Covid clusters among the vaccinated are killing our back-to-normal dreams

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.

The initial studies of Pfizer's vaccine, the mostly widely used in the US, showed it was more than 90% effective in preventing symptomatic disease. But that's not what was seen in the gold miners; half ended up with symptoms like a fever. The vaccines may still have helped, though. None of the miners became seriously ill, even though most were older than 50 and some had risk factors like high blood pressure and diabetes.

More evidence comes from India, where health-care workers were eligible for the AstraZeneca vaccine starting in early 2021. But when a team from the UK and India looked at covid-19 cases in these workers, they found significant numbers of vaccine breakthrough infections" at three Delhi hospitals, including a superspreading event that infected 30 people.

The breakthrough infections were much more likely to be caused by the delta variant, they say, than any of the older strains. The older variants were never able to cause a cluster of more than two linked cases among the health-care workers. But the researchers found 10 delta outbreaks that did so.

The reason the delta variant is different is that it transmits more easily; one reason is that the strain may be evading" prior immunity, say researchers. That could help explain outbreaks among vaccinated people, and it also means that if you've already had covid-19, you could more easily get it again. The UK-India team estimated that natural protection against infection dropped by as much as half when people were exposed to delta.

Covid on Cape Cod

In the US, the Provincetown outbreak may have taken hold during the July 4 Independence Week," when the town hosts thousands of visitors. As July wore on, investigators learned of hundreds of covid-19 cases, and sequencing labs in Boston determined they were caused by delta.

The Provincetown outbreak set off alarm bells at the CDC because vaccines didn't seem to prevent the virus from spreading person to person, even though most were vaccinated, according to the Washington Post, which obtained an internal CDC presentation that described delta as being as contagious as chicken pox.

Another key clue came from PCR tests run on about 200 people in the Provincetown cluster. Researchers found that the amount of virus in someone's airway-and hence what the person might launch into the word with every cough and sneeze-was roughly the same, no matter whether people were vaccinated or not.

That doesn't prove that vaccinated people transmit just as much, says Monica Gandhi, an infectious disease researcher at the University of California, San Francisco. She says that PCR tests detect virus fragments as well as live germs, so vaccinated people might be shedding less live virus or be infectious for less time. Gandhi adds that even with variants circulating, vaccines are still effective so far at preventing most major illness.

Nevertheless, we are seeing more mild, symptomatic cases," she says, as well as transmission among the vaccinated.

For the CDC, the new information posed a difficult communication problem: how to tell everyone the vaccine party might be over. In May, it had said that fully vaccinated Americans could dispense with masks and social distancing in most circumstances.

But by July 25, local officials in Provincetown had reintroduced an indoor mask mandate for the town, covering indoor restaurants, offices, bars, and dance floors, and said they would begin testing wastewater. Two days later, the CDC followed suit, recommending that in high-transmission areas everyone wear a mask in indoor public settings.

Because of the delta variant, much of the US may soon qualify as being a high-risk area. Since a low in June, covid-19 cases have risen more than sixfold.

Correction: Seventy-three percent of Massachusetts residents linked to the Provincetown covid-19 cluster had been vaccinated. An earlier version of this article incorrectly said the figure applied to the whole cluster; health authorities did not report the vaccination status of out-of-state residents, who account for around 40% of cases.