1 infected camper likely led to 115 coronavirus cases at an overnight camp where people didn't have to wear masks or social distance

• 76% of campers and staffers at a faith-based retreat for high school boys in Wisconsin came down with COVID-19 after a single camper developed symptoms.
• The camp implemented some mitigation strategies, like requiring the 152 attendees to quarantine at home for a week before travel, but didn't require mask wearing or social distancing.
• Attendees who didn't contract COVID-19 had tested positive for antibodies prior to arrival, providing evidence of their potential protective power.
• Outbreaks can, and have been, prevented in congregate settings like summer camps when a comprehensive mitigation strategy is in place.

An overnight camp in Wisconsin became a coronavirus superspreader setting this summer after a single camper, who'd previously tested negative for COVID-19, came down with symptoms the day after his arrival.

He later tested positive for the disease, and over the course of three weeks, 76% of his 152 comrades and counselors (116 people), contracted it too, the Centers for Disease Control and Prevention reported Friday. The attendees, who were there for a high-school faith-based retreat, weren't required to wear masks or practice social distancing.

The findings underscore why a comprehensive mitigation plan rather than a piecemeal one is critical in curbing the spread of the novel coronavirus, particularly in congregant settings like camps, colleges, and nursing homes.

They also provide evidence for the potential protective power of antibodies, since the 24 students who remained healthy had received positive COVID-19 antibody blood tests within the three weeks before their arrival.

The attendees were told to get tested before arrival and wear masks during travel, but it wasn't enough

Prior to their arrival, campers and staffers — a group that included people from 21 states and territories and two foreign countries — were required to show they'd either had a positive antibody test in the past three months or a negative COVID-19 test within the week.

They were also instructed to quarantine at home the week before traveling and wear masks while on the road or in the air to their destination.

But once at the retreat, attendees weren't required to wear masks or keep their distance from one another. Students took classes outside while seated less than six feet apart, and groups of four to eight slept in dorms together with bunk beds, shared bathrooms, and shared common areas.

When the boy came down with symptoms July 3, the day after arrival, he was isolated in a private room and 11 of his close contacts were quarantined together in their own dorm. By July 7, the contacts were released from quarantine, with the camp saying they'd received negative COVID-19 tests. Public health officials couldn't verify that the tests were conducted or the results were negative.

Meantime, six of those contact and 18 other students reported mild symptoms. They were given masks but leaders didn't conduct contact tracing or isolate the campers.

On July 13, one of the original close contacts tested positive for COVID-19 and two days later, the Wisconsin Department of Health Services launched an investigation. At that point, so many students felt sick the camp was unable to implement recommended mitigation measures like isolating everyone with symptoms.

After conducting rounds of testing in late July and early August, WDHS concluded 76% of attendees had confirmed (51%) or probable (25%) COVID-19, and 80% of those tested for antibodies had positive results. Fortunately, no one had a severe illness or required hospitalization.

Campers should have quarantined for 2 weeks before arrival, not 1, the CDC says

The outbreak may have been prevented, or at least more contained, had attendees been required to quarantine for 14 days, rather than seven, prior to their arrival, and quarantined with cohorts for 14 days after their arrival, the report authors write.

But the report is encouraging in that the 24 people who had antibodies against the virus before attending the camp never tested positive for COVID-19.

"These findings provide preliminary evidence that detectable antibodies might provide protection against new SARS-CoV-2 infections for an unknown duration," the authors write. "A robust COVID-19 mitigation plan developed in collaboration with public health authorities is important for preventing and containing similar outbreaks at overnight camps and residential schools."

Past reports show similar gatherings can be safe, if more mitigation measures are in place

The Wisconsin camp outbreak has similar features to an outbreak in an overnight camp in Georgia in June. In that case, at least 44% of teh nearly 600 campers and staffers got COVID-19 after a single counselor came down with the chills and left camp the next day.

The camp established most recommended protocols including requiring attendees to provide a negative COVID-19 test result before arriving, but only counselors were required to wear masks and windows and doors weren't kept open. Activities like singing, cheering, and sleeping in cabins with an average of 15 people probably didn't help either.

But not all such settings are doomed. Four overnight camps in Maine with a total of 1,022 attendees from 41 states and international locations successfully staved off outbreaks, another CDC report found, by implementing strategies like quarantining attendees in groups for 14 days after arrival and isolating the only three asymptomatic cases found.

"Although no single intervention can prevent SARS-CoV-2 transmission, a multilayered use of [non-pharmaceutical interventions] allowed camps to prevent transmission and quickly identify campers or staff members with SARS-CoV-2 infection to successfully mitigate spread," the study authors write.