Two reports of so-called coronavirus breakthrough infections — in which fully vaccinated people get the illness anyway — suggest that the vaccines still offer strong protection against severe disease even in the face of variants.
The cases, which were detailed Wednesday in the New England Journal of Medicine, were those of two women out of more than 400 fully vaccinated study participants who were tested for Covid-19 weekly. Both women developed mild cases and recovered quickly.
A co-author of the study, Dr. Robert Darnell, a professor and senior physician at Rockefeller University in New York City, said the two cases aren't cause for alarm.
"They certainly didn't need to be hospitalized," he said. "They had at-home cases of Covid-19."
As the number of fully vaccinated people increases in the U.S., so, too, will reports of breakthrough infections rise. The Centers for Disease Control and Prevention said Friday that it had received reports of around 5,800 breakthrough infections out of more that 77 million fully vaccinated people.
Breakthrough infections can occur because no vaccine is 100 percent effective. Such cases remain very rare.
CDC officials are gathering more data about breakthrough cases to determine whether there are any patterns. Among the questions is whether certain variants are more likely to play a role in breakthrough cases.
Both of the cases in the new report were sequenced, and both were found to share certain mutations with the variants first identified in the U.K. and New York. However, neither included all the mutations to match the previously identified variants. (Variants of the virus can include a number of mutations.)
Experts cautioned that because the report detailed just two cases, it's too early to draw conclusions about which variants are most likely to lead to breakthrough infections.
One of the samples included a mutation called E484K, which is also found in the variants from South Africa, Brazil and New York City. It is thought to help the virus evade the body's immune response to a degree.
Jeremy Kamil, a virologist at LSU Health Shreveport in Louisiana, said he wasn't surprised that the mutation was detected, as lab data suggest it would play a role in breakthrough cases.
"If you ask scientists what mutations you'd expect to see in a breakthrough infection, I think the No. 1 answer you'd get would be E484K," said Kamil, who wasn't involved with the new study.
Two other studies, published Wednesday in the CDC's Weekly Morbidity and Mortality Report, touched on breakthrough infections in nursing homes. One report identified 22 breakthrough infections across 78 Chicago-area nursing homes, which had fully vaccinated nearly 15,000 residents and staff members from December through March. In two-thirds of the breakthrough cases, the infections were asymptomatic, although several people developed mild to moderate symptoms, the report said. Two patients were hospitalized, and one person died.
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The second report focused on a Covid-19 outbreak at a nursing home in Kentucky in March. Twenty-six residents and 20 staff members tested positive, including 18 residents and four staff members who had been fully vaccinated. Sequencing of the cases detected the same E484K mutation as in the New York cases.
However, those who had been vaccinated were still 87 percent less likely to develop symptoms than those who were unvaccinated.
"The results from this study are quite telling that vaccination resulted in decreased likelihood of infection and symptomatic disease in a high-risk population" like a nursing home, said Jason Kindrachuk, an assistant professor of medical microbiology and infectious diseases at the University of Manitoba in Winnipeg.
Similarly, the women described in the New England Journal of Medicine report also had mild symptoms, said Kindrachuk, who wasn't involved with the new reports. "The vaccines did exactly what we had assumed based on the clinical trial data and the real world data: They protected from severe disease."
One of the patients in the New England Journal of Medicine report, a healthy 51-year-old woman, tested positive for Covid-19 on March 10, 19 days after her second dose of the Moderna vaccine. She said she followed guidelines, including masking and social distancing, but still developed symptoms, including a sore throat, congestion and a headache. The day after her test, she lost her sense of smell. All of her symptoms went away a week later.
The second patient, a 65-year-old woman with no risk factors for severe Covid-19, tested positive March 17, 36 days after her second dose of the Pfizer-BioNTech vaccine. She got sick two weeks after her unvaccinated partner was diagnosed with Covid-19.
Her symptoms included fatigue, sinus congestion and a headache. As in the first case, her symptoms went away after just a few days.
While data from the CDC suggest that breakthrough infections are rare, Darnell said it would be prudent for fully vaccinated people to get tested for Covid-19 if they develop symptoms that resemble the illness.
"If you do get sick after vaccination and it looks like, smells like and sounds like Covid-19, it may be Covid-19," he said.