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This award-winning blog supplements the articles in Hospital Infection Control & Prevention.

Mask Up: CDC Returns to Vaccinated Masking After Delta Breakthrough Infections

By Gary Evans, Medical Writer

In a disturbing pandemic development, 74% of cases in a large Massachusetts outbreak had been fully immunized with COVID-19 vaccines, the Centers for Disease Control and Prevention reported on July 30, 2021. The primary etiologic agent was the Delta variant of SARS-CoV-2, which now represents some 80% of U.S. cases.

"During July 2021, 469 cases of COVID-19 associated with multiple summer events and large public gatherings in a town in Barnstable County, Massachusetts, were identified," the CDC reported. "Approximately three quarters (346; 74%) of cases occurred in fully vaccinated persons (those who had completed a two-dose course of mRNA vaccine [Pfizer-BioNTech or Moderna] or had received a single dose of Janssen [Johnson & Johnson] vaccine ≥ 14 days before exposure)."

There were no deaths in the vaccinated group, but as a result of that and other reports, the CDC is recommending a return to indoor masking for fully vaccinated people in areas with “substantial or high transmission” of COVID-19.

The CDC referred to a map of U.S counties for determining whether any given local area has high or substantial transmission. Overall, 52% of U.S. counties were designated as high transmission and 17% as substantial. Thus, vaccinated people in 69% of U.S. counties should go back to wearing masks indoors. Most of these counties are in the South and traditional red states, where COVID-19 vaccination rates are also low.

“In recent days I have seen new science with the data from outbreak investigations showing that the Delta variant behaves uniquely different than past strains of the virus that causes COVID-19,” CDC Director Rochelle Walensky, MD, MPH, said at a press conference.

Data from several states and other countries indicate that on “rare occasions some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus others,” she said. “This new science is worrisome and unfortunately warrants us to update our recommendations. Vaccinated individuals continue to represent a very small amount of transmission occurring around the country.”

The CDC added the caveat that data "from this report are insufficient to draw conclusions about the effectiveness of COVID-19 vaccines against SARS-CoV-2, including the Delta variant, during this outbreak. As population-level vaccination coverage increases, vaccinated persons are likely to represent a larger proportion of COVID-19 cases. Second, asymptomatic breakthrough infections might be underrepresented because of detection bias."

Vaccination reduces the changes of symptomatic breakthrough infection with the Delta variant by seven-fold, and the risk of being hospitalized by 20-fold, Walensky said.

For more on this story, see the next issue of Hospital Infection Control & Prevention.