The U.S. declared measles eliminated in 2000 "thanks to a very high percentage of people receiving the safe and effective measles, mumps, and rubella vaccine," the CDC said, but now U.S. coverage among children of kindergarten age is below the 95 percent coverage target, which creates a higher risk for outbreaks of the virus.

Texas reported 90 cases this week, while New Mexico reported nine. A majority of the cases have been in a Texas county where vaccination rates have lagged behind the rest of the state.

This is the tip of the iceberg,” said Rekha Lakshmanan, chief strategy officer for The Immunization Partnership in Houston, a nonprofit that advocates for vaccine access. “I think this is going to get a lot worse before it gets better.