Measles' return is no mystery: At its root is the falling vaccination rate.

Around 90% of the U.S. population has received the MMR vaccine, which protects against measles, mumps and rubella, and in some regions of the country, the rate is below 60%. Since about 2019-2020, that overall number has dropped below the 95% needed for herd immunity. It is necessary to keep that rate nationally, but maintaining herd immunity at the local level is equally important in order to prevent measles from finding pockets of unvaccinated communities.

Countries that remain free of continuous transmission for 12 months are deemed to have eliminated measles — a designation the U.S. achieved in 2000. The Pan American Health Organization was scheduled to decide in April whether the U.S. should lose that designation, but the organization postponed its meeting until November.

Current trends suggest that both the U.S. and Mexico, which has also been battling the disease, may lose this status — as Canada did in November 2025. All three countries have seen their vaccination rates fall below the 95% threshold, and their outbreaks may share epidemiological links.