In another dispiriting setback for the United States’ 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 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 fueling 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.

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.

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