It’s rare and expected, but some fully vaccinated people are catching COVID-19

Sarah Graham is one of about 6,000 unlucky people in the United States who were fully vaccinated against the coronavirus but have been diagnosed with what epidemiologists call “breakthrough cases.”Suzanne Kreiter/Globe Staff

Sarah Graham has a chronic inherited lung disease, so the 25-year-old public relations account executive was relieved to qualify for Pfizer-BioNTech’s COVID-19 vaccine before most people her age.

Graham received her second shot on March 19 and soon after resumed activities she had considered too risky, including exercising ― while wearing a mask ― at an LA Fitness club in her hometown of Stoneham.

But last Friday her head began to throb. She developed a low-grade fever, sore throat, and slight pain in her chest. Her live-in boyfriend, who wasn’t yet vaccinated, had been diagnosed with COVID-19 the week before. He tried to keep his distance, but their condominium is small. Graham got tested and learned the next day that she, too, had been infected.

“Me: fully vaccinated. Me: tests positive,” she tweeted Saturday. “This happened with the swine flu,” she added, referring to another virus she caught, in high school, despite getting inoculated. “My luck.”

Graham, who already feels better, is one of fewer than 6,000 unlucky people in the United States who were fully vaccinated against the coronavirus but have been diagnosed with what epidemiologists call “breakthrough cases.” These infections are very rare and entirely expected; the three vaccines cleared for emergency use provided robust protection in clinical trials, but they aren’t perfect.

Me: fully vaccinated
Me: tests positive

This happened with the swine flu. My luck 😅

— Sarah Graham (@SarahGrahamPR) April 17, 2021

The Pfizer and Moderna two-shot vaccines prevented 95 percent and 94.1 percent of symptomatic cases, respectively, in late-stage studies, while the Johnson & Johnson one-shot vaccine prevented 72 percent of moderate and severe cases in the United States, and 66 percent globally. The CDC considers people fully vaccinated two weeks after the second Pfizer and Moderna doses, and two weeks after the J&J shot.

“We’ve said since the beginning that these vaccines are unbelievably effective, but they are not 100 percent, and when there’s still such a high rate of COVID in the community, these breakthrough infections are bound to happen,” said Dr. Megan Ranney, an emergency room physician at Rhode Island Hospital in Providence. Ranney did not know how many such cases have occurred at her hospital but said she has treated only one fully inoculated patient for the coronavirus.

As of April 13, more than 75 million people in the United States had been fully vaccinated, according to the Centers for Disease Control and Prevention, and about 5,800 breakthrough cases had been reported to the agency. Some 29 percent of the cases were asymptomatic, said the CDC. Only 7 percent resulted in hospitalization; 1 percent of the patients — 74 people — died. The totals indicate that the vaccines were more than 99.9 percent effective.

The CDC and the Massachusetts Department of Public Health are tracking breakthrough cases and collecting viral samples for genomic sequencing to determine which viral strain was responsible. The CDC isn’t providing a state-by-state breakdown of cases, and the state health department was unable to provide a total on Tuesday.

Nonetheless, data from two hospitals in Massachusetts that started inoculating their own employees in December underscore how infrequent infections are after vaccinations.

Dr. Richard Ellison, the epidemiologist for UMass Memorial Medical Center in Worcester, said his hospital fully immunized more than 7,400 employees with the Pfizer and Moderna vaccines by April 10. Roughly 80 percent received the Pfizer vaccine, and the rest got Moderna’s. About two of every 1,000 employees later tested positive for COVID-19, he said.

The number of breakthrough cases was proportionate to the overall distribution of the Pfizer and Moderna vaccines, Ellison said. Both use the same messenger RNA technology to teach cells to create a part of the coronavirus and stimulate the immune system to make antibodies.

Boston Medical Center studied the rates of COVID-19 infections among health care workers there who received at least one dose of the Pfizer or Moderna vaccines but not necessarily the two shots required to be fully vaccinated. A total of 7,109 workers received at least one shot as of Feb. 23, according to a study published last month that has yet to be peer reviewed.

Postvaccination cases of COVID-19 occurred in 1.3 percent of the workers who had received at least one dose and in 0.3 percent who had received both doses but hadn’t necessarily reached the two-week mark after the second dose to be fully inoculated. The rate of infection among 3,481 BMC health care workers who had not yet been vaccinated was much higher, 9.5 percent, according to the study.

All the physicians at teaching hospitals who recounted seeing breakthrough cases said they were generally mild. Some patients had no symptoms but got tested because they were exposed to someone with COVID-19, typically a member of their household.

“Even though there have been these breakthrough cases, even fewer of them have been severe enough to require hospitalization, and among those who were hospitalized, many had multiple other medical problems,” said Dr. Paul Sax, clinical director of the division of infectious diseases at Brigham and Women’s Hospital in Boston. “It wasn’t as if they were just getting severe COVID-19.”

Vaccinated people who do test positive for the virus tend to have lower amounts of viral particles in their bodies than infected people who weren’t inoculated, Sax said. He did not know how many fully vaccinated patients have been diagnosed with COVID-19 at Brigham and Women’s, but said some were employees. He was unaware of any deaths.

As expected, vaccinated people seemed more vulnerable to catching COVID-19 if they were taking medicines to suppress their immune systems, Sax said. That includes people receiving immunotherapy for cancer or taking immunosuppressant drugs to keep their bodies from rejecting transplanted organs.

Massachusetts General Hospital has also seen “a number of breakthrough cases among both patients and employees,” according to Dr. Erica S. Shenoy, associate chief of the infection control unit at the hospital. She didn’t have specific numbers.

As of Monday, the Pfizer and Moderna vaccines accounted for more than 95 percent of the doses distributed in the United States, while the J&J vaccine made up less than 4 percent. Distribution of the J&J vaccine was paused on April 13 while drug regulators investigated the cases of six women who developed rare but severe blood clots after receiving a shot, including one who died.

Despite her rueful tweet about catching COVID-19, Graham, the public relations executive, said she has no doubt she would be far sicker if she hadn’t received the Pfizer shots.

She has felt exhausted, sleeping until 11 a.m. instead of her usual 5 a.m. But Graham’s fever never went above 100 degrees and she hasn’t had a cough. She was starting to feel better Tuesday and likened her symptoms to the mononucleosis she had while attending Melrose High School. It’s nothing compared with the repeated bouts of pneumonia that have landed her in the hospital as a result of her chronic lung condition, she said.

“I’d give it a two compared to the ten of pneumonia,” Graham said. “I know my symptoms would have been way worse without [the vaccine]. I still feel very thankful.”