Measles Climbing Area

Measles was declared eliminated from the United States in 2000, by the World Health Organization (WHO) due to the success of vaccination efforts. However, it continues to be reintroduced—especially when unvaccinated people travel out of the United States to places where measles is present. Anti-vaccination sentiment continues to exacerbate the reemergence of measles outbreaks.

Measles is one of the most contagious infectious diseases. If not immunized, a person exposed to someone with measles has a 95% chance of becoming infected. During the early stage of an outbreak in an unvaccinated population, each infected person spreads the disease to an average of 12 to 18 other people. The recommended measles vaccination protocol is to receive two doses, at least one month apart. One dose of the vaccination is 93 percent effective at preventing measles, while two doses is 97 percent effective.

The United States has experienced variable trends in measles outbreaks in the 21st century, with major outbreaks occurring in 2014–2015, 2019, and in 2025–2026. In 2025, the CDC reported 2,288 confirmed cases across 45 jurisdictions and 49 separate outbreaks. Experts say cases likely are significantly undercounted as many go unreported.

Since 2000, the largest single outbreak in the United States has been the 2025–2026 South Carolina measles outbreak with 997 total cases as of April 2026.

The vast majority of people infected in the 21st century were not vaccinated and lived in close-knit communities where the immunization rate is lower than average. There is concern that the WHO may rescind the US's measles elimination status.