CLEVELAND — Researchers presented new genomic and wastewater surveillance data at the Centers for Disease Control and Prevention's Epidemic Intelligence Service conference indicating that recent measles outbreaks in the United States were substantially larger than official case counts showed. Nearly two thousand measles cases have been confirmed in the country in the first four months of 2026.

Genomic sequencing of the Arizona-Utah measles outbreak suggested it involved at least six and a half times as many infections as reported, and transmission likely circulated for weeks to months before the first case was confirmed. Wastewater surveillance in Oregon detected the measles virus 100 times across 23 counties where only six cases were officially reported.

"Not everybody has a severe set of symptoms. Sometimes the measles can look like the common cold, and we know that measles is very infectious. People who spend time with somebody who has a measles infection are very likely to contract measles themselves," said Dr. Scott Hadland, Chief of Adolescent Medicine at Mass General Brigham Children's Hospital. About nine in ten unvaccinated people exposed to measles will become infected.

For most people, measles causes fever and rash and typically lasts one to two weeks, though a minority of cases cause respiratory complications requiring intensive care. Measles infection can also cause brain infections that may manifest years or even a decade after infection, and some of those brain infections are untreatable. The virus can additionally cause immune amnesia, in which it erases immune memory against previously encountered infections, an effect that can last up to two to three years.

People who receive two doses of the measles vaccine have approximately 97 percent protection against measles infection, Hadland said, and the MMR vaccine prevents both measles infection and the immune amnesia effect. The two-dose MMR vaccine is considered safe, he said, and most American families choose to vaccinate against measles. He said a rising percentage of people declining the vaccine can erode community-wide protection that keeps those too young or too ill safe.

"Measles vaccination rates need to be about 95 percent in a community to protect that community against transmission," Hadland said. That threshold is known as herd immunity.

South Carolina recently declared its measles outbreak over. The United States achieved measles elimination status in 2000, and the Pan American Health Organization will determine in the fall whether the country loses that status.