Pennsylvania health officials reported 890 measles cases across 39 counties this year as of Sept. 25, including 173 hospitalizations and four deaths. Fewer than 1% of cases involved vaccinated people, and 31% were among patients under 18.
Health officials say the two-dose MMR vaccine is 97% effective at preventing measles. The virus can spread through the air, and contaminated air can remain infectious for up to two hours.
The outbreak is Pennsylvania’s largest in decades. Nationwide, more than 3,600 measles cases have been reported across 47 states this year, a 35-year high.
