Measles in the United States, 2020 to Early 2024

This report covers measles cases through March 28, 2024.

Measles is a highly infectious illness with fever and rash; more than 90% of susceptible people exposed to it catch it. High two-dose coverage with measles, mumps and rubella (MMR) vaccine allowed the United States to declare measles eliminated in 2000, meaning no endemic spread for at least 12 months with a well-performing surveillance system. Imported cases keep arriving, though, and in 2019 two long outbreaks in undervaccinated communities in New York State and New York City threatened elimination; they accounted for 29% of all U.S. cases reported from 2001 to 2019. CDC reviewed surveillance data from January 1, 2020, to March 28, 2024, to see whether elimination had held and to put a new rise in cases in context.

The cases

CDC was notified of 338 confirmed cases in 30 jurisdictions.

Confirmed measles cases in the United States by month of rash onset, January 2020–March 2024. CDC.

• Age: the median patient was 3 years old (range 0–64), and 191 (58%) were aged 16 months to 19 years.
• Vaccination: 309 (91%) were unvaccinated (68%) or of unknown status (23%); 29 (9%) had at least one MMR dose. Of the 309, 259 (84%) were eligible for vaccination, 40 were 6 to 11 months old and not yet due for routine MMR, and 10 were under 6 months.
• Hospitalization: 155 patients (46%) were hospitalized, 109 of them under age 5, and 142 of the 155 were unvaccinated or of unknown status. No deaths were reported.

Where cases came from

Of the 338 cases, 326 (96%) were linked to an importation and 12 (4%) had no known source. Of the import-associated cases, 200 (61%) were in U.S. residents who were eligible for vaccination but unvaccinated or of unknown status.

Ninety-three cases were imported directly from abroad: 34 in foreign visitors and 59 in U.S. residents, 53 of whom were eligible but unvaccinated or of unknown status. The most common sources were WHO's Eastern Mediterranean (48) and African (24) regions, which had the highest measles rates in 2021–2022. In the first quarter of 2024, six imported cases came from the European and South-East Asia regions, 50% more than the yearly average of two from each region in 2020–2023.

Is surveillance working?

A well-performing system should have at least 80% of cases import-associated, with complete information on at least eight of 10 key indicators, and laboratory-confirmed. The U.S. system met that bar:

• all but two of the 338 investigations had at least 80% of key indicators, and those two had 70%;
• 314 cases (93%) were lab-confirmed, 298 of them by rRT-PCR, and 221 of those were genotyped: 80% genotype B3 and 20% genotype D8, with 22 and 13 distinct sequences respectively; no sequence was detected for more than 15 weeks;
• where the date was known, 58% of cases were reported to a health department on or before the day the rash appeared.

The cases formed 92 transmission chains: 62 isolated cases, 10 two-case chains and 20 outbreaks of three or more (seven of them in 2024). Outbreaks had a median of six cases (range 3–86) and lasted a median of 20 days; the longest chain in 2020–2023 lasted 63 days. In no chain did more than 21 days, the maximum incubation period, pass between cases, suggesting no missed links.

Elimination held, but the risk is growing

With no sustained spread for 12 months and a well-performing surveillance system, U.S. measles elimination was maintained as of the end of 2023. Keeping it prevents the illness, deaths and costs that would come if measles took hold again. But the surge in early 2024 is a renewed threat.

Worldwide, the number of countries with large or disruptive outbreaks rose 123%, from 22 to 49, between November 2022 and October 2023, and first-dose measles vaccine coverage fell from 86% in 2019 to 83% in 2022, leaving almost 22 million infants susceptible. More global measles means more importations.

At home, the risk of widespread transmission remains low thanks to high overall immunity. But national two-dose MMR coverage among kindergartners had been below the Healthy People 2030 target of 95% for three years, leaving about 250,000 kindergartners a year susceptible. Coverage was below 90% in 12 states and the District of Columbia, and in the 2022–23 school year kindergarten exemptions topped 5% in 10 states. Clusters of unvaccinated people make large outbreaks possible, as in central Ohio in 2022, where 94% of patients were unvaccinated and 42% were hospitalized.

What is needed

• Raise routine MMR coverage, especially in close-knit and undervaccinated communities, using county- and zip code–level coverage data to target efforts.
• Get vaccinated with MMR before international travel.
• Identify communities at risk and investigate suspected cases quickly.

The findings are limited because some importations may have been missed, ruled-out measles investigations are not reported nationally, and the date of first report was missing for 35% of cases.

Sources

• Mathis AD, Raines K, Masters NB, et al. Measles — United States, January 1, 2020–March 28, 2024. MMWR Morb Mortal Wkly Rep 73(14). https://www.cdc.gov/mmwr/volumes/73/wr/mm7314a1.htm