On October 4, 2019, this report was posted online as an MMWR Early Release.
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.
TABLE
Number and vaccination status of measles cases, by age group — United States, January
1– October 1, 2019
Age group
Measles cases
no. (%)
Vaccination status
no. (%)*
Unvaccinated
Vaccinated
Unknown
0–5 mos
43 (3)
43 (100)
0 (0)
0 (0)
6–11 mos
116 (9)
110 (95)
5 (4)
1 (1)
12–15 mos
118 (9)
106 (90)
12 (10)
0 (0)
16 mos–4 yrs
274 (22)
238 (87)
33 (12)
3 (1)
5–17 yrs
339 (27)
295 (87)
26 (8)
18 (5)
18–29 yrs
144 (12)
49 (34)
41 (28)
54 (38)
30–49 yrs
160 (13)
25 (16)
22 (14)
113 (71)
≥50 yrs
55 (4)
6 (11)
3 (5)
46 (84)
Overall
1,249
872 (70)
142 (11)
235 (19)
* Received ≥1 dose of measles, mumps, and rubella vaccine.
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.
FIGURE
Number of reported measles cases (N = 1,487), by week of rash onset — United States,
September 30, 2018–October 1, 2019
Abbreviations: NYC = New York City; NYS = New York State.
The figure is a histogram, an epidemiologic curve showing the number of reported measles
cases (N = 1,487), by week of rash onset in the United States, during September 30,
2018–October 1, 2019.
Discussion
A total of 1,249 measles cases have been reported in the United States in 2019, with
most cases associated with large and closely related outbreaks in New York City and
the rest of New York State. Consistent with previous outbreaks that have occurred
since measles was declared eliminated in the United States in 2000, most of the other
U.S. outbreaks reported in 2019 were of limited size and duration because of high
population immunity and rapid implementation of outbreak control measures by local
and state public health authorities. In contrast, the two sustained outbreaks in NYC
and NYS were larger and lasted longer because of a combination of three important
risk factors for measles transmission: 1) pockets of low vaccination coverage and
variable vaccine acceptance; 2) relatively high population density and closed social
nature of the affected community; and 3) repeated importations of measles cases among
unvaccinated persons traveling internationally and returning to or visiting the affected
communities. These two almost year-long outbreaks placed the United States at risk
for losing measles elimination status. Robust responses in NYC and NYS with multiple
partners involved vaccination efforts, including administration of approximately 60,000
MMR vaccine doses in the affected communities; tailored communication campaigns; partnerships
with religious leaders, local physicians, health centers, and advocacy groups; and
use of local public health statutory authorities. These efforts ended transmission
before the 12-month elimination deadline, with the most recent cases reported with
rash onset on July 15, 2019, in NYC and August 19, 2019, in the rest of NYS. Both
jurisdictions have since passed two incubation periods for measles with no additional
reported cases associated with these outbreaks as of October 1, 2019; however, continued
vigilance is important to ensure that elimination is sustained.
Increased global measles activity and existence of undervaccinated communities place
the United States at continual risk for measles cases and outbreaks (
6
). Control measures for measles outbreaks have been in place for decades in the United
States to limit transmission and prevent reestablishment of endemic transmission (
7
,
8
). Core elements include a highly sensitive surveillance system with multiple feedback
loops between providers, laboratories, local and state public health authorities,
and CDC. These measures are coupled with rapid activation of local and state public
health departments in response to every measles case to determine the source of infection,
identify susceptible contacts, and implement control measures, including postexposure
prophylaxis, exclusion and quarantine, and community-wide vaccination. High national
MMR vaccination coverage remains the foundation for preventing more widespread measles
transmission (
9
). The limited size and duration of 24 of the 26 outbreaks reported during September
2018–September 2019 indicate that high baseline vaccination coverage and standard
measles control measures effectively controlled most outbreaks in the United States.
Measles outbreaks in undervaccinated, close-knit communities pose challenges that
require considerations beyond standard control measures. To identify and protect communities,
routine assessments, including school audits and use of electronic immunization information
systems to ascertain local vaccination coverage and vaccine access, could help identify
critical gaps and resource needs. Because health-seeking behaviors in members of close-knit
communities are routinely informed by discussions with like-minded community members,
establishing strong community partnerships before outbreaks occur can foster overarching
goals to protect the community against public health threats. Public health authorities
might also benefit from identifying trusted community liaisons who can assist with
case and contact investigations so that standard control measures can be rapidly implemented.
Undervaccinated, close-knit communities are not unique to the United States and exist
around the world. These communities are at high risk for outbreaks of vaccine-preventable
diseases, which threaten the health and safety of vulnerable persons within, as well
as outside of, these communities. Therefore, public health authorities need to identify
pockets of undervaccinated persons to prevent these outbreaks, which require substantial
resources to control. A preventive strategy to build vaccine confidence is important,
especially one that uses culturally appropriate communication strategies to offset
misinformation and disseminate accurate information about the safety and importance
of vaccination in advance of outbreaks.
Summary
What is already known about this topic?
Measles was eliminated in the United States in 2000. High national coverage with measles,
mumps, and rubella vaccine and rapid implementation of measles control measures prevent
widespread measles transmission.
What is added by this report?
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.
What are the implications for public health practice?
Ensuring high rates of measles immunization in all communities is critical to sustaining
measles elimination.