LANSING, Mich. (AP) — The number of measles cases in Michigan has reached 41, including the first reported in Washtenaw County.

The Michigan Department of Health and Human Services updated the numbers Monday with two additional cases. Thirty-nine cases have been confirmed in Oakland County.

Authorities are trying to determine possible exposure sites in Washtenaw County in southeastern Michigan. They’re also urging people again to get vaccinated if they haven’t received the vaccine in the past. Measles is highly contagious and is spread by personal contact and through the air.

The state considers the vaccine to be highly effective and safe. A single dose protects about 95% of children. After two doses, almost 100% are immune.

The number of measles cases in Michigan is the highest since 65 in 1991.