Lubbock, Texas - A measles outbreak has killed a second child in Texas, authorities said Sunday, with almost 650 people now infected as the highly contagious disease spreads.

"We are deeply saddened to report that a school-aged child who was recently diagnosed with measles has passed away," Aaron Davis, vice president of UMC Health System, told AFP.

The child had been receiving treatment for "complications of measles" in hospital, he said, adding they were "not vaccinated against measles and had no known underlying health conditions."

As the US grapples with its worst measles outbreak in years, President Donald Trump's health secretary, Robert F. Kennedy Jr., has alarmed health experts with his past rhetoric downplaying the importance of vaccines, as well as the mass firing of workers at health agencies.

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On Sunday, however, RFK Jr. conceded that "the most effective way to prevent the spread of measles is the MMR vaccine."

He added that his Health and Human Services (HHS) department and the Centers for Disease Control and Prevention (CDC) were supporting distribution of the shots in Texas.

Kennedy, who said he had traveled to Texas to comfort the child's family, also tallied "642 confirmed cases of measles across 22 states, 499 of those in Texas" as of Sunday.

Questioned by journalists aboard Air Force One, Trump appeared to downplay the outbreak as "so far a fairly small number of people relative to what we're talking about."

But he added that if it "progresses, we'll have to take action very strongly," without giving further details.

The CDC has recorded cases stretching from Alaska to Florida, as well as in New York City.

Texas had reported its first measles death, also of a child, in late February – marking the first US fatality from the disease in nearly a decade.

The death of a New Mexico adult last month was also classified by the CDC as a measles-related fatality.

The vast majority of measles cases tallied by the CDC – 97% – are patients not vaccinated against the measles, it said on April 3.

Some 196 of them were under five years old, 240 were aged 5-19, and an additional 159 were aged 20 years or older, with a few others of unknown age.

The CDC, which defines an "outbreak" as three or more related cases, has recorded six outbreaks so far in 2025. Some 93% of the confirmed cases are related to those outbreaks.

"For comparison, 16 outbreaks were reported during 2024 and 69% of cases (198 of 285) were outbreak-associated," it said on its website.

"This unfortunate event underscores the importance of vaccination," Davis, of UMC Health System in Texas, wrote in an email regarding the latest death. "We encourage all individuals to stay current with their vaccinations to protect themselves and the broader community."