The Centers for Disease Control and Prevention (CDC) released a report Friday detailing a recent COVID-19 outbreak in Provincetown, Massachusetts, which was cited by the agency as a contributing factor to its recent change in mask guidance.

Over the July 4 weekend, gatherings in Provincetown contributed to at least 900 cases, and roughly 75% of those who tested positive in the cluster were vaccinated. The Provincetown report concludes that spread among highly-vaccinated populations does occur and suggests that authorities reimplement mask mandates and other mitigation measures, even for the vaccinated.

The Provincetown report was a major warning sign for CDC officials, who earlier this week reversed course and once again recommended that vaccinated Americans wear face coverings in some indoor settings. A leaked internal document from the CDC included preliminary data from the Provincetown outbreak in its summary of information on breakthrough cases of the delta variant.

The vaccines are working. Of the 900 cases related to the Provincetown cluster, there have been no deaths, 7 hospitalizations, and the symptoms are largely mild. Our positivity peaked at 15% on 7/15 and was only 4.8% yesterday. The outbreak is contained and Provincetown is safe.

— Alex Morse (@AlexBMorse) July 30, 2021

CDC Director Rochelle Walensky said the finding in Massachusetts was “concerning and was a pivotal discovery leading to CDC’s updated mask recommendation” in a statement released Friday. “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.”

Despite the apparently substantial spread among vaccinated individuals that occurred in Provincetown, the data clearly indicates that the vaccines work at preventing serious illness and death in nearly all cases. Of the more than 900 cases tied to the cluster, only seven required hospitalization, and there have been zero reported deaths nearly a month after the spawn of the outbreak. (RELATED: Previously Redacted Email Between Peter Daszak, Anthony Fauci Suddenly Sees Light Of Day)

But the high titers is what’s causing concern that even vaccinated can transmit delta, even if they don’t become noticeably ill because the vaccine is protecting them. Also need to factor in that vaccinated adults may be less cautious, so more likely to come in contact with virus

— Scott Gottlieb, MD (@ScottGottliebMD) July 30, 2021

The report suggests that the viral load carried by vaccinated individuals may be similar to that of unvaccinated individuals, but that does not necessarily mean they are just as contagious or will suffer the same severity of symptoms, according to former Food and Drug Administration Commissioner Scott Gottlieb.

Officials examined data from Provincetown between July 3 and July 17. Despite issuing the new mask guidance on Tuesday the agency did not release the Provincetown report until Friday. The public had no access to key data driving the masking decision until an internal CDC presentation was leaked to The Washington Post late on the night of July 29.

“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,” Walensky added Friday.