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Why are breakthrough COVID cases rising in Mass.? Experts blame waning immunity and surging Delta

A health care worker filled out a COVID-19 vaccination card.ROBYN BECK/AFP via Getty Images

There’s no doubt about it: COVID-19 cases among the fully vaccinated are on the rise in Massachusetts.

Between July 10 and 17, the state recorded about 700 so-called breakthrough cases. Between Aug. 7 and 14, it tallied almost 2,700 — raising the cumulative number of breakthrough infections to nearly 13,000.

Although most vaccinated individuals have mild cases, some still land in the hospital. As of Wednesday, almost 30 percent of the 439 patients hospitalized for COVID-19 in Massachusetts were fully vaccinated.

Experts attribute the state’s uptick in breakthrough cases to a number of factors, including the diminishing protection against COVID-19 offered by the vaccines.


Federal health officials released a statement Wednesday confirming that “available data make very clear” that vaccine protection “begins to decrease over time following the initial doses” — prompting plans for booster shots in coming months.

A study of adults in New York City released Wednesday by the CDC found that vaccine effectiveness against infection declined from 91.7 to 79.8 percent between May 3 and July 25. Vaccine effectiveness against hospitalization was stable throughout the 12 weeks, at 95.3 percent.

The rise in breakthroughs in Massachusetts is “a combination of the immunity starting to wane from the original course of the vaccine, and then Delta,” said Dr. David Hamer, an infectious diseases doctor at Boston Medical Center. The variant has a high viral load that is sometimes “exceeding the protective value of the vaccines,” he explained.

Dr. Philip Landrigan, director of the Global Public Health Program at Boston College, believes the rise in breakthroughs “reflects the fact that the Delta variant is loose in the population, and it reflects the fact that there’s a lot of virus circulating around.”

Massachusetts has had over 1,000 confirmed cases a day since Aug. 5. Whenever there are more infections, there will “inevitably” be more breakthrough cases, said Dr. Daniel Kuritzkes, who heads the Division of Infectious Diseases at Brigham and Women’s Hospital.


“Even if you have a highly effective vaccine, if you have high community prevalence of an infectious agent, you’re going to see a substantial number of cases in the vaccinated,” Kuritzkes explained.

As of Aug. 14, the Massachusetts Department of Public Health had recorded a total of 12,641 breakthrough cases — about 0.29 percent of the state’s almost 4.4 million inoculated individuals.

Among the state’s breakthrough cases, there were 496 hospitalizations and 124 deaths, accounting for 4 percent and 1 percent of recorded cases, respectively. About 55 percent of those who were hospitalized and 73 percent of those who died had underlying conditions that made them susceptible to severe disease, according to DPH. The median age of those who died is 81.7 years.

In reality, there have also likely been more mild and asymptomatic breakthrough cases than the figure recorded by the state.

“This isn’t some kind of unbiased, systematic survey of people that’s picking up all infections,” Kuritzkes said. “We’re picking up a selected subset of infections, with a predominance of people who have symptoms or are hospitalized. We really don’t know what the denominator is.”

Health care providers are required to report only breakthrough cases that result in hospitalization and death, according to a memo from DPH in May. DPH identifies the remainder of breakthrough cases by “matching electronically reported laboratory results with immunization reports.”


“Anybody who’s doing self testing or home testing is not going to be included,” Kuritzkes said. Nor will the individuals who have symptoms they consider too mild to procure a test, said Hamer.

Asymptomatic cases are also almost impossible to count. With the Delta variant of the virus, about 25 percent of breakthrough infections are asymptomatic, according to Hamer.

Though rising cases across the state are cause for concern, Kuritzkes urged individuals to keep the breakthrough cases “in perspective.”

“Most people who get vaccinated, if they come in contact with coronavirus, are not going to get infected. They’re not going to get sick,” he said.

Even if they become sick, they are much less likely to be hospitalized or die.

“We’re still seeing great protection and very low rates of hospitalization and death in these vaccine breakthrough cases relative to what we saw a year ago when we had no vaccines,” Landrigan said. “Vaccines are still far, far and away the most effective protection against this disease.”

The uptick in breakthrough cases should still serve as are a reminder that vaccinated individuals should continue taking precautions, said Landrigan.

“The rising incidence of breakthrough cases is the strongest argument for people wearing masks when they go into social circumstances,” he said.

“Even people who are vaccinated need to be cautious again,” Kuritzkes agreed. He advises vaccinated individuals to wear masks indoors whenever they don’t know the vaccine status of those around them, and thinks indoor restaurants and bars are “problematic.”


Anyone who received Pfizer or Moderna should also plan to receive a third shot about eight months after their second dose, starting the week of Sept. 20. For now, pending more data, health officials have not decided on the booster timeline for those who received the Johnson & Johnson vaccine.

“A booster shot will be needed to maximize vaccine-induced protection and prolong its durability,” federal health officials wrote on Wednesday.

Experts emphasized that breakthrough cases are not a reason to refuse vaccination.

“It’s like saying that you won’t wear a seat belt, just because somebody had a collision and died despite the fact that they were wearing a seat belt,” Kuritzkes said. “Even if it’s not perfect, the vaccines are still extraordinarily effective.”

Camille Caldera was a Globe intern in 2022.Follow her on Twitter @camille_caldera.