"Reaching this milestone is particularly striking because we are only in July, and measles cases are still being reported throughout the country," said William Moss, epidemiology professor at Johns Hopkins Bloomberg School of Public Health.

Young people, ranging from infants to age 19, made up 69 per cent of cases tracked through 23 July, with children younger than five comprising a fifth of those infected this year, the CDC said.

A tenth of those younger than five who were infected ended up hospitalized, along with 3 per cent of those aged five to 19, the CDC added. Measles-related hospitalizations this year stand at 151 so far.