the hospital,” Schaffner says, even with the Delta variant.

Breakthrough infections are generally less intense; they’re likely to resemble a cold or a mild case of the flu. They might include some aches, a runny nose, sneezing, a sore throat, and vomiting, nausea or diarrhea. “For most breakthrough cases, we can treat at home, and people are getting better much faster without any medical intervention,” Mullins says. For more serious issues like shortness of breath or chest pain, get medical attention right away.

Which groups are considered high risk for breakthrough infections?

People with weakened immune systems, such as organ transplant patients, don’t mount as strong a response to vaccination, so they might have an increased risk of breakthrough infections, and possibly more severe ones. Other potentially high-risk groups include people with autoimmune conditions who take immunosuppressant drugs, adults over age 65, and anyone with diabetes or obesity.

How can a breakthrough infection impact your immunity to COVID? Will it make you less susceptible to future variants?

Not necessarily. Scientists still don’t know for sure, and more research is needed. “But having a breakthrough infection probably boosts your immunity against COVID in general,” Septimus says. 

People with “long COVID” report residual symptoms that persist for weeks or months. Could this happen from a breakthrough infection?

“We don’t think so,” says Septimus, “but we don’t know for sure.” This is another area in need of additional research, so stay tuned.

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