(CNN) The number of cases of measles in the United States this year has surpassed 1,000, the US Department of Health and Human Services said Wednesday. The agency said there have been 1,001 cases so far this year.

That's 20 more cases than the US Centers for Diseases Control and Prevention reported on Monday in its weekly national update. This year, which is barely half over, has the greatest number of cases in a single year in nearly three decades.

"The Department of Health and Human Services has been deeply engaged in promoting the safety and effectiveness of vaccines, amid concerning signs that there are pockets of undervaccination around the country," said US Health and Human Services Secretary Alex Azar said in a statement

"The 1,000th case of a preventable disease like measles is a troubling reminder of how important that work is to the public health of the nation," Azar continued.

The number of cases this year is the highest since 1992 when there were 2,237 cases of the highly contagious illness reported in the United States. It's the highest number of cases in a single year since it was eliminated nationwide in 2000, meaning it was no longer continuously transmitted in the country.

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