A new study from Rockefeller University found just two people tested positive for COVID-19 after being fully vaccinated against the coronavirus.

The study — published in the New England Journal of Medicine — reviewed 417 employees from Rockefeller University who received both doses of the Pfizer or Moderna vaccines.

Two of the 417 — or about 0.5% — tested positive for COVID-19.

“We have characterized bona fide examples of vaccine breakthrough manifesting as clinical symptoms,” the researchers wrote in the study, according to CNN. “These observations in no way undermine the importance of the urgent efforts being taken at the federal and state levels to vaccinate the U.S. population. They also lend support to efforts to advance a new vaccine booster (as well as a pan-coronavirus vaccine) to provide increased protection against variants.”

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But — and this is an interesting note — the researchers said the infections came from coronavirus variants, which had multiple changes to the original virus, according to CNN.

Health officials have been clear the none of the COVID-19 vaccines are 100% effective in stopping the novel coronavirus. Dr. Robert Darnell, a senior physician at Rockefeller University in New York City, told NBC News that the study’s findings aren’t a cause for concern.

  • “They certainly didn’t need to be hospitalized,” he said. “They had at-home cases of COVID-19.”

COVID-19 breakthrough cases

Experts have said there are COVID-19 cases among the fully vaccinated — but it’s really rare. Still, vaccine breakthrough cases are expected because vaccines don’t guarantee 100% protection against the novel coronavirus.

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Dr. Francesca Torriani, who is an infectious disease researcher at the University of California, San Diego, told NPR that the patients who have gotten COVID-19 despite being vaccinated usually suffer from less severe illness.