Version May 2024
INTRODUCTION — Measles is a highly contagious viral illness characterized by fever, malaise, rash, cough, coryza, and conjunctivitis [1]. Measles has been targeted for eradication given the favorable biologic characteristic that humans are the only reservoir [2]; however, due to social and political factors and high transmissibility, elimination has been achieved in very few areas of the world [3,4].
The epidemiology and transmission of measles will be reviewed here. Issues related to clinical manifestations, diagnosis, treatment, and prevention of measles are discussed separately. (See "Measles: Clinical manifestations, diagnosis, treatment, and prevention" and "Measles, mumps, and rubella immunization in adults" and "Measles, mumps, and rubella immunization in infants, children, and adolescents".)
EPIDEMIOLOGY
Incidence — Measles occurs worldwide and remains a leading cause of mortality especially among children ≤5 years of age [5-8]. Precise worldwide incidence estimates are difficult to obtain because of heterogeneous surveillance systems and probable under-reporting [5]. Before the introduction of the measles vaccine, over two million deaths occurred annually. Availability of measles vaccination beginning in the 1960s immediately impacted disease incidence, reduced associated mortality rates, and altered the global distribution (figure 1). Measles occurs predominantly in areas with low vaccination rates, particularly resource-limited settings [9]. However, even in resource-rich settings, outbreaks of measles have occurred in settings where vaccination uptake has declined, allowing for transmission of imported measles virus from unvaccinated and infected travelers [10,11]. The burden of measles globally and the impact of vaccination are discussed in detail elsewhere. (See 'Measles control' below.)
Individuals at risk — Individuals at risk for measles include children too young to be vaccinated, those who have not been vaccinated for medical or other reasons, those who have not received a second dose of measles vaccine, and those for whom the vaccine failed to elicit a protective immune response (a very small fraction of those immunized with two doses of vaccine).
Travel to areas where measles is endemic or contact with ill persons arriving from these countries increases the risk of exposure to measles [12].
TRANSMISSION — Measles is highly contagious; the attack rate in a susceptible individual exposed to measles is 90 percent [13,14]. Transmission occurs via person-to-person contact as well as airborne spread. Infectious droplets from the respiratory secretions of a patient with measles can remain airborne for up to two hours [14,15]. Therefore, the illness may be transmitted in public spaces, even in the absence of person-to-person contact. Measles transmission between airplane passengers in airports and during flight has been described [16-18], and large outbreaks can occur in areas of crowding such as schools and densely populated communities.
The incubation period for measles is 6 to 21 days (median 13 days) [19]. Subclinical illness is unusual. The period of contagiousness is estimated to be from five days before the appearance of rash to four days afterward. The period of maximum contagiousness is thought to be during the late prodrome phase, when the patient is febrile and has respiratory symptoms. Patients with measles-associated subacute sclerosing panencephalitis are not contagious [20]. (See "Measles: Clinical manifestations, diagnosis, treatment, and prevention".)
In temperate areas, the peak incidence of measles occurs in the late winter and early spring. However, cases occur throughout the year and, in some regions, no seasonal incidence is apparent.
Outbreak control measures — A measles outbreak has been defined by the United States Centers for Disease Control and Prevention as "a chain of transmission with three or more confirmed cases linked in time and space" [21]. Prevention of spread in the setting of an outbreak depends upon prompt administration of vaccine to all susceptible individuals. (See "Measles, mumps, and rubella immunization in adults", section on 'Outbreak settings'.)
ROLE OF PROTECTIVE IMMUNITY — Natural measles infection is thought to confer lifelong immunity. Immunity due to measles vaccination is also highly protective against clinical infection [22]. During an outbreak in the Netherlands, for example, unvaccinated individuals were 224 times more likely to become infected compared with vaccinated individuals [23].
Children of mothers vaccinated against measles have lower concentrations of transplacentally acquired maternal antibodies to measles (and therefore lose protection afforded by maternal antibodies at an earlier age) than children born to mothers with immunity acquired from natural infection [24-27]. As a result, measles occurs more commonly among children <12 months in countries with high measles vaccine coverage [6,28].
In resource-limited settings, a younger age of measles infection has been noted among children born to HIV-infected mothers than children born to HIV-uninfected mothers. In this setting, the titer of transplacentally acquired measles antibody may be reduced. (See "Measles: Clinical manifestations, diagnosis, treatment, and prevention", section on 'Immunocompromised patients'.)
MEASLES CONTROL
Global data — Ideally, immunization efforts should focus on control, followed by outbreak control, then elimination, and finally eradication [29]. Population immunity of >95 percent is needed to stop ongoing measles transmission [30].
Tremendous progress has been made toward reducing the contribution of measles to childhood morbidity and mortality worldwide, largely through the commitment to achieve two-dose immunization strategies against measles in all regions of the world [31].
To reduce measles morbidity and mortality globally, in 2000 the World Health Assembly adopted the World Health Organization (WHO)/United Nations International Children's Emergency Fund (UNICEF) Global Immunization Vision and Strategy, which identified 47 priority countries to focus on measles mortality reduction efforts; jointly, these nations account for approximately 98 percent of measles deaths [6]. These efforts continue with renewed commitments and iterations to strategies updated in the WHO measles and rubella strategic framework: 2012-2030 [32].
Global coverage with a first dose of measles vaccine increased from 72 to 85 percent between 2000 and 2010 [31] and again in 2019 with 122 countries (63 percent of WHO member states) achieving ≥90 percent measles-containing vaccine first-dose coverage, a 42 percent increase from 86 (45 percent) countries in 2000.
It is well known that measles elimination is not achievable without two doses of measles vaccine, and much progress has occurred in this area. The estimated global coverage with measles-containing vaccine second-dose (MCV2) nearly quadrupled between 2000 and 2019, from 18 to 71 percent, largely due to an 86 percent increase in the number of countries providing MCV2 (from 95 to 177 countries between 2000 and 2019) [33].
Data from 2000 to 2016 indicated an 87 percent reduction in the annual reported measles incidence (from 145 to 19 cases per one million persons) and an 84 percent reduction in annual estimated measles deaths (from 550,000 to 89,780) [34]. During this time, measles vaccination prevented approximately 20.4 million deaths; 2016 marked the first year that global measles deaths dropped below 100,000; however, there were more than 140,000 deaths from measles in 2018 [35] and over 207,000 deaths in 2019, the highest number reported in over 20 years. Most measles deaths occurred in Africa and India [36]. The rise in mortality was associated with large outbreaks in several countries; however, all WHO regions reported an increase in cases starting in 2019 attributed to lower vaccine rates. As a result, the number of measles cases increased 556 percent between 2016 and 2019, from 132,490 to 869,770 (the most reported cases since 1996) [33,37].
Following the onset of the COVID-19 pandemic in January of 2020, a record number of children missed at least one measles vaccine dose. In 2021 alone, nearly 40 million children missed a vaccine, and an estimated 9 million cases and 128,000 deaths occurred worldwide, with 22 countries experiencing large outbreaks [38,39]. Continued efforts to reduce measles morbidity and mortality globally have been established through the Global Measles and Rubella Strategic Plan [40].
Further information on measles control is available at the Measles and Rubella Initiative website and the WHO website.
The Americas — The WHO-designated Region of the Americas in partnership with the Pan American Health Organization met its goal of eliminating endemic measles transmission in 2002 [7,41,42]. However, eradication has yet to be achieved because cases continue to occur due to measles virus importation, emphasizing the importance of maintaining routine vaccination [30]. Between 1990 and 2008, measles cases in the region fell from 250,000 to 203 cases [41]. The Region of the Americas was the first to be declared by the International Expert Committee free of rubella in 2015 and measles in 2016; continued measles cases are expected primarily due to importation. In 2018, there were 16,999 cases of confirmed measles reported from ten countries in the Region of the Americas, with the highest numbers in Brazil and Venezuela [43,44].
United States — In the decade before the measles vaccination program began, there were as many as 500,000 reported cases of measles per year in the United States; by one estimate that also accounted for unreported infections, there may have been as many as four million cases per year [45]. About 48,000 were hospitalized, 1000 were chronically disabled, and nearly 500 died.
The live attenuated measles vaccine was introduced in 1967 and, by 1985, had prevented about 52 million cases of measles, 5200 deaths, and 17,400 cases of permanent mental damage attributable to measles [45]. In 1989, the United States Public Health Service recommended a two-dose schedule to immunize the 5 to 20 percent of individuals who had not responded to the first dose of the vaccine [46]. With this immunization schedule, the number of cases fell by approximately 99 percent, and measles is no longer considered an endemic disease in the United States [47-49]. Endemic spread of measles was eliminated in 2000 [50], and by 2002, measles was declared eliminated from the Americas [51].
However, cases began to increase in 2008, and there have been several yearly outbreaks (three or more cases linked in time or place). The annual median number of measles cases and outbreaks in the United States more than doubled during 2009 to 2014 compared with 2001 to 2008 [52]. Cases continue to occur in under- or unvaccinated populations associated with exposure to imported cases (figure 2 and figure 3) [53-57]. Among infected individuals who were United States residents, 85 percent were not vaccinated or had unknown vaccination status but were considered eligible for vaccination [58].
Between January and December 2019, 1282 measles cases were confirmed in the United States [59], the largest number of cases reported in a single year since 1992 and since the year 2000 when measles was declared eliminated. Among infected individuals, 89 percent of cases occurred in unvaccinated persons, and 86 percent of all cases have been associated with outbreaks in close-knit, under-vaccinated communities including two outbreaks in New York [59]. Of the 81 cases that were imported from other countries, 64 percent were imported by traveling United States residents most of whom were not vaccinated.
There were only 13 cases of measles confirmed in 2020, likely attributable to reduced exposure in the context of the COVID-19 pandemic. However, in 2021 and 2022, the case counts increased. In 2022, there were 121 cases, and a single outbreak in Ohio accounted for at least 85 cases, including 36 hospitalizations; all cases were in children who were not fully vaccinated [60,61].
The United States Centers for Disease Control and Prevention also estimates that, due to missed vaccination visits during the COVID-19 pandemic, there are up to 1.5 million missed measles vaccinations since March 2020, further jeopardizing measles elimination in the United States [38].
Canada — Measles was eliminated in Canada in 1998, however outbreaks continue to occur, with increased cases in 2007 and 2011 due to importation [50,62]. In 2007, an outbreak including 94 measles cases occurred in Quebec, resulting in transmission within several unrelated networks of unvaccinated individuals despite an estimated overall population immunity of 95 percent [50]. Subsequently, in 2011, Quebec sustained the largest outbreak since elimination, with 21 measles importations linked to a large outbreak in France and 725 cases [62]. A super-spreading event triggered by one importation resulted in sustained transmission and 678 cases. The overall incidence was 9 per 100,000; the highest incidence occurred among adolescents (75 per 100,000), among whom 22 percent had received two vaccine doses. Two-dose recipients had a milder illness and lower risk of hospitalization than single-dose recipients or unvaccinated individuals. In 2016, 2017, 2018, and 2019, there have been 11, 45, 18, and 113 cases reported, respectively [63,64].
European Region — In 2010, all 53 countries in the WHO European Region (EUR) confirmed their commitment to eliminate measles as a priority in the European Vaccine Action Plan 2015-2020 [65]. Between 2009 and 2017 efforts to improve vaccination coverage were successful, with 90 percent of individuals in the region receiving a second vaccine dose. As a result, by the end of 2017, 70 percent of EUR countries were verified to have eliminated endemic measles. However, in 2018, there was a subsequent increase in measles (89.5 cases per million population) due to large outbreaks in several EUR countries including some countries that had achieved elimination [65].
In 2018, more than 82,000 cases were reported in the EUR. Eleven countries (Albania, France, Georgia, Greece, Israel, Italy, Kyrgyzstan, Romania, Russia, Serbia, and Ukraine) reported more than 1000 infections. The largest number occurred in Ukraine (more than 53,000 cases) [65].
The increase in cases has been primarily attributed to lack of vaccination of susceptible populations. Reasons for lack of vaccination include religious or philosophical beliefs, lack of health care access, and antivaccination movements [23,66-70].
Between June 2020 and May 2021, there have been 160 measles cases reported, equating to an incident rate of 0.17 per 1 million people [71].
African Region — The WHO-designated African Region has established several goals toward measles elimination, yet this region still accounted for 51 percent of all measles-associated deaths in 2013 [72]. The region set a goal to reduce measles deaths by 98 percent in 2012 compared with year 2000 estimates [73]; in addition, in 2011 a goal of measles eradication by 2020 was set [69]. The launch of these initiatives and previous efforts have improved measles immunization coverage rates for the first dose in children from 71 to 74 percent from 2013 to 2015 [74]. However, only 15 percent of member states had coverage ≥95 percent. Furthermore, efforts to provide a routine second dose have lagged behind, with only 42 percent of member states providing this. Nevertheless, the improved vaccination rates were associated with a 60 percent decline in measles cases (from 71,500 to 28,200 cases between 2013 and 2016) [74]. The reported cases remained below 100,000 between 2014 and 2017, but rose again to 125,426 and 618,595, in 2018 and 2019, respectively. Measles outbreaks continue to occur, and failure to vaccinate has been identified as the primary reason, with only some countries in the region reporting ranges of 26 to 99 percent of children receiving two doses of measles vaccine [75].
Western Pacific Region — The WHO-designated Western Pacific Region established a target year of 2012 for measles elimination in 2005; this goal that has not been met [31,76]. For some nations in the Region, measles elimination is hampered by inadequate surveillance and inadequate public health services. Between 2000 and 2013, there was an 82 and 88 percent decline in cases and mortality rates, respectively [72]. A surge in cases was noted in 2008, but dramatic declines were noted after this and, as of mid-2009, 24 of the 37 countries and areas that comprise the Region were thought to have eliminated or nearly eliminated measles. The Republic of Korea declared measles eliminated in 2006. The majority of measles cases occurred in China in 2008 [77].
An outbreak occurred in Japan from 2007 to 2008, which led to secondary imported cases in Canada and the United States [78]. After a record low of 5.9 cases per million population in 2012, measles incidence increased during 2013 to 2016 to a high of 68.9 cases per million population in 2014 (because of outbreaks in the Philippines and Vietnam, as well as increased incidence in China) and then declined to a historic low incidence of 5.2 per million population in 2017. During this period, countries that achieved measles elimination had a resurgence due to importations. Cases continue to occur in the region, largely due to outbreaks in the Philippines with 30,572 and 63,284 measles cases in 2018 and 2019, respectively. Despite this, the region has a two-dose measles vaccine coverage rate reaching an all-time high of 93 percent with all countries in the region except Vanuatu [79].
Eastern Mediterranean Region — The Eastern Mediterranean Region set a goal of measles elimination by 2010 in 1997 and revised this to 2015; this has not been achieved. Implementing measles control has been a challenge in some areas, in part due to civil unrest, natural disasters, and inadequate public health systems [42,80]. Overall, measles cases have declined; in the early 1980s, there were approximately 200,000 cases identified annually [80], but, in 2016, 57,800 cases were identified [72], with further continued declines with 7588 cases reported in 2020, with the majority in Pakistan and Sudan. Measles-related deaths are estimated to have fallen by 83 percent from 2000 to 2016, with resurgence in cases noted starting in 2008 mainly because of large outbreaks in several countries with conflicts and insecurity [81]. In 2016, 63 percent of the 23 member countries achieved ≥90 percent of one dose of measles vaccine and 93 percent of the member countries offered a two-dose schedule [34]. In 2018, among the 22 countries (except Somalia) that provide a routine second dose of measles-containing vaccine, coverage was more than 95 percent in 10 countries, 90 to 94 percent in 3 countries, and less than 90 percent (range 40 to 82 percent) in 8 countries.
Southeast Asia Region — In 2013, the Southeast Asia Region (SEAR) adopted the regional goal of measles elimination by 2020, which was revised with a target of 2023. The incidence has declined steadily from 15.2 to 5.4 per million population from 2017 to 2020. Four countries account for the majority of current cases with 28,322 cases reported in 2019 and 5744 cases from January through June of 2020 [82]. The SEAR improved measles vaccine coverage between 2000 to 2013 by 24 percent; 78 percent of member states offered two doses of vaccine which was associated with a 63 percent reduction in measles deaths [72]. There was a rise in measles vaccination coverage from 84 to 87 percent between 2013 and 2016, with 64 percent of countries achieving ≥90 percent coverage, and 64 percent with two-dose coverage [34]. In 2019, five countries (Bhutan, Democratic People’s Republic of Korea, Maldives, Sri Lanka, and Timor-Leste) were verified as having eliminated measles. This Region had approximately 39,000 measles deaths in 2016, representing the highest mortality rate globally (40 percent). This Region includes India, identified as the only WHO-designated "priority country" that has not implemented the WHO measles control strategies [6,42,83].
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Here are the patient education articles that are relevant to this topic. We encourage you to print or email these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
Basics topic (see "Patient education: Measles (The Basics)")
SUMMARY
●General epidemiology – Measles is a highly contagious viral illness characterized by fever, malaise, rash, cough, coryza, and conjunctivitis that occurs worldwide; it remains endemic in areas with low vaccination rates, particularly in the resource-limited settings. It has been targeted for eradication given the favorable biologic characteristic that humans are the only reservoir. However, due to social and political factors and high transmissibility, elimination has been achieved in very few areas of the world. (See 'Introduction' above.)
●Effect of vaccine on age at onset of infection – In the prevaccine era, ≥90 percent of children acquired measles by age 15. Following implementation of routine childhood vaccination at age 12 to 15 months, the age of peak measles incidence during epidemics in the United States shifted to less than 12 months. This is the time at which transplacentally acquired maternal antibodies are no longer protective in the child if the mother has vaccine-induced immunity. (See 'Epidemiology' above.)
●Transmission – Measles is highly contagious; the attack rate in a susceptible individual exposed to measles is 90 percent. The period of contagiousness is estimated to be from five days before the appearance of rash to four days afterward. Infectious droplets from the respiratory secretions of a patient with measles can remain airborne for up to two hours. Therefore, the illness may be transmitted in public spaces, even in the absence of person-to-person contact. (See 'Transmission' above.)
●Risk factors for acquisition – Individuals at risk for measles include children too young to be vaccinated, those who have not been vaccinated for medical or other reasons, those who have not received a second dose of measles vaccine, and those for whom the vaccine failed to elicit a protective immune response (a very small fraction of those immunized). Travel to areas where measles is endemic or contact with ill persons arriving from these countries increases the risk of exposure to measles. (See 'Individuals at risk' above.)
●Efficacy of vaccination – The live attenuated measles vaccine was introduced in the United States in 1967; since that time, the number of cases has fallen by approximately 99 percent. Measles is no longer considered an endemic disease in the United States, although increasing numbers of outbreaks due to measles importation and spread through communities with low vaccination rates continue to occur. The impact of the global coronavirus disease 2019 pandemic on missed measles immunizations may jeopardize measles elimination efforts in the United States and around the world. (See 'United States' above.)
ACKNOWLEDGMENTS — The UpToDate editorial staff acknowledges Jorge Barinaga, MD, MS, and Paul Skolnik, MD, FACP, FIDSA, who contributed to an earlier version of this topic review.
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