It’s called mild COVID. But vaccinated people with breakthrough infections can still feel pretty sick.

Andrew Kinsey knew that even after being vaccinated against the coronavirus, there was a chance he could still fall ill with COVID-19.

He just never expected to feel this lousy from a case doctors call mild.

For nearly a week, Kinsey felt like he had been “run over by a truck.” He struggled to walk a few steps and to stay awake through episodes of the TV show “Doomsday Preppers.” He returned to work last Monday as a corporate litigator but needs midday naps.

“The vaccine appears to have worked to protect my lungs, so that kept me from having life-threatening symptoms, but at the same time, a so-called mild course can be . . . sort of the sickest I’ve ever been in my life,” said Kinsey, who is 38 and lives with his wife and three children in Pennsylvania. “It’s important for people to know that what they picture in their head of a bad cold isn’t necessarily what will actually happen even if they get a mild course.”

Kinsey and other vaccinated people who develop breakthrough cases of COVID-19, the illness caused by the virus, are learning a mild case may not seem so mild to the person enduring the infection. Those cases can be as modest as a few days of sniffles, but, in other circumstances, can spawn debilitating headaches and fatigue. Symptoms can persist longer than the usual cold.

But public health authorities and scientists stress that research overwhelmingly shows that coronavirus vaccines are keeping people out of the hospital and that most breakthrough cases are mild or moderate.

[‘Be vigilant’: Vaccine breakthrough COVID-19 cases in Alaska are surprising, usually less severe and part of the reality of the pandemic for now]

Seven vaccinated people who ended up sicker than they expected shared their stories and said they did not want to cast doubt on vaccines - because they believe their outcome would have been much worse had they not been inoculated. Instead, they said they want to help fellow vaccinated people weigh their risks as they decide when to wear a mask and whether to attend a wedding or travel for vacation. They also do not want people to assume a mild case is trivial.

Kinsey is re-examining how he weighs risk this upcoming school year after his family’s battle with the virus. He’s not sure how he and his wife, Lisa, who is also vaccinated, were exposed. They are generally cautious and wear masks to protect their daughter Sarah, who is too young for vaccines at age 8 and has significant medical issues.

Sarah also contracted the virus and was hospitalized for nearly two weeks, later mostly recovering while her vaccinated siblings stayed healthy. For at least several months, the family expects to have protection from natural and vaccine-induced antibodies. But Kinsey says his experience was a reminder of the urgency of paying close attention to the changing understanding of the virus.

Matt Longman, who is 41 and lives in Tucson, Ariz., said he had a 103-degree fever, experienced aches in his elbows and toes like he had never encountered before and could not stop shaking even after wrapping himself in three blankets. Longman fears he would have ended up in the hospital had he not been vaccinated, especially because his immune system is weakened from migraine treatments.

“It was definitely scary feeling as sick as I did, but throughout it, I kept thinking if I hadn’t been vaccinated, this would be so much worse, and I can’t imagine what it’s like for people who are unvaccinated,” said Longman, who works in communications for a pharmaceutical company.

Longman said he wishes he had worn a mask to the grocery store because he suspects that is where he was exposed.

[Alaska surpasses record for COVID hospitalizations, reports 6 deaths, including woman in her 20s]

Public health experts warn that the novel coronavirus fueling this pandemic probably will never be eradicated and instead is destined to become another endemic respiratory virus that can be kept in check with vaccines - similar to influenza. But even people who get their flu shot occasionally are bedridden with the flu.

Studies have confirmed that coronavirus vaccines are highly effective in keeping people out of hospitals, even as the highly contagious delta variant rampages throughout the United States. And vaccinated people are still far less likely to develop infections — recently published data shows that unvaccinated people in Los Angeles County were five times as likely to become infected with the coronavirus and 29 times as likely to be hospitalized as people who were fully immunized.

But with breakthrough cases, limited data and research make it hard to pinpoint what share ends up with significant symptoms that do not require hospitalization.

The Centers for Disease Control and Prevention reported about 27 percent of breakthrough cases reviewed between January and April were asymptomatic. The public health agency stopped collecting widespread data on mild and moderate infections among the fully vaccinated in May, prompting criticism from some experts who say those cases should be monitored even if they are less serious.

One study published in the New England Journal of Medicine found about one-third of 39 Israeli health-care workers who developed breakthrough infections had no symptoms, while one-fifth reported “long COVID” symptoms six weeks after their diagnosis. None were hospitalized.

Experts say the rising number of breakthrough cases during a national surge is expected. That is because the vaccine’s protection in a pandemic is a bit like a raincoat: It will keep you dry when you are walking in a drizzle, but you can still get wet in a relentless tropical storm. It is storming in the United States as the delta variant sends new cases surging to levels not seen since last winter.

Ilan Shapiro, medical director of a network of Los Angeles community health centers, advises vaccinated patients to remain vigilant and wear masks - not so much for themselves but to protect children and people who are older and immunocompromised and may face higher risk from breakthrough infections. A study of an outbreak in Provincetown, Mass., found that the people who were infected were overwhelmingly vaccinated, leading experts to conclude the immunized can still spread the virus in some cases.

“I let them know it’s better to think they are not vaccinated,” Shapiro said. “It can still attack you.”

Vaccinated people throughout the United States are reassessing their tolerance of risk as governments recommend everyone wear masks in indoor public settings. Some people tolerate the relatively low risk of breakthrough infections and trust vaccines to prevent the worst outcomes if they end up sick.

Tyler Black, 26, said he suspects he contracted a breakthrough case shortly after joining a gym in an attempt to stay healthy. The virus left him with what he described as the longest illness of his life and the worst headaches ever. He said he was never in danger of being hospitalized. He is thankful the vaccines appeared to protect his wife from getting sick.

“There wasn’t really anything that hit me that I’d actually give up,” said Black, a Boston-area dental student. “It’s more like doing the safe practices would enable me to do more things safely, like masks and handwashing.”

Part of the risk calculation for the vaccinated involves the potential for lingering health consequences from the virus.

Steven Deeks, an infectious-disease physician at the University of California at San Francisco who studies long-haul COVID, said there is little data to say how worried people should be, but he cautioned people should not assume they will be fine.

“I wouldn’t tell people to shut down based on this theoretical risk,” said Deeks, who lets his son who is too young to qualify for a vaccine hang out with friends. “But on the other hand, I’m telling people, ‘Let’s give science time to figure out what these breakthrough infections can mean to your long-term health and until then, be careful.’ "

Most vaccinated Americans will have an opportunity to gain extra protection as the Biden administration mobilizes to start providing booster shots to the general public in September, pending approval by drug regulators. Third shots are already available to the immunocompromised.

Anna Durbin, professor of global health at the Johns Hopkins Bloomberg School of Public Health, said Americans worried about mild or moderate cases of COVID can lower their risks by wearing masks, washing their hands frequently and social distancing instead of rushing for a booster shot.

“There’s no doubt if you give a booster shot, you’ll reduce the number of mild to moderate infections in vaccinated people for a time,” Durbin said. “But I’m not sure preventing mild to moderate colds in people right now today should be our goal. It should be vaccinating as many people in the world right now.”

Erin Goodyear, 28, is recovering from a breakthrough infection contracted after traveling to Arkansas for a 10-year high school reunion while cases were spiking.

She felt secure enough to attend the reunion, held in a well-ventilated venue, and to stay with her vaccinated parents. Goodyear does not regret her decision because of the opportunity to reconnect with old friends. But she wishes she had avoided certain activities, such as the drag show in a poorly ventilated and packed basement bar she attended after the reunion.

After returning home to the District of Columbia, Goodyear experienced an illness she described as a two-day cold that left her weak but not worried enough to get a coronavirus test until a friend suggested it. She also was concerned she might have infected her parents, who are vaccinated and did not become sick. Now, she has her guard up and plans to wear a mask, socialize outdoors and skip bars.

“Maybe my risk tolerance will change in the fall and winter when I’m lonely and want to see people, and I may have to do that inside more,” said Goodyear, who works for a nonprofit. “The physical recovery and mental and emotional recovery did take a toll, and I just don’t want to go through that again either.”

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The Washington Post’s Joel Achenbach and Ben Guarino contributed to this report.