Number of cases rises by 63 % in December . Last year , the United States had its most number of measles cases -- at 644 -- since 2000 , the CDC says on its website . Most are related to an outbreak at Penn Station . 644 new cases in New York City 's , but schools are making no changes for attendance .
Number of cases rises by 63 % in December . Most are related to an outbreak at Penn Station . 644 new cases in New York City 's , but schools are making no changes for attendance .