COVID-19 vaccines do not provide 100% protection against the virus, and in rare cases, people who are fully vaccinated may still develop the disease. Those so-called breakthrough cases may be driven by rapid virus changes, and ongoing testing of immunized individuals will be necessary to help prevent future outbreaks, researchers say.

Investigators at Rockefeller University in New York City followed the cases of two fully vaccinated people in their own medical research community. These study participants tested positive for the coronavirus after receiving two doses of either the Pfizer or Moderna vaccines.

One person initially was asymptomatic but then developed symptoms. The other person had symptoms before being tested. Both patients recovered at home, “an outcome consistent with evidence suggesting vaccination is effective in preventing severe disease,” the researchers noted.

Investigators found multiple mutations in viral samples from both individuals. Those included the E484K variant in one participant, first identified in South Africa and Brazil, and the S477N variant, a mutation that has been detected in New York since November, in the other. 

“These patients got vaccinated, had great immune responses, and nonetheless broke through with a clinical infection,” said Robert B. Darnell, M.D., Ph.D. The findings highlight the small ongoing risk among vaccinated individuals, and suggests that they may continue to spread the virus, he said.

“The idea that we could be entirely done with testing in the post-vaccine world is probably not a good one right now,” said Darnell, a well-respected neuroscientist

People who develop respiratory symptoms, even after vaccination, still should consider being tested for COVID-19, he said. Likewise, testing should be considered after exposure to individuals with known infection despite vaccination status, he noted.

“Given the scope of the pandemic, there’s a huge amount of virus in the world right now, meaning a huge opportunity for mutations to develop and spread,” Darnell added. “That is going to be a challenge for the developers of vaccines over the next months and years.”

Full findings were published in the New England Journal of Medicine.