The tally of COVID-19 cases linked to a conference of disease detectives hosted by the Centers for Disease Control and Prevention in April has reached at least 181, the agency reported.

Roughly 1,800 gathered in person for this year's annual Epidemic Intelligence Service (EIS) Conference, which was held on April 24 to 27 in a hotel conference facility in Atlanta where the CDC's headquarters are located. It was the first time the 70-year-old conference had in-person attendees since 2019. The CDC agency estimates an additional 400 attended virtually this year.

By the last day of the event, a number of in-person attendees had reported testing positive for COVID-19, causing conference organizers to warn attendees and make changes to reduce the chance of further spread. That reportedly included canceling an in-person training and offering to extend the hotel stays of sick attendees who needed to isolate.

But in the days that followed, the CDC received reports of more cases, and it teamed up with the Georgia Department of Public Health to carry out a rapid assessment. As of May 2, the agency had tallied 35 cases linked to the conference.

The rapid assessment team surveyed attendees from May 5 to 12, and 1,443 conference attendees responded to the survey. Of those who responded, some attended the conference virtually, but the CDC said over 80 percent attended in person.

Overall, 181 or (13 percent of the total survey takers) reported testing positive for COVID-19, and 52 percent of the COVID-positive responders indicated it was their first known bout of COVID-19. Nearly all of the survey takers, 1,435 (99.4 percent), reported having received at least one COVID-19 vaccine. But 70 percent of the survey takers reported going unmasked during the gathering. The CDC notes that the conference occurred when transmission levels were low, during which the CDC does not recommend wearing masks.

The agency highlighted that none of the infected conference attendees were hospitalized, though 49 respondents (27 percent) reported taking antiviral medications for their infection.

"[T]he findings of this rapid assessment support previous data that demonstrate that COVID-19 vaccines, antiviral treatments, and immunity from previous infection continue to provide people with protection against serious illness," the agency wrote. "CDC continues to recommend that everyone ages six months and older stay up to date with all COVID-19 vaccines, including receiving an updated vaccine."

Still, according to an advisory seen by The Washington Post, the CDC is warning attendees of an upcoming conference the agency is holding at the same hotel venue in June about the outbreak at the event in April. The CDC is encouraging attendees of the June event to wear their "own high-quality masks and, if possible, also carry COVID-19 rapid tests with them." Spokesperson Kristen Nordlund said that the agency will also have masks on hand.