The city's Health Department is now investigating what it calls a "cluster of COVID-19 cases" linked to the festival. So far, 16 people are part of the cluster.
"Anyone who attended this festival should get tested immediately, regardless of whether or not they have been vaccinated," Dr. Dave Chokshi, the health commissioner, said in a statement. "This is especially urgent if attendees are experiencing symptoms."
Health officials have identified another eight people who were probably exposed before attending the festival but were "potentially contagious and could have exposed others."
The outdoor festival was held from Friday, Sept. 3, through Sunday, Sept. 5. Organizers encouraged fans to wear masks but didn't require them. Attendees had to show either proof of vaccination or a negative COVID-19 test result within three days. Anyone testing positive right before the festival was eligible for a ticket refund.
But "vaccinated" meant that an attendee only had to show proof of getting the first dose no later than the day of entry.
"Proof of one Covid-19 vaccination dose is acceptable meaning you can get your first dose the same day you attend the festival to enter as a vaccinated guest," the festival's website states.
New York City residents can find a testing site at nyc.gov/covidtest.
"Those that attended the festival but reside outside [New York City] should visit their local health department's website to find testing options near them," the city's Health Department said.