You probably think of measles as a childhood illness from decades ago. For most of your life, in most of the US, it has been. That is no longer true in one state right now.
Pennsylvania has now recorded more than 1,000 confirmed measles cases and five deaths in 2026. Fewer than 1 percent of the people who got sick had been fully vaccinated. Every single person hospitalized or killed by the virus was unvaccinated.
It is the largest measles outbreak in a single US state in more than 30 years.
Who the outbreak is killing
The five deaths include two unvaccinated adults, ages 18 and 40, and two infants who were too young for the measles vaccine. A fifth person recently died in Lancaster County, which has been the center of the outbreak.
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About 200 people have been hospitalized across more than 40 Pennsylvania counties.
The outbreak started in April 2026 west of Philadelphia and spread from there. Pennsylvania is now reporting the most measles cases of any state in the country.
Across the US, the CDC has counted more than 4,000 measles cases in 2026. That is the highest annual total since 1991.
Vaccination has fallen below the level that stops the virus
The CDC says a community needs about 95 percent vaccination coverage to keep measles from spreading. The rate among US kindergartners has slipped from 95.2 percent before the pandemic to 92.4 percent in the 2025 to 2026 school year.
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A record 4.2 percent of US kindergartners now hold a vaccine exemption, the highest share on record.
Measles is one of the most contagious diseases known. It spreads through the air when an infected person coughs or sneezes, and the virus can hang in the air of a room for up to two hours after that person has left.
How well the MMR vaccine actually works
The measles, mumps and rubella vaccine, usually called MMR, is one of the most effective vaccines in routine use. According to the CDC, one dose is about 93 percent effective at preventing measles, and two doses push that to 97 percent.
The CDC recommends the first MMR dose for children between 12 and 15 months, and the second between ages 4 and 6. Adults who were never vaccinated, or who are not sure, can talk to their doctor about catching up. Health departments and pharmacies in every US state offer the shot.
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This article is made and published by Aksel Kryhlmand, who may have used AI in the preparation.