In the first year of the pandemic, at least four people in Michigan were infected with a version of the coronavirus observed mostly in mink, the Centers for Disease Control and Prevention confirmed on Monday.
The cluster, which previously included just three cases, represents the first known instance of possible animal-to-human transmission of the virus in the United States.
Two of the infected were employees of a Michigan mink farm that experienced a coronavirus outbreak in October 2020. The other two had no known links to the farm, suggesting that the mink variant may have been circulating more widely among area residents at the time.
Samples of the virus collected from all four people contained two mutations that scientists have hypothesized may be signs of adaptation to mink, Dr. Casey Barton Behravesh, who directs the C.D.C.’s One Health Office, said in an email on Monday.