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Keep an eye on severe COVID cases among breakthrough infections, experts say

Severe illness and death in vaccinated individuals is exceedingly rare, data show

A health care worker administered a dose of a COVID-19 vaccine earlier this year.KEVIN HAGEN/NYT

As COVID-19 cases begin to tick up in Massachusetts, including among those who have been vaccinated, experts say the most important breakthrough cases to watch are those that bring serious illness or death.

Such severe cases are by far the exception and not the rule, data show, as the vaccines in use in the US have proven highly effective at preventing serious disease.

But as public health officials weigh the current surge in cases nationwide, particularly those tied to the now-dominant Delta variant, they say infections in vaccinated individuals who aren’t experiencing symptoms are far less cause for concern than the potential risks to the unvaccinated.


In Massachusetts, which boasts one of the nation’s highest vaccination rates, it should be no surprise that some positive COVID-19 test results are emerging in people who were vaccinated, they said.

While the COVID-19 vaccines are highly effective, it is still possible for those who have been vaccinated to test positive for the virus, and, in very rare cases, suffer severe disease. It’s those cases that are the most critical to attend to, experts said.

“There is a lot of media hype about breakthrough infections,” said Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center. “But really it is the unvaccinated people who should be worried right now much more so than the vaccinated people.”

As far as tracking the data, she added, “there’s a really good argument to not pay attention to [asymptomatic cases] at all,” though she noted that researchers still don’t know how common it is for asymptomatic individuals with breakthrough infections to transmit the virus to others. “And then what about the mild infections? That is the vaccine still doing what it is supposed to do.”

It’s important to track those who have symptoms as a way of assessing how well the vaccine is working, Doron said. But “we don’t need to act on mild infection.”


Instead, she said, the focus should be on ensuring as many people as possible get vaccinated.

As of July 17, the most recent date for which data was available, there were 5,166 COVID-19 cases in 4.3 million vaccinated individuals in Massachusetts. Very few of those who were infected after being vaccinated — 80 people — died.

Upon request, state health officials provide weekly data detailing how many infections have been recorded among the ranks of the vaccinated in Massachusetts and whether they led to hospitalization, death, both, or neither. State officials are considering adding that information to their COVID dashboard every week, they said Friday.

Providers in Massachusetts report data on breakthrough infections directly to the US Centers for Disease Control and Prevention, which then provides those numbers to Massachusetts, state health officials said.

The CDC said earlier this year that it “transitioned to investigating only breakthrough cases that involve patients who are hospitalized or patients that have died,” a decision that has rankled some.

Senator Edward J. Markey sent a letter to the CDC this week asking federal officials about that decision, and urging the agency to “remain vigilant and transparent in its surveillance of breakthrough cases.”

“I encourage the CDC to continue taking steps that ensure our national response to this global pandemic is comprehensive and that patients, families, and caregivers across the country have the necessary information, including on breakthrough cases, to care for themselves and others and local and state officials can track outbreaks or hotspots,” he wrote.


But Stephen Kissler, who uses mathematical models to study infectious disease at Harvard’s T.H. Chan School of Public Health, said it’s the symptomatic cases that merit the most attention from researchers, especially given the CDC’s limited resources.

As more and more Americans get vaccinated, the novel coronavirus will remain part of life here, receding until it becomes a threat more like the seasonal flu, he said.

“We don’t track every symptomatic case of flu, we don’t track every symptomatic case of the other viruses that cause the common cold. COVID ideally will resemble those other viruses much more closely,” Kissler said. “It makes a lot of sense to be closely tracking anybody who ends up hospitalized or worse. . . . I think we can start to relax a little bit our surveillance for these milder cases and certainly asymptomatic ones.”

Emma Platoff can be reached at emma.platoff@globe.com. Follow her on Twitter @emmaplatoff.