Infectious Diseases

Measles is a highly contagious viral infection. It affects the respiratory system and is most often associated with the characteristic red measles rash, but it can also result in severe complications, such as pneumonia and encephalitis.

Measles is preventable through the measles, mumps, and rubella (MMR) vaccine; two doses of a measles-containing vaccine are 97% effective against infection. However, due to measles’ contagious nature, the population immunity level needs to exceed 95% to achieve herd immunity and protect communities from ongoing spread of the virus.

In 2000, the United States was declared measles-free. Recently, however, declining MMR vaccination rates and a global increase in measles activity have led to persistent transmission of measles in the US. Measles is often called a “canary in the coal mine” disease because a rise in measles cases indicates when population vaccination levels have declined.

According to a study from the Yale School of Public Health, the estimated cost of measles in the United States in 2025 was $244.2 million. This is expected to increase with decreasing vaccination coverage.

The Yale School of Public Health also publishes weekly situation reports on the measles outbreak.

The plots below show the spread of measles in the United States, characterized by many years of sporadic, geographically-limited outbreaks, followed by consistent transmission since early 2025. Data on measles cases are from the CDC and Johns Hopkins University. MMR vaccination data are obtained from the Washington Post and HealthMap. Exemption rates are from Fattah & Stoffel et al. (JAMA 2026).