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CDC: 74% Of People Infected in Massachusetts COVID-19 Outbreak Were Vaccinated

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Key Takeaways

  • The majority of people infected in a July COVID-19 outbreak in Massachusetts were fully vaccinated against the virus.
  • This report suggests that people who are fully vaccinated can still spread the virus to others.
  • Still, doctors stress the importance of the COVID-19 vaccine to prevent serious illness and hospitalization.

A new report issued by the Centers for Disease Control and Prevention (CDC) last Friday found that 74% of people who were infected in a COVID-19 outbreak in Massachusetts earlier this summer were fully vaccinated against the virus.

The data also found that people who are fully vaccinated and get infected can carry as much of SARS-CoV-2, the virus that causes COVID-19, in their noses as those who are unvaccinated. As a result, they can spread the infection to others.

The study, which was published in the CDC’s Morbidity and Mortality Weekly Report on July 30, detailed the July 2021 COVID-19 outbreak in Barnstable County, Massachusetts—specifically Provincetown. Several large public events in the area led to 469 COVID-19 cases in people who traveled to the area between July 3 and 17. Of the 469 people infected, 346 (or 74%) were fully vaccinated against the virus.

Genomic testing on 133 patients found that 90% had the Delta variant. The majority of patients with breakthrough infections—79%—experienced symptoms, including cough, headaches, sore throat, muscle aches and pains, and fever.

Of those who had breakthrough infections:

  • 159 (46%) had the Pfizer-BioNTech vaccine
  • 131 (38%) had the Moderna vaccine
  • 56 (16%) had the Johnson & Johnson vaccine

Among five people who were hospitalized in the outbreak, four were fully vaccinated. No deaths were reported in the outbreak.

The new data was released just days after the CDC updated its guidance to recommend that fully vaccinated people wear a mask in public indoor spaces in areas where the spread of COVID-19 is substantial or high.

“Findings from this investigation suggest that even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status, given the potential risk of infection during attendance at large public gatherings that include travelers from many areas with differing levels of transmission," the researchers wrote.

The Delta Variant Is More Transmissible

The Delta variant is still hitting unvaccinated people the hardest. But these recent findings suggest that the variant may be causing vaccinated people to carry higher levels of the virus than previously understood.

While the Delta variant caused just 3.1% of cases of the virus in late May, it is now responsible for 82.2% of cases—making it the dominant strain.

“It is astounding to the degree that the Delta variant has taken over so quickly,” Stanley Weiss, MD, professor of medicine at the Rutgers New Jersey Medical School and professor of biostatistics and epidemiology at the Rutgers School of Public Health, tells Verywell.

This isn’t unexpected, though. “The Delta variant is evidence of the fact of viruses are going to evolve to become more efficient at spreading between people, which is their goal,” Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Verywell.

What This Means For You

While the COVID-19 vaccine can largely protect you against serious forms of the virus, breakthrough cases can happen—particularly with the Delta variant. Getting fully vaccinated against COVID-19 and wearing a mask when you’re indoors in areas of substantial or high spread of the virus will go a long way toward helping to protect you.

You Should Still Get Vaccinated

Even though breakthrough infections can occur, experts still stress the importance of getting vaccinated against COVID-19.

“The vaccine might be less effective for preventing infection with the Delta variant, but it still offers excellent protection against the need for hospitalization and against death,” Richard Watkins, MD, an infectious disease physician and professor of internal medicine at the Northeast Ohio Medical University, tells Verywell.

The data suggest that booster shots may be needed at some point, Weiss adds. “The federal government hasn’t issued guidance for that yet, but I hope they will in the near future,” he says.

And, even though some people in the general public have taken the report to suggest that the vaccine doesn’t work, Adalja says it’s actually the opposite.

"The vaccine is efficacious of stopping what matters,” Adalja says. “It underscores the need to get the vaccine into the arms of the unvaccinated, especially those at high risk for hospitalization.”

“The Delta variant is more infectious, and we need to be better protected against it,” Weiss adds. “For those people who have not yet been fully vaccinated, it’s time that they should start to protect themselves.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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  1. Centers for Disease Control and Prevention. Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 2021. July 30, 2021.

  2. Centers for Disease Control and Prevention. Interim Public Health Recommendations for Fully Vaccinated People. July 28, 2021.

  3. Centers for Disease Control and Prevention. Variant Proportions. July 17, 2021.