Scientists who studied a big COVID-19 outbreak in Massachusetts concluded that vaccinated people who got so-called breakthrough infections carried about the same amount of the coronavirus as those who did not get the shots. Health officials on Friday released details of that research, which was key in this week's decision by the Centers for Disease Control and Prevention to recommend that vaccinated people return to wearing masks indoors. The authors said the findings suggest the CDC's mask guidance should be expanded to include the entire country, even outside of hot spots. The findings have the potential to upend past thinking about how the disease is spread. Previously, vaccinated people who got infected were thought to have low levels of virus and to be unlikely to pass it to others. But the new data shows that is not the case with the delta variant. The outbreak in Provincetown a seaside tourist spot on Cape Cod in the county with Massachusetts' highest vaccination rate " has so far included more than 900 cases. About three-quarters of them were people who were fully vaccinated. Leaked internal documents ... suggest the CDC may be considering other changes in advice on how the nation fights the coronavirus, such as recommending masks for everyone and requiring vaccines for doctors and other health workers. People with breakthrough infections make up an increasing portion of hospitalizations and in-hospital deaths among COVID-19 patients.