In 2000, measles was declared eliminated from the United States,” according to the Centers for Disease Control and Prevention (CDC). That statement appeared in the same document reporting a large multistate outbreak of measles linked to an amusement park in California and that “has spread to more than a dozen other states.”

The CDC explains:

Elimination is defined as the absence of endemic measles virus transmission in a defined geographic area, such as a region or country, for 12 months or longer in the presence of a well-performing surveillance system. However measles cases and outbreaks still occur every year in the United States because measles is still commonly transmitted in many parts of the world, including countries in Europe, Asia, the Pacific, and Africa. An estimated 20 million people become infected with measles worldwide each year, of whom 146,000 die.

Since 2000, when measles was declared eliminated from the U.S., the annual number of cases has ranged from a low of 37 in 2004 to a high of 644 in 2014. The majority of cases have been among people who are not vaccinated against measles. Measles cases in the United States occur as a result of importations by people who were infected while in other countries and from transmission that may occur from those importations. Measles is more likely to spread and cause outbreaks in U.S. communities where groups of people are unvaccinated. ■