CDC New Study

July 30, 2021
CDC New Study

New Case Study

The CDC releases a new report on breakthrough infections – when someone fully vaccinated becomes infected with the virus that causes COVID-19.
What To Know & Why It Matters:

The Basics:

  • What: A case study in Barnstable County, Mass. (located on the island of Cape Cod).
  • Why: On July 3rd, Barnstable had 0 reported cases per 100,000 people of COVID-19. Later in the month, after the Independence Day holiday, the cases increased dramatically – to 177 per 100,000.
  • Take-Away: The CDC, working with the Massachusetts Dept. of Health, tracked more than 400 COVID-19 cases. Nearly three-quarters of cases happened in fully-vaccinated people.


  • 469 people tested positive.
  • 346 (74%) people were fully vaccinated.
  • Nearly 80% of breakthrough cases were symptomatic.
  • Delta variant found in 90% of the cases sequenced (120/133).
  • 85% of the infections happened in men; median age 40.
  • Five people were hospitalized. Four were fully vaccinated; one was unvaccinated. "As of July 27, no deaths were reported" (CDC).
“This shows the delta is formidable. We can’t take one report of packed bars and extrapolate and say the sky is falling. The sky is not falling. But it does say the vaccine is not infallible.”

Virologist Larry Corey, Fred Hutchinson Cancer Research Center. Health officials say COVID-19 vaccines continue to show good results in preventing severe disease or death in most cases. But while breakthrough cases are expected (esp. in areas with high vaccination rates, such as Mass.), this study also raises questions about waning vaccine immunity.

Something To Consider:

Significant Take-Away: In a limited sample of those infected, vaccinated and unvaccinated individuals carried similar amounts of the virus in their nose, raising questions about whether both can spread the virus similarly. This is different from previous research which concluded that did not seem to be the case with other variants (such as alpha).

Reinfections: We do not have a data point of how many of these individuals had previous COVID-19 infections. This would add to a bigger picture on immunity.

The study's authors noted several limitations – including that breakthrough infections, as with infections in unvaccinated people, may be underrepresented and that further study is "required to confirm" that the viral load is the same between fully vaccinated and unvaccinated individuals.

CDC Report on Outbreak Infections:

by Jenna Lee,