MACKINAC ISLAND, Mich. (AP) — At least 15 people who attended a public affairs conference last week on Michigan’s Mackinac Island have tested positive for COVID-19. They include U.S. Transportation Secretary Pete Buttigieg.

The gathering is put on each year by a business group, the Detroit Regional Chamber, and attracts more than 1,000 public officials, journalists and others. Participants were required to show proof of a COVID-19 vaccination or a negative test.

Buttigieg disclosed his infection on Twitter, saying he had mild symptoms.

Mackinac Island’s wastewater tested “fairly high” for evidence of COVID-19 just before Memorial Day. Gov. Gretchen Whitmer so far has tested negative.