On Nov. 8, 2020, research from the U.S. had shown that white-tailed deer were being infected with SARS-CoV-2, the virus that causes COVID-19 in humans. Antibodies were found in 40% of deer that were tested from January to March 2021 across Michigan, Pennsylvania, Illinois and New York state.

A second unpublished study had detected the virus in 80% of deer sampled in Iowa between November 2020 and January 2021. Human-to-deer and deer-to-deer transmission were believed to be driving the rapid spread of the disease within white-tailed deer populations across the U.S.