On October 4, 2019, this report was posted online as an MMWR Early Release.

During January–September 2019, 1,249 U.S. measles cases were reported, the highest annual number since 1992. Eighty-nine percent of measles patients were unvaccinated or had an unknown vaccination status, and 10% were hospitalized. Eighty-six percent of cases were associated with outbreaks in underimmunized, close-knit communities, including two outbreaks in New York Orthodox Jewish communities that threatened measles elimination status in the United States.

During January 1–October 1, 2019, a total of 1,249 measles cases and 22 measles outbreaks were reported in the United States. This represents the most U.S. cases reported in a single year since 1992 (1), and the second highest number of reported outbreaks annually since measles was declared eliminated* in the United States in 2000 (2). Measles is an acute febrile rash illness with an attack rate of approximately 90% in susceptible household contacts (3). Domestic outbreaks can occur when travelers contract measles outside the United States and subsequently transmit infection to unvaccinated persons they expose in the United States. Among the 1,249 measles cases reported in 2019, 1,163 (93%) were associated with the 22 outbreaks, 1,107 (89%) were in patients who were unvaccinated or had an unknown vaccination status, and 119 (10%) measles patients were hospitalized. Closely related outbreaks in New York City (NYC) and New York State (NYS; excluding NYC), with ongoing transmission for nearly 1 year in large and close-knit Orthodox Jewish communities, accounted for 934 (75%) cases during 2019 and threatened the elimination status of measles in the United States. Robust responses in NYC and NYS were effective in controlling transmission before the 1-year mark; however, continued vigilance for additional cases within these communities is essential to determine whether elimination has been sustained. Collaboration between public health authorities and undervaccinated communities is important for preventing outbreaks and limiting transmission. The combination of maintenance of high national vaccination coverage with measles, mumps, and rubella vaccine (MMR) and rapid implementation of measles control measures remains the cornerstone for preventing widespread measles transmission (4).

Measles cases are classified according to the Council of State and Territorial Epidemiologists’ case definition for measles (5). Cases are considered internationally imported if at least part of the exposure period (7–21 days before rash onset) occurred outside the United States and rash occurred within 21 days of entry into the United States, with no known exposure to measles in the United States during the exposure period. An outbreak of measles is defined as a chain of transmission of three or more cases linked in time and place as determined by local and state health department investigations.

During January 1–October 1, 2019, a total of 1,249 measles cases were reported in 31 states and New York City,† including 1,211 (97%) among U.S. residents. Median patient age was 6 years (interquartile range [IQR] = 2–22 years); 13% were infants aged <12 months (not routinely recommended to receive MMR vaccine), 31% were children aged 1–4 years, 27% were school-aged children aged 5–17 years, and 29% were adults aged ≥18 years (Table). Among all measles patients, 1,107 (89%) were unvaccinated or vaccination status was unknown, and 142 (11%) had received ≥1 MMR vaccination. Most cases (1,054, 84%) were laboratory-confirmed; among 714 (57%) cases for which specimens were available for molecular sequencing, genotypes B3 (49, 7%) and D8 (665, 93%) were identified. Overall, 119 (10%) patients were hospitalized (median age 6 years, IQR = 1–33 years; 20% were infants aged <12 months), 60 (5%) had pneumonia, and one (0.1%) had encephalitis; no deaths were reported to CDC. Eighty-one cases were imported from other countries§ including 52 (64%) cases in U.S. residents returning from travel abroad. Among these 81 internationally imported measles cases, 73 (90%) were in unvaccinated persons or persons for whom vaccination status was unknown.

In 2019, 22 outbreaks occurred in 17 states (seven were multistate outbreaks); outbreaks accounted for 1,163 (93%) of all reported cases. Eight outbreaks that occurred in underimmunized, close-knit communities accounted for 85% of all cases; outbreaks associated with NYS and NYC accounted for 934 (75%) of all cases. The median outbreak size and duration were six cases (range = 3–646 cases) and 27.5 days (range = 5–230 days), respectively. The median age of patients with outbreak-related cases was 6 years (IQR = 2–19 years). Most outbreak-related cases occurred in persons who were unvaccinated, or in those for whom vaccination status was unknown (1,032, 89%). Most (57, 70%) of the 81 internationally imported cases were not associated with outbreaks.

Beginning in late 2018, two closely related outbreaks within Orthodox Jewish communities were reported in NYC and NYS. The first began in NYC with an internationally imported case in a returning U.S. traveler on September 30, 2018; this outbreak lasted 9.5 months and included 702 cases. The second outbreak, which began in NYS with an internationally imported case in a foreign visitor on October 1, 2018, lasted 10.5 months and included 412 cases. The NYC outbreak included 53 cases reported by four other jurisdictions, and the NYS outbreak included four cases reported by two other jurisdictions. Among the 1,487 cases reported to CDC during September 30, 2018–October 1, 2019, 1,397 (94%) cases were associated with 26 outbreaks, and 1,114 (75%) were related to outbreaks in NYC and NYS (Figure). Compared with the NYC and NYS outbreaks, the 24 other U.S. outbreaks reported during the same period were of smaller sizes (median = six cases; range = 3–79 cases), and shorter durations (median = 27 days; range = 5–82 days). Median age was similar between the NYC (median = 4 years; IQR = 1–14 years) and NYS (median = 5 years; IQR = 2–14 years) outbreaks, but lower than that in the other U.S. outbreaks (median = 19 years; IQR = 8–25 years). The proportion of unvaccinated patients and patients with unknown vaccination status was similar in NYC (89%), NYS (91%), and other U.S. (87%) outbreaks. The NYC and NYS outbreaks were associated with multiple internationally imported cases (eight in NYC and 10 in NYS), whereas the other U.S. outbreaks were associated with a median of one internationally imported case.