The Centers for Disease Control and Prevention (CDC) has released “some” of the evidence behind a reversal in guidance issued Tuesday advising fully vaccinated populations to wear masks in public indoor areas in places with high and substantial viral spread.The data, released in the agency’s Morbidity and Mortality Weekly Report, stemmed from recent outbreaks in Barnstable County, Massachusetts, in which 74% of some 469 COVID-19 cases occurred in fully vaccinated individuals. Sequencing from 133 patients suggested the delta variant was behind nearly all (89%) of samples.CLICK HERE TO FIND A COVID-19 VACCINE NEAR YOUNearly 80% of the breakthrough infections resulted in symptomatic illness, with four of five hospitalizations among the fully vaccinated and no reported deaths. Two patients hospitalized had an underlying health condition. What’s more, in a statement accompanying the report, CDC Director Dr. Rochelle Walensky reported “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,” Walensky said. “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.”The report notes so-called Ct values from 127 fully vaccinated people were “similar” to 84 patients who were unvaccinated, partially vaccinated or had an unknown vaccination status with an average values of 22.77 versus 21.54, respectively. Ct refers to the number of cycles needed to amplify viral RNA to reach a detectable level. However Ct values can be impacted by other factors and might offer a “crude correlation to the amount of virus present in a sample,” the report reads.CLICK HERE FOR COMPLETE CORONAVIRUS COVERAGEOf the breakthrough cases, 46% had received Pfizer’s vaccine, 38% had Moderna’s shot and 16% had Johnson & Johnson’s one-shot vaccine.The report notes that thousands were drawn to the area between July 3-17 for multiple summer events and large public gatherings. On July 3, the state’s department of public health reported a 14-day average of zero cases per 100,000 persons per day in residents of Barnstable County. By July 17, the 14-day average incidence had increased to 177 cases per 100,000 persons. The report suggests the area was grappling with high levels of viral spread as defined by the CDC by July 17.”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,” Walensky said.Earlier this spring, the agency released data on so-called “healthcare provider cohort studies,” estimating vaccine efficacy across dozens of states.Fox News’ Alexandria Hein and Tyler Olson contributed to this report.