In another dispiriting setback for the nation’s efforts to stamp out the Coronavirus, 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 Centres for Disease Control and Prevention to recommend that vaccinated people return to wearing masks indoors in parts of the US  where the Delta variant is fuelling infection surges. The authors said the findings suggest that the CDC’s mask guidance should be expanded to include the entire country, even outside of hot spots.