BOSTON — While millions of people in Massachusetts have been vaccinated against COVID-19, public health officials are still reporting thousands of “breakthrough” cases, where individuals become infected even after they’ve been vaccinated.

There have been more than 4,300 breakthrough cases reported in the state as of May 31, according to the state Department of Public Health.

The agency notes that most of the cases were not severe and account for only a fraction of more than 3.2 million people who’ve been fully vaccinated.

Public health officials say while the COVID-19 vaccines are about 95% effective, some people will inevitably get sick after getting their shots.

“The vaccines appear to have a high level of protection, but there still a small proportion of people that are going to break through with symptomatic infections,” said Dr. David Hamer, an infectious disease expert and professor at Boston University’s School of Public Health and School of Medicine. “This appears to happen in only a fraction of the people who have been fully vaccinated.”

He said the latest data suggests many of the breakthrough cases are asymptomatic, and the vaccinated individuals who get sick have relatively low viral loads.

Hamer said more research is needed to determine exactly why the vaccine doesn’t work on some individuals and what risk factors might be involved.

One group that should be concerned about breakthrough cases are people with compromised immune systems, such as patients undergoing cancer treatments.

“There is a growing body of evidence that these people do not respond that well to the vaccine,” he said.

The U.S. Centers for Disease Control and Prevention reported more than 10,200 breakthrough cases nationwide as of the end of April. But in May the federal agency shifted its focus to investigating only severe breakthrough cases. Of those reported to date, it has identified about 2,400 cases, including 439 deaths.

“There will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from COVID-19,” the CDC said in a recent advisory.

Many of the individuals who died after being vaccinated were already hospitalized and or had pre-existing conditions that contributed to their death, the CDC said.

The federal agency, which is tracking how well the COVID-19 vaccines work, points out that because many breakthrough cases involve asymptomatic individuals, the number of actual cases may be a lot higher than reported.

Dr. Howard Koh, a professor at Harvard University’s T.H. Chan School for Public Health and former state public health commissioner, said despite public concerns about breakthrough cases the vaccines have proven to be the best defense against COVID-19 infections and allowing the nation to return to normal.

“The end of this public health marathon is within sight,” he said. “We’re in the final sprint, and need to keep the momentum going, but we always have to be vigilant because this virus has pummeled us so many times in the past”

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites. Email him at cwade@cnhi.com

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