They were gold miners in French Guiana, revelers in Cape Cod, and Indian health-care workers. Even though they inhabit worlds apart, they ended up having two things in common. All were vaccinated against covid-19. And they all became part of infection clusters.
In recent weeks, cases like these are proving that covid-19 transmission chains and superspreading events can occur even in groups where nearly everyone is vaccinated, setting off alarms among health officials and torpedoing hopes of a quick return to business as usual in the US.
In May 2021, the CDC had told vaccinated Americans they could safety go unmasked, but on Tuesday the agency reversed course, saying vaccinated people should wear masks in indoor public settings.
The reason was what investigators learned from an outbreak in Provincetown, Massachusetts, a seaside town on Cape Cod, which in early July hosted a rowdy parade and crowded weeks of pool parties. Since then, Massachusetts health investigators say, there have been more than 500 cases of covid-19 linked to those events in state residents, 73% of which are in people who were vaccinated. Including people from other states, the infection cluster involves over 900 people.
The Provincetown outbreak was caused by the so-called delta variant, which now accounts for most cases in the US. In a statement released today, Rochelle Walensky, head of the CDC, said the “pivotal discovery” was that vaccinated people infected with delta in Provincetown appear to have just as much virus in their systems as those who are unvaccinated.
“High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus,” she said.
The recommendation suggests a rapid return to a layered approach of countermeasures, including masks and social distancing, which could also complicate school reopenings starting next month in the US.
Infection at a gold mine
Investigations around the world have been building evidence of outbreaks among the vaccinated for weeks. For instance, a scientific team in Paris and French Guiana recently described how covid-19 tore through a South American gold mine in May, even though nearly all the miners had received Pfizer’s vaccine.
Despite being inoculated, 60% became infected by a variant called gamma. That surprised the scientists so much that they checked to see if the vaccines had been damaged in shipping, but they weren’t.