Although CDC data indicates breakthrough infections are exceptionally rare—and that most people who do get infected after vaccination tend to have no symptoms or very mild ones—some breakthrough infections result in severe cases of Covid-19.

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How common are breakthrough infections?

CDC in May stopped tracking asymptomatic cases of Covid-19 among people who have been fully vaccinated. However, according to unpublished CDC data obtained by ABC News, out of more than 156 million vaccinated Americans, there have been only about 153,000 symptomatic breakthrough cases nationwide as of last week—accounting for roughly 0.098% of those fully vaccinated.

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"The risk [of Covid-19] to fully vaccinated people is dramatically less than that to unvaccinated individuals," Matthew Ferrari, director of the Center for Infectious Disease Dynamics at Pennsylvania State University, said. "The occurrence of breakthrough cases is expected and, at this point, is not a level that should raise any concerns about the performance of the currently available vaccines."

Moreover, severe breakthrough infections remain exceedingly rare, according to the White House Covid-19 Task Force. According to the task force, 97% of all patients hospitalized with Covid-19 are unvaccinated.

Overall, according to public health experts, most of the hospitalizations and deaths that do occur in vaccinated individuals happen in older patients and those with serious underlying health conditions that may have led to their vaccines being less effective.

John Brownstein, CIO at Boston Children's Hospital, said that while any tally of breakthrough infections will likely be an undercount—given the decline in Covid-19 testing rates and the mild nature of breakthrough cases—the vaccines remain highly effective, and people should not be overly concerned by news around occasional breakthrough "outbreaks."

"While anecdotal cases and clusters can conjure concern around the vaccine, when put in the larger context of how many people have been vaccinated and the sheer volume of cases in the unvaccinated population, we recognize that the vaccines are working and how rare breakthroughs actually are," he said.

What it feels like to have a breakthrough infection

To learn more about the experience of fully vaccinated people who did contract Covid-19, the Boston Globe spoke to a several individuals whose breakthrough infections varied widely.

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For instance, JD Moore, a 20-year-old from West Roxbury, Mass., said when he started feeling sick, he initially wrote it off as a sinus infection. But when his doctor advised he get a Covid-19 test just to be safe, it came back positive.

Ultimately, however, Moore said his case was mild. It had "a little bit more symptoms than the common cold, but nothing too crazy," he said.

On the other hand, Cathy Mertz, a 60-year-old from Needham, Mass., had much stronger symptoms. Mertz takes medication that leaves her immunocompromised. When she got Covid-19, Mertz said she got "hit pretty hard" and experienced "virtually every symptom." Even 10 days after infection, she said she was still sick with a slight fever and a "very tired, very foggy" mind.

Similarly, Jeff Davis, a 40-year-old from Cambridge, Mass., said on his first day of symptoms, he felt "exhausted, I was achy, I had a little bit of a sniffle. It's kind of like the early onset of a flu—something feels off."

But the next day, Davis said he woke up feeling significantly worse. "I was completely maxed out. I had to take like multiple naps. I had all the symptoms: loss of appetite, headache, congestion, fever, sore throat," he said. "It just kind of hit me like a wall."

And while the individuals the Boston Globe spoke with generally recovered, recent research has found that some people with breakthrough infections could develop prolonged symptoms.

Specifically, a study in the New England Journal of Medicine looked at breakthrough infections among 1,497 fully vaccinated health care workers at Sheba Medical Center in Israel from Jan. 20 to April 28. It found that 19% of the 39 workers who experienced breakthrough infections had symptoms that lasted at least six weeks. Those symptoms included a loss of smell, a persistent cough, fatigue, weakness, difficulty breathing, and muscle pain.

For instance, the Boston Globe also profiled Jojo Jacobson, Covid-19 patient who was originally vaccinated in April. Jacobson said that despite recovering from the worst of her illness, she has yet to regain her sense of smell. Still, although she still wears her mask and avoids crowds, Jacobson said the vaccine provided her a "sense of reassurance."

"I knew that I wouldn't have to go to the hospital. I knew that it was likely to be just like a bad cold and be over in a few days," she said. "That was really reassuring." (Mitropoulos, ABC News, 7/26; Mandavilli, New York Times, 7/30; Caldera, Boston Globe, 7/29; Bean, Becker's Hospital Review, 7/29)