Measles Outbreaks: How the Disease Returns and Spreads

Why is measles so easy to spread?

Measles is an infectious disease caused by a virus. The supplied source describes it as one of the most contagious infectious diseases. When it reaches a group where many people are not immune, a single infection can lead to many more.¹

This matters because an outbreak can affect more than the first person who becomes ill. The source says most people infected in US outbreaks in the 21st century were unvaccinated and lived in close-knit communities where vaccination rates were below average.¹

What does measles elimination mean?

The World Health Organization declared measles eliminated from the United States in 2000 after vaccination efforts reduced its ongoing spread.¹ Elimination does not mean the virus has disappeared worldwide or that it can never return. The source notes that measles can be brought into the country, including by unvaccinated travelers returning from places where the disease is present.¹

If the virus enters a community with too little protection, it may spread there. The supplied source raises the possibility that the United States could lose its elimination status, but does not establish that a change has happened.¹

How does vaccination help?

The source describes a two-dose measles vaccination protocol, with at least a month between doses. It reports that one dose is 93% effective at preventing measles and two doses are 97% effective.¹ Vaccination reduces the chance of infection and helps limit opportunities for the virus to spread.¹

These figures describe protection against measles, not a guarantee that infection is impossible. For questions about an individual vaccination record or health circumstances, readers can speak with a health professional or consult an official public-health source.

What can past US outbreaks teach us?

Measles outbreaks have occurred in the United States after elimination was declared. The source identifies major outbreaks in 2014–2015, 2019, and 2025–2026.¹ In 2015, 147 cases were linked to exposure at Disneyland in California. New York City and Rockland County also had outbreaks in 2018 and 2019; New York City reported 423 confirmed cases between October 2018 and April 2019.¹

These examples show how measles can spread in particular places when the virus encounters groups with insufficient protection. The source does not explain or verify the reported New York state disaster emergency, so this background article does not describe its cause or details. Its information about recent case counts may also change over time.