About three-fourths of people infected in a Massachusetts Covid-19 outbreak were fully vaccinated against the coronavirus with four of them ending up in the hospital, according to new data published Friday by the Centers for Disease Control and Prevention. The new data, published in the U.S. agency's Morbidity and Mortality Weekly Report, also found that fully vaccinated people who get infected carry as much of the virus in their nose as unvaccinated people, and could spread it to other individuals. "This finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation," CDC Director Dr. Rochelle Walensky said. "The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones." On Tuesday, the CDC reversed course on its prior guidance and recommended fully vaccinated Americans who live in areas with high Covid infection rates resume wearing face masks indoors. The guidelines cover about two-thirds of the U.S. population. While the delta variant continues to hit unvaccinated people the hardest, some vaccinated people could be carrying higher levels of the virus than previously understood and are potentially transmitting it to others, Walensky told reporters. The data published Friday was based on 469 cases of Covid associated with multiple summer events. Approximately three-quarters, or 74%, of the cases occurred in fully vaccinated people.