Infectious Diseases

Measles in the United States: Outbreaks, Vaccination, and Risk

A clear, evidence-based guide to measles in the U.S., including outbreaks, symptoms, complications, vaccination, and what to do if exposed.

4 min read

Intro

Measles is a highly contagious viral illness that was declared eliminated in the United States in 2000. In recent years, however, outbreaks have returned due to declining vaccination coverage in some communities.

This guide explains how measles spreads, why outbreaks occur, how vaccination protects individuals and communities, and what to do if exposure is suspected.

Key Points

  • Measles is one of the most contagious human viruses.
  • Two doses of the MMR vaccine provide strong, long-lasting protection.
  • Outbreaks occur when vaccination coverage drops below ~95%.
  • Complications can be severe, especially in children and immunocompromised people.
  • Early recognition and isolation are critical to stopping spread.

What Is Measles?

Measles is caused by the measles virus, which spreads through airborne respiratory droplets. The virus can remain infectious in the air for up to two hours after an infected person leaves a room.

People without immunity who are exposed have a very high likelihood of becoming infected.

Symptoms and Timeline

Typical symptoms appear 7–14 days after exposure and often follow this sequence:

  1. Fever, cough, runny nose, red eyes
  2. Koplik spots (small white spots in the mouth)
  3. Widespread red rash spreading from face downward

An infected person is contagious from four days before to four days after the rash appears.

Complications and Risks

While measles is sometimes dismissed as a childhood illness, complications are common and can be serious:

  • Pneumonia
  • Encephalitis (brain inflammation)
  • Severe dehydration
  • Long-term immune suppression
  • Death

Young children, pregnant individuals, and people with weakened immune systems face the highest risk.

Why Outbreaks Are Occurring in the U.S.

Measles outbreaks do not require nationwide vaccine collapse.

They occur when:

  • Local vaccination coverage drops
  • Unvaccinated individuals cluster geographically or socially
  • The virus is imported through travel

Because measles is so contagious, even small immunity gaps can sustain transmission.

Explainer: Why Elimination Status Matters

“Measles elimination” means there is no continuous domestic transmission — outbreaks are contained quickly and do not become self-sustaining.

Losing elimination status would indicate:

  • Persistent immunity gaps
  • Delayed outbreak detection
  • Breakdown in coordinated prevention

It is a system warning, not a technical label.

Vaccination and Prevention

The MMR (measles–mumps–rubella) vaccine is the primary protection against measles.

  • First dose: 12–15 months
  • Second dose: 4–6 years

Two doses provide ~97% protection.

High community vaccination rates protect vulnerable people who cannot be vaccinated.

Guidance is issued by organizations such as the CDC and the WHO.

What to Do After Exposure

If you believe you’ve been exposed to measles:

  • Avoid public places immediately
  • Contact a healthcare provider before visiting a clinic
  • Follow isolation guidance if advised
  • Monitor symptoms for 21 days

Early public-health response limits further spread.

FAQ

Is measles dangerous if I’m vaccinated?
Severe illness is very unlikely after two MMR doses.

Can adults get measles?
Yes — especially if unvaccinated or unsure of immunity.

Is measles still common in the U.S.?
Overall incidence remains low, but outbreaks occur regularly in under-vaccinated areas.

Further Reading

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