By Eduardo Cuevas

Nearly a third of all U.S. measles cases in the past four years happened during a three-month stretch in 2024, according to a report from the Centers for Disease Control and Prevention.

The rise in measles, a highly infectious virus, is troubling, experts warned. Officials attribute it to the drop in the U.S. vaccination rate for the deadly and preventable virus amid a global surge in cases. For now, the risk of widespread transmission remains low due to existing immunity and robust public health responses to contain outbreaks, according to the report published Thursday.

The CDC documented nearly 340 measles cases since January 2020. Almost 100 of the infections happened in 2024, prior to March 28, with cases occurring in more than a dozen states. Since then, there have more than a dozen cases as of April 4 that aren’t included in the report.

“Most of the outbreaks that we’ve seen during this period have been small and short due to high population immunity and rapid response by state and local health departments to control these outbreaks,” said Adria Mathis, a study author and the CDC’s lead epidemiologist for measles surveillance.

At the start of the year, the CDC warned clinicians about signs of the virus amid the global uptick. The latest report shows measles spread mostly by unvaccinated U.S. residents who traveled abroad and then brought measles to schools or hospitals they visited.

Measles was once thought to be a disease of the past. Public health officials in 2000 declared measles eliminated in the U.S. after decades of people getting vaccinated which are highly effective at preventing the virus. While vaccines are still widely used, Mathis said, the report notes that more measures must be taken to curb the rise in cases and prevent further transmission within the U.S.

Many Americans have chosen not to vaccinate their children, which CDC officials said contributed to recent outbreaks of cases. The nationwide measles, mumps and rubella (MMR) vaccination rate among American kindergartners is around 93%, below the 95% goal thought to provide adequate protection against measles.

The median age of a measles patient is 3, CDC's report said. About 90% of recent cases were in patients who weren't vaccinated or those for whom vaccination status was unknown.

New measles cases tend to primarily be caused by lack of vaccinations and an uptick in travel after the pandemic, experts said.

“This is akin to turning back the clock to the bad old days,” said Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center.

How deadly is measles?

Before vaccines, Schaffner said between 400 and 500 Americans died annually of the disease. Measles leaves about one-fifth of children hospitalized and about 1 in 1,000 with brain swelling. One to 3 children out of 1,000 die after contracting the disease.

Recent cases, Schaffner explained, are a result of parents delaying routine MMR vaccines, or withholding vaccines altogether from their children, which are typically required at U.S. public schools.

“The only way it can come back is if we stop vaccinating and permit the virus in other parts of the world to be imported into the United States and then spread,” he said.

The CDC report confirmed nearly all cases were introduced from abroad by people entering the U.S., primarily from other areas that the World Health Organization defines as the Eastern Mediterranean and African regions.

The nation's largest outbreak in 2024 has occurred at a Chicago migrant shelter, and city officials have identified over 60 cases so far. Local health officials have said it likely spread through local transmission. Cases haven't been linked to international travel, including among people who recently crossed the southern U.S. border, the city's top health official said in March.

U.S. residents traveling abroad accounted for two-thirds of imported cases. The report said imported cases were likely underreported.

“It doesn't bode well for what the future might bring,” said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy. “Because we're just going to continue to see more and more international travel. Meanwhile, we're continuing to see even more widespread measles transmission around the world. And that combination is a very, unfortunately, deadly combination.”

The CDC report noted the U.S. needs to increase vaccination coverage, including for young children before international travel and among at-risk communities with low uptake.