Important context from this Washington Post article (gift link):

The share of U.S. counties where 95 percent or more of kindergartners were vaccinated against measles — the number doctors say is needed to achieve overall protection for the class, known as “herd immunity” — has dropped from 50 percent before the pandemic to 28 percent, according to The Post’s examination of the public records from 44 states and the District of Columbia.

Dr. Lucky Tran:

Note that today the CDC claimed that MMR vaccination rates are still high, as data shows only a 0.1% overall decline. But this data lacks the granular detail of the Washington Post investigation. State-wide totals often mask localized pockets of low immunization, where outbreaks occur.