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A dramatic increase in measles cases reported in the first few months of 2024 threatens the elimination status of the disease in the U.S., a new CDC report found.

Of 338 measles cases reported to the CDC from Jan. 1, 2020 through March 28, 2024, a total of 97 cases (29%) occurred during the first quarter of this year, Adria Mathis, MSPH, of the CDC's National Center for Immunization and Respiratory Diseases in Atlanta, and colleagues wrote in .

In comparison, a mean of just five measles cases were reported during the first quarter of each year during 2020-2023, the authors pointed out.

Twenty measles outbreaks have occurred since 2020, and seven of those outbreaks occurred in the first quarter of 2024. For comparison, four outbreaks were reported during all of 2023.

Some experts speculate that vaccine hesitancy may be behind the recent increase in measles cases in the U.S. and data seem to confirm that hypothesis. For 3 consecutive years, the national two-dose measles/mumps/rubella (MMR) vaccine coverage has remained below the 2030 target of 95% of the population, the estimated percentage necessary to confer immunity to prevent sustained measles transmission, the study authors noted.

"Clusters of unvaccinated persons placed communities at risk for large outbreaks," wrote Mathis and colleagues, pointing to a 2022 measles outbreak in central Ohio. In that instance, 94% of the people who contracted measles were unvaccinated and 42% were hospitalized.

Most of the cases occurring in 2024 occurred in individuals less than 20 years of age who were unvaccinated or whose vaccination status was unknown. Among the 338 cases reported since 2020, 91% have occurred in people who were unvaccinated or whose vaccination status was unknown.

The vast majority of measles cases (96%) reported since 2020 were brought into the U.S. by travelers or people from other countries. Of these, 61% occurred among U.S. residents who were eligible for vaccination but hadn't received the vaccine or whose vaccination status was unknown. Among the 28% of cases that were directly imported from other countries, 37% occurred in foreign visitors and 63% occurred in U.S. residents, the majority of whom were unvaccinated or whose vaccination status wasn't known.

Measles has been considered eliminated in the U.S. since 2000, but prolonged outbreaks in 2019 threatened that status. The report emphasized that the risk for widespread measles transmission in the U.S. remains low because of high population immunity.

However, "enhanced efforts are needed to increase routine U.S. vaccination coverage," the authors wrote. Given that a majority of cases were imported, they recommended vaccination before international travel, in addition to identifying "close-knit and undervaccinated" communities and rapidly investigating suspected measles cases.

Since March 28, even have been reported. According to the most recent CDC data, as of April 4, 2024, there has now been a total of 113 measles cases in 18 jurisdictions across the country this year.

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