Childhood vaccination rates against measles, mumps, and rubella have been declining in much of the U.S. since the start of the COVID-19 pandemic, a new study has found.
The study, which was published in JAMA on June 2, analyzed measles, mumps, and rubella (MMR) vaccination rates by county where data were available. Researchers at Johns Hopkins University collected county-level data on MMR vaccination rates for kindergarteners from each state’s health department website for the school years before the pandemic (2017-2018) and after (2023-2024). In states where that data were not available, researchers analyzed the most comparable data instead.
Of the 2,066 counties in 33 states that researchers analyzed, 78% of them reported a decline in vaccination rates. Researchers found that the county-level mean vaccination rate decreased from almost 94% before the pandemic to about 91% after the pandemic—falling well below what public-health experts generally consider to be the threshold for herd immunity against measles of about 95%.
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Lauren Gardner, senior author of the study and director of Johns Hopkins University’s Center for Systems Science and Engineering, says the goal of this research was to get a “more comprehensive, higher resolution understanding of the vaccination landscape” in the country.
The U.S. Centers for Disease Control and Prevention (CDC) “has national- and state-level data, but we know that vaccination patterns can vary significantly within a state and even at a higher resolution than that across communities, within even the county,” Gardner says. For places with a low vaccination rate, “if measles gets introduced into these regions, there’s more likely to be an outbreak. And that’s what we’re seeing.”
As of May 29, a total of 1,088 confirmed measles cases were reported by 33 jurisdictions in the U.S., according to the CDC. Of those cases, 96% were in people who were either unvaccinated or whose vaccination status was unknown. So far in 2025, there have been 14 reported outbreaks and three people confirmed to have died of measles-related complications, all of whom were unvaccinated. Before this year, the last confirmed measles-related death in the country occurred in 2015.
Getting vaccinated is the best way to be protected against measles. The MMR vaccine is both safe and effective, and the CDC has said that most people who get it will be protected against measles for life.
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The new study found that only four of the 33 states that researchers analyzed—California, Connecticut, Maine, and New York—reported a rise in the median county-level vaccination rates. Researchers didn’t investigate the causes behind disparate rates, and Gardner says that many factors—including the pandemic’s impact on access to health care, different populations, the quality of reported data, and vaccine hesitancy—play a role in a community’s vaccination rates. But she also notes that the four states that reported an increase in the measles vaccination rate are among the few that don’t allow non-medical exemptions for school immunization requirements, such as the MMR vaccine.
Gardner says that Hawaii stood out to her in the study because it experienced a significant drop in measles vaccine coverage; before the pandemic, its rate was around 95%, but after the pandemic, it plummeted to roughly 80%. Wisconsin had the lowest vaccination rate, on average, among the states studied; its most recent rate was in the 70s.
Gardner says that a state’s average vaccination rate might be misleading because the rate can vary greatly within communities. It would be ideal, she says, if researchers could get even more granular with the data and analyze vaccination rates school by school.
“The higher granularity you get, the more you actually see problems—and then can also respond to them and figure out where it’s actually important to go and target and try and help get vaccination rates up,” Gardner says.
