A 900-person Covid delta cluster in Massachusetts includes 74% vaccinated people

An alarming Washington Post article says the Covid-19 delta variant is "so contagious that it acts almost like a different novel virus, leaping from target to target more swiftly than Ebola or the common cold."

Ars Technica says an internal CDC report paints a grim picture:

An analysis of a COVID-19 cluster of around 900 people in Massachusetts—74 percent of whom are vaccinated—is among the alarming data that spurred the Centers for Disease Control and Prevention to reverse course on masks this week.

According to an internal CDC document first obtained by The Washington Post, data on the Provincetown, Massachusetts, cluster showed that vaccinated people carried surprisingly high levels of the delta coronavirus in their noses and throats. More importantly, vaccinated people were found to be spreading the dangerous virus variant to other vaccinated people. Nationwide, the CDC estimated that there are 35,000 symptomatic breakthrough infections per week among 162 million fully vaccinated Americans.