Charts: What you need to know about breakthrough cases in Mass.

The state reported 3,431 new breakthrough cases this week, and another 26 deaths among vaccinated individuals. 

Priya Sarin Gupta administers a COVID-19 vaccine to Bladimiro Sanchez, 66, outside an MGH vaccination van during a mini-festival for teens at the La Colaborativa food pantry in Chelsea, MA Oct. 06, 2021. (Nathan Klima for The Boston Globe)

With widespread community transmission, breakthrough cases of COVID-19 continue to show up, as experts have predicted, but they’re much less likely to lead to hospitalization and death. Plus, so far only about 1% of vaccinated people have tested positive for COVID-19.


Dr. Sabrina Assoumou, an infectious disease physician at Boston Medical Center, referenced data recently published by the Centers for Disease Control and Prevention, which shows that vaccinated people are six times less likely to test positive for COVID-19, and 11 times less likely to die from it, compared to those who are unvaccinated.

“These vaccines work remarkably well for protecting against hospitalization and death, but they’re not 100% effective,” she told “When we have high levels of community transmission of the virus it’s more likely we’ll get infections after vaccination. So, if we want to see less of these breakthrough cases, what we need to do is continue to vaccinate the unvaccinated.”


The state released updated data Tuesday on the number of COVID-19 cases, hospitalizations, and deaths among vaccinated individuals in Massachusetts. There have been a total of 47,929 breakthrough cases of COVID-19 as of Oct. 16, an increase of 3,431 from Oct. 9. Between Oct. 2 and 9, 4,034 breakthrough cases were reported, so 603 fewer breakthrough cases were reported this week.

There were 4,686,467 people vaccinated as of Oct. 16, meaning 1.02% have reported a breakthrough case of COVID-19; this is the first week it’s cracked 1%. The rate has been steadily increasing — it was 0.23% on Aug. 7. 

“When we think about infections after vaccination, they’re relatively uncommon,” Assoumou said. “The numbers have been creeping up in Massachusetts but it’s close to 1% so we have to look at the big picture here and how common these are, and vaccination is a way to get us out of this mess.”

Though this reflects the overall trend of how many vaccinated people are reporting cases, it’s easier to understand on a week-by-week level. So, for example, how many of the new cases reported this week were breakthrough cases.

Between Oct. 10 and 16, 8,657 people tested positive for COVID-19 (110 less than last week, and continuing a downward trend). The same week, 3,431 breakthrough cases were reported, meaning 39.6% of COVID-19 cases in that period were breakthrough infections. The week before that, 46% of COVID-19 cases were breakthrough cases.


“If we have a high community level of protection, then you’re protecting the vulnerable, you’re protecting the children who are less than 12…individuals with underlying conditions who did everything we asked them to do but may not have as robust an immune response and are not protected,” Assoumou said. “This is a true statement for all vaccinations — that’s why you vaccinate, to protect not only yourself.”

Hospitalizations and deaths are also increasing but at a much lower rate. Tuesday’s data showed a total of 1,566 hospitalizations and 371 deaths, an increase of 138 and 26 respectively from Oct. 9. Out of vaccinated individuals, 0.03% have been hospitalized and 0.008% have died. According to the data, 3.27% of breakthrough cases resulted in hospitalization and 0.77% in death.

A spokesperson from the state Office of Health and Human Services told all available data continue to support that all three vaccines are highly protective against severe disease and death from all known variants of COVID-19. As of Oct. 9, the median age of those who died from a breakthrough case of COVID-19 was 81.2 years, and 73% had underlying conditions making them more likely to develop severe disease. Similarly, 61% of patients hospitalized with breakthrough infection had underlying conditions.


“One of the important messages of this pandemic – which we don’t seem to be learning – is that we’re all connected,” she said. “Colin Powell’s story is a story of someone not getting vaccinated impacting him, ultimately, because we’re in a country where there’s a lot of community transmission. It’s not only a humanitarian imperative, but also from a national security perspective, we need to vaccinate the world.”


This discussion has ended. Please join elsewhere on