A vial of the measles, mumps, and rubella (MMR) vaccine is pictured at the International Community Health Services clinic in Seattle, Washington, U.S., March 20, 2019. Picture taken March 20, 2019. REUTERS/Lindsey Wasson

The province's chief medical officer of health is urging Ontarians to make sure they are immunized against the measles amid the most severe outbreak in the province in more than a decade.