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Massachusetts public health officials have for the first time disclosed how many people in the state have tested positive for COVID-19 despite being fully vaccinated, and the total underscores the remarkable effectiveness of the three available vaccines.

As of April 27, the Department of Public Health had identified 1,798 people who developed what officials call “breakthrough cases.” That’s less than 0.1 percent of the roughly 2.4 million people who were fully vaccinated.

Someone is considered fully vaccinated two weeks after getting the second Pfizer-BioNTech or Moderna shots and two weeks after getting the single-dose Johnson & Johnson shot.

“This is very good news,” said Dr. Eric Rubin, an immunologist at the Harvard T.H. Chan School of Public Health and member of the advisory committee to the Food and Drug Administration that cleared the three vaccines for emergency use in the United States. “The vaccines aren’t perfect but they’re awfully good, and these data suggest that they’re working as well in the real world as they did under the ideal conditions of the original trials.”

Rubin, who is also editor-in-chief of the New England Journal of Medicine, added that “while we expected that some people who are vaccinated would contract the illness, most evidence suggests that these people are getting much less seriously ill.”

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The state percentages conform to national data compiled by the Centers for Disease Control and Prevention. As of April 20, about 87 million people nationwide had been fully vaccinated, and the agency had been informed of 7,157 breakthrough infections.

Nearly a third of the breakthrough infections nationwide were asymptomatic, according to the CDC data. Only 7 percent of the infections involved hospitalization, and many of those individuals had no coronavirus symptoms or were hospitalized for reasons unrelated to COVID-19. Only 1 percent of the breakthrough cases involved patients who died, and some of those deaths were unrelated to COVID-19, according to the CDC.

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None of the breakthrough cases reported to the Massachusetts health department were fatal. State data on hospitalizations were not available.

Infectious disease specialists recently told the Globe that vaccinated people have seemed more vulnerable to rare breakthrough infections if they were taking medicines to suppress their immune systems. That includes people receiving immunotherapy for cancer or taking immunosuppressant drugs to keep their bodies from rejecting transplanted organs.

The CDC says on its website that its tally of breakthrough cases nationally undoubtedly represents an undercount, since the agency relies on voluntary reporting from state health departments. In addition, some breakthrough cases likely go uncounted because of limited testing and because some illnesses are asymptomatic or mild.

Nonetheless, the agency maintains that breakthrough cases are rare and that people should get vaccinated as soon as possible.


Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.