A COVID-19 outbreak struck a recent CDC epidemiology conference, infecting at least 181 attendees, officials said.

The U.S. Centers for Disease Control and Prevention’s four-day Epidemic Intelligence Service Conference was held at an Atlanta hotel in late April, hosting 1,800 people in person. Of 1,443 attendees who responded to a post-event survey, 13% said they later tested positive for the coronavirus, according to findings released last week.

Among those who were infected, slightly more than half said they were unaware of any prior bouts with the virus.

CDC guidance in effect during the conference, based on local COVID-19 conditions in the Atlanta area, said people could choose to wear masks if they wished, but stopped short of recommending the practice for everyone. Nearly three-quarters of survey respondents said they did not wear a face covering during the conference.

Health officials have routinely said mask use reduces the risk of infection, particularly in crowded indoor settings where air circulation might be poor.

The survey data showed a greater chance of infection the longer a person attended the conference, which was held April 24-27.

“Specifically, respondents who tested positive reported attending the conference on average for all four days, and the risk of infection was 70% greater among those who attended for three or more days versus those who attended for two or fewer days,” the CDC said in a statement.

No one was hospitalized as a result of the infections, according to survey data. Virtually all survey respondents said they had received at least one dose of COVID-19 vaccine, and about 1 in 4 said they received antiviral drugs to help treat the infection.

“Again, the 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 CDC statement continued.