A June outbreak at an Illinois church camp exposed more than 1,100 people to the coronavirus.
Eighty-seven campers and staffers got infected, all but eight of whom were unvaccinated. No vaccinated people were hospitalized.
A total of 180 infections were linked to the outbreak.
Nearly 300 teenagers piled into buses and vans in June to attend an overnight church camp in Illinois. Some were vaccinated, but others weren't, despite being eligible.
The camp didn't require proof of vaccination or a negative COVID-19 test, nor did it ask campers to wear masks during their five-day stay. The kids slept in large boarding facilities with around 100 people each. They dined together in the cafeteria and participated in group activities both indoors and outdoors.
Within days, a COVID-19 outbreak had ignited. Several camp staffers then attended a nearby men's conference, leading to a second outbreak there. Together, the events exposed more than 1,100 people to the coronavirus across at least four states, according to a new report from the Centers for Disease Control Prevention.
Eighty-seven campers and staffers - roughly a quarter of those in attendance - were infected. But just eight of the infected people at the camp were vaccinated, while the rest were not.
Out of 180 people whose infections were linked to either the conference or camp outbreaks, five people were hospitalized - all of them unvaccinated.
Previous CDC findings suggest that unvaccinated people are five times more likely to be infected and 29 times more likely to be hospitalized than vaccinated people.
The camp and conference were ideal breeding grounds for the Delta variant, the CDC said, since the close living quarters and lack of masks made it easy for the virus to spread. The report estimated that 87% of COVID-19 cases linked to the two events were caused by Delta.
"These findings underscore the risk for COVID-19 outbreaks at camps and large events where prevention strategies are not implemented," the CDC wrote.
Vaccinated people did pass on the virus, but it's unclear how much
The CDC report found that vaccinated people at the Illinois camp likely played some role in spreading the virus to the conference and beyond.
Scientists are still figuring out the extent to which vaccinated people can fuel transmission, though.
An August CDC report suggested that vaccinated people infected with Delta carry just as much virus as unvaccinated people, though that's not always a foolproof indicator of how contagious they are. But a study from Imperial College London, which hasn't been peer reviewed, found the opposite: On average from May to July, vaccinated people in the UK had lower viral loads than their unvaccinated counterparts.
Meanwhile, a Singapore study that's also awaiting peer review found that viral loads dropped off faster in vaccinated people than unvaccinated people - a sign that vaccinated people may be contagious for less time.
For now, the CDC still recommends masking indoors in areas with high transmission rates, which constitutes almost all of the US right now.
Vaccinated campers were largely protected from illness
Before vaccines became available, camps that didn't enforce masking and social distancing fared even worse than the one in Illinois.
In June 2020, 44% of attendees at an overnight camp in Georgia tested positive for COVID-19 - compared to 26% of attendees at the Illinois camp. The Georgia camp's cabins held large groups, and only staffers wore masks.
By contrast, the CDC identified just three COVID-19 cases across four overnight summer camps in Maine last September. Those camps enforced strict protocols, including mask mandates, testing, and quarantine policies. Ultimately, none of the infected campers passed the virus to anyone else.
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