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They were gold miners in French Guiana, revelers in Cape Cod, and Indian health workers. Although they inhabit different worlds, they ultimately have two things in common. All were vaccinated against covid-19. And they all became part of infectious clusters.

In recent weeks, cases like this have proven that covid-19 transmission chains and proliferation events can occur even in groups where almost everyone is vaccinated, raising alarm among health officials and undermining hopes for a quick return to business as usual in the United States.

In May 2021, the CDC told vaccinated Americans that they could be exposed, but the agency changed course on Tuesday, saying vaccinated people should wear masks in public areas.

The reason was that investigators learned from the outbreak in Provincetown, Massachusetts, a seaside town on Cape Cod that hosted a rowdy parade and crowded weeks of pool parties in early July. Since then, more than 800 COVID-19 cases have been reported associated with these events, according to medical researchers, 74% of which are in people who have been vaccinated.

The Provincetown outbreak was caused by the so-called delta variant, which now accounts for the majority of cases in the United States. In a statement released today, Rochelle Walenski, head of the CDC, said the “key finding” was that vaccinated people infected with delta in Provincetown appear to have as much virus in their bodies as unvaccinated people.

“A high viral load suggests an increased risk of transmission and raises concerns that, unlike other options, vaccinated people infected with delta can transmit the virus,” she said.

The recommendation suggests a quick return to a tiered approach to countermeasures, including masks and social distancing, which could also make it difficult to open schools in the United States from next month.

Goldmine contamination

For several weeks, investigations around the world gathered evidence of outbreaks among vaccinated people. For example, a research team from Paris and French Guiana recently described how covid-19 broke through a South American gold mine in May, even though nearly all miners received the Pfizer vaccine.

Despite being vaccinated, 60% contracted a species called gamma. This surprised the scientists so much that they checked to see if the vaccines were damaged in transit, but they are not.