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On July 27th, CDC updated its guidance for fully vaccinated people, recommending that everyone wear a mask in indoor public settings in areas of substantial and high transmission, regardless of vaccination status. This decision was made with the data and science available to CDC at the time, including a valuable public health partnership resulting in rapid receipt and review of unpublished data.

Today, some of those data were published in CDC’s Morbidity and Mortality Weekly Report (MMWR), demonstrating that Delta infection resulted in similarly high SARS-CoV-2 viral loads in vaccinated and unvaccinated people. High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus. This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones.

This outbreak investigation and the published report were a collaboration between the Commonwealth of Massachusetts Department of Public Health and CDC. I am grateful to the commonwealth for their collaboration and rigorous investigation. I would also like to humbly thank the residents of Barnstable County who leaned in to assist with the investigation through their swift participation in interviews by contact tracers, willingness to provide samples for testing, and adherence to safety protocols following notification of exposure.

This outbreak investigation is one of many CDC has been involved in across the country and data from those investigations will be rapidly shared with the public when available. The agency works every day to use the best available science and data to quickly and transparently inform the American public about threats to health.