The second person in the United States to test positive for the omicron variant had recently returned from an anime convention in New York with 53,000 other attendees. He has not recently been out of the country and was fully vaccinated.
From Ars Technica:
According to the Minnesota Department of Health and the CDC, the man is a resident of Hennepin County and traveled to New York City to attend the Anime NYC 2021 convention at the Javits Center from November 19 to 21. The man had been vaccinated. He developed mild symptoms on November 22 and sought COVID-19 testing on November 24. He has since recovered from the infection.
It's particularly concerning that the resident returned from the large convention, which may have been ideal for SARS-CoV-2 spread. In a tweet on November 30, Anime NYC President Peter Tatara reported that 53,000 fans attended the event this year and that the convention struggled with "challenges with crowding."