New York health officials on Thursday reported a polio case, the first in the U.S. in nearly a decade.

U.S. health officials redefined what counts as close contact with someone who has COVID-19 to include briefer but repeated encounters. The change was triggered by a study of a correctional officer who caught the coronavirus after multiple brief encounters with prisoners who later tested positive.

A study by researchers in Singapore became the latest to estimate that somewhere around 10 per centof new infections may be sparked by people who carry the virus but have not yet suffered its flu-like symptoms.