The rising number of measles cases this year is by far the most since successful vaccination led to elimination of the disease in this country almost two decades ago. Cases appear in the United States every year when travelers bring it into the country.

At least 971 cases have been reported this year across 26 states, according to the Centers for Disease Control and Prevention. The ongoing major outbreaks may soon mean measles can no longer be considered eliminated from this country, the agency said.

Declining vaccination rates have increased the risk of outbreaks when the virus arrives, and allowed it to persist in some communities.