This article was originally published on May 1, 2019. Please see publish date for most recent update.

Measles was officially declared eliminated in the U.S. in 2001, thanks to widespread measles vaccine uptake. Around the world, measles cases also fell 80% from 2000-2016, with deaths dropping from 550,000 to 90,000 per year. However, ongoing measles outbreaks persist globally, with 23,066 cases reported in Yemen from June 2023–Nov. 2023 alone. Furthermore, due to an increase in vaccine hesitancy and global travel, surges in cases and even outbreaks have been reported in areas of the world where measles has been eliminated. For example, the U.K. is currently experiencing a large measles outbreak, with over 200 confirmed cases, despite previous elimination of the disease. Measles cases also continue to be detected in the U.S. on an annual basis, with the most recently confirmed outbreak taking place in 2019.

Number of Measles Cases Reported by Year

Between Jan. 1-Dec. 31, 2019, 1,274 individual cases were confirmed in 31 states. This was the greatest number of measles cases in the U.S. since 1992. Since that time, the U.S. experienced relatively stable case numbers. However, cases have once again begun rebounding, driven by a combination of poverty, warfare, tight vaccine supplies, interruption in vaccine administration during the COVID-19 pandemic and, in some countries (including the U.S.), hesitation about vaccination

In Jan. 2024, a patient in Philadelphia presented with measles to Children’s Hospital of Philadelphia, following recent international travel. Other patients at the hospital were exposed, and 1 subsequently infected patient returned to daycare before the recommended quarantine period (21 days) had ended, which lead to a daycare-associated outbreak.

As of Jan. 25, the U.S. Centers for Disease Control and Prevention (CDC) had already reported 9 cases (in 4 U.S. jurisdictions: Georgia, Missouri, New Jersey, and Pennsylvania) in 2024. Since that report, there have been additional confirmed cases in Ohio and California.

On February 1, 2024, Los Angeles Department of Public Health announced the first case of measles in Los Angeles since 2020. The person had traveled through Los Angeles International Airport on an international flight and visited a fast-food chain, while contagious. Investigation is still ongoing to identify times and locations of exposure and to contact trace.

Why is it So Difficult to Stop Transmission?

Measles is one of the most contagious pathogens, with 90% of people, with no prior immunity from vaccination or previous measles infection contracting measles if exposed. The virus spreads through contact with infected throat or nasal secretions and can remain airborne for up to 2 hours (meaning anyone who is in the same location within 2 hours of an infected person can still become infected).

In addition to being highly contagious, the measles virus can also be fatal and poses the greatest risk to unvaccinated, young children, killing 1 or 2 out of every 1,000 children who contract it, according to the CDC. Pregnant people, older individuals, and those who are immunocompromised are also at higher risk for severe complications, including permanent hearing loss, pneumonia, encephalitis,  intellectual disabilities or other severe complications.

Notably, the symptoms of measles overlap with many respiratory viruses, and during respiratory season, when a high level of multiple respiratory viruses are circulating, other common viruses may be suspected first, creating a higher risk of an infected person unknowingly spreading the virus.

Common, early non-specific symptoms include:

• High fever.
• Cough.
• Runny nose.
• Conjunctivitis.

The characteristic measles rash does not appear until 3-5 days after initial symptom onset, which means that the virus can be spread from an infected person before the rash is present.

Measles Diagnostics

When measles is suspected, serum and a urine sample, nasopharyngeal swab or a throat swab should be collected. Measles can be diagnosed using serology (specifically measles IgM) or real time polymerase chain reaction (RT-PCR) methods. Genotyping may be used to help trace transmission and differentiate between measles infection and the measles vaccine strain, which is not contagious person-to-person.

Measles testing for active infection is typically performed in public health laboratories or reference laboratories. Any suspected cases must be reported to local public health authorities.

ASM Resources on the Measles Virus

• Measles and Immune Amnesia.
• Diagnosing Measles in the Post-Elimination Era.
• Promoting Lab Equity with Laboratory Developed Tests.
• ASM Commends Committee Hearing on Vaccines and Preventable Outbreaks.
• ASM Applauds Subcommittee Hearing on Measles Outbreaks in the U.S.

To speak with a measles expert, email us at communications@asmusa.org.