Organizers said Mr. McGinn might have contracted Omicron elsewhere. “He was just one person who was at our event,” said Kelly Comboni, president of Left Field Media, which organized Anime NYC. “There have been no other mass cases reported from our event.”

Returning to Minnesota a few days before Thanksgiving, Mr. McGinn felt unusually tired. His slight cough was probably his asthma, he figured. After a long sleep — some 14 hours — he felt fine.

Then Mr. McGinn heard from a friend from the convention who lived in North Carolina and had tested positive for the virus.

On Nov. 23, he took an at-home Covid test, which came up positive. He also went to a large testing site for a P.C.R. test.

Other friends from the convention, all vaccinated, reported that they too had been infected.

“One guy had a bad day, but for the most part, mild symptoms for everyone,” Mr. McGinn said. “It was a stay home, get a blanket and watch a movie kind of thing.”

Mr. McGinn said he had no idea who infected whom, or where.

By the evening of Dec. 1, Minnesota health officials were convinced that a batch of samples they had recently analyzed for mutations included their first case of Omicron. A case investigator, Kathy Como-Sabetti, called Mr. McGinn to learn whom he might have exposed to the new variant. Mr. McGinn told her about the anime convention, with its crowds. “I kind of went, ‘Wow, well, this changes our story,’ ” she said.

Minnesota officials immediately called the New York City Health Department with the bad news. “They took it very much in stride,” Ms. Como-Sabetti said.