ARTICULO MEDICO:Epidemiology and transmission of measles

Epidemiology and transmission of measles
Authors
Jorge L Barinaga, MD, MS
Paul R Skolnik, MD, FACP, FIDSA
Section Editors
Martin S Hirsch, MD
Sheldon L Kaplan, MD
Deputy Editor
Elinor L Baron, MD, DTMH
Disclosures
All topics are updated as new evidence becomes available and our peer review process is
complete.
Literature review current through: May 2013. | This topic last updated: abr 26, 2013.

INTRODUCTION — Measles is a highly contagious viral illness characterized by fever, malaise, rash, cough, coryza, and conjunctivitis [1]. There are no known measles virus reservoirs outside of humans [2]. The epidemiology and transmission of measles and its complications will be reviewed here. The clinical manifestations, diagnosis, prevention, and treatment are discussed separately. (See "Clinical presentation and diagnosis of measles" and "Prevention and treatment of measles".)

EPIDEMIOLOGY AND GLOBAL ERADICATION — Measles occurs worldwide; control efforts have substantially altered the global distribution [3]. Measles incidence has decreased substantially in regions where vaccination has been instituted; measles in the developing world has been attributed to low vaccination rates [4].

In developed countries during the prevaccine era, more than 90 percent of children acquired measles by age 15 [5-7]. 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 six months of age. This is approximately the time at which transplacentally acquired maternal antibodies are no longer present if the mother has vaccine-induced immunity [8-10]. Worldwide, measles is a significant cause of morbidity and mortality. Precise incidence estimates are difficult to obtain because of heterogeneous surveillance systems and probable under-reporting [11]. In 2000, measles was estimated to cause approximately 31 to 39.9 million illnesses worldwide with an estimated 733,000 to 777,000 deaths, making it the fifth most common cause of death in children under 5 years of age [11-13]. The World Health Assembly adopted the WHO/UNICEF Global Immunization Vision and Strategy, which included a goal of 90 percent reduction in global measles mortality between 2000 and 2010 [14,15]. The WHO identified 47 “priority countries” to focus measles mortality reduction efforts; these nations jointly account for approximately 98 percent of measles deaths. The strategy in these nations includes the following measures: (1) measles immunization with a goal of >90 percent national coverage and >80 percent per-district coverage with two doses of vaccine; (2) surveillance activities, including case investigation and laboratory testing in all suspected cases; and (3) clinical management of measles cases, including administration of vitamin A [12,16].

A model to estimate progress to reaching the above goals suggested that the estimated annual global measles incidence fell by 66 percent between 2000 and 2010, from 4.6 to 1.6 cases per 1000 population [17]. During the same period the estimated annual global measles mortality fell by 74 percent; most measles deaths occurred in Africa and India. Further information on measles control is available at the Measles Initiative website, including information for the general public, technical documents, and multimedia presentations [18].

Achievements in global measles control

The Americas — The WHO-designated Region of the Americas appears to have met its goal of eliminating endemic measles transmission; the last endemic case occurred in 2002 [13,19,20]. This regional success illustrates the feasibility of global measles eradication [20]. Imported cases continue to occur, emphasizing the importance of routine vaccination to maintain immunity. Between 1990 and 2008, measles cases fell from approximately 250,000 to 203 cases [19,21].

A large measles outbreak occurred in 2011 in Quebec, Canada; there were 21 measles importations and 725 cases [22]. 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.

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, there may have been as many as 4 million cases per year [23]. About 48,000 were hospitalized, 1000 were chronically disabled and nearly 500 died.

The Food and Drug Administration approved a measles vaccine in 1963. Since that time the number of cases has fallen by approximately 99 percent, and measles is no longer considered an endemic disease in the United States [24-26].

Measles imported by travelers to the US is well described [27-29]; of 692 measles cases reported during 2001-2010, 87 percent were import-associated [27]. Since 2008 most imported United States cases have been from the European Region [30]. (See 'European Region' below.)

During 2011, 222 cases of measles were reported in the United States, an increase compared with a median of 60 cases reported during 2001-2010 [31]. Among infected patients who were United States residents, 85 percent were not vaccinated or had unknown vaccination status, but were considered eligible for vaccination. (See "Autism and chronic disease: Little evidence for vaccines as a contributing factor", section on 'Lack of evidence for association between autism and MMR'.) European Region — The WHO-designated European Region established a goal of measles elimination by the year 2015 (revised from 2010 because of ongoing measles outbreaks) [32].

Beginning in late 2009, there has been a sharp increase in measles cases in the Region, primarily in Western Europe in 2011. Of the approximately 26,000 cases reported for 2011, an estimated 54 percent occurred in France [30]. 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 healthcare access, and anti-vaccination movements [30,32-36].

African Region — The WHO-designated African Region improved coverage rates children from 57 to 73 percent with the first dose of measles vaccine for nine-month-olds between 2001 and 2008.

Additionally, “catch-up” vaccinations targeting a wide age range were offered in 43 of the 46 African Region nations by the end of 2008. The improved vaccination rates were associated with a 93 percent reduction in measles cases, from 492,116 cases in 2001 to 32,278 cases in 2008. Measles outbreaks continue to occur, and failure to vaccinate has been identified as the primary cause. However, given the overall success, the African Region recently established several goals, including the year 2012 goal of reducing measles deaths by 98 percent compared with year 2000 estimates [37], and the goal of measles eradication by 2020 [35].

Western Pacific Region — In 2005, the WHO-designated Western Pacific Region established a target year of 2012 for measles elimination. For some nations in the Region, measles elimination is hampered by inadequate surveillance and inadequate public health services. 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. China and Japan are thought to have had the majority of measles cases in recent years [38]. Japan had a measles epidemic from 2007–2008, which led to secondary imported cases in Canada and the United States [39]. The Region is estimated to have had 147,986 measles cases in 2008 [21]. 

Eastern Mediterranean Region — In 1997, the WHO-designated Eastern Mediterranean Region set a goal of measles elimination by 2010. Implementing measles control has been a challenge in some areas, in part due to civil unrest, natural disasters, and inadequate public health systems [20,40]. Overall, however, measles cases have declined; in the early 1980s there were approximately 200,000 cases identified annually [40], but in 2008 12,120 cases were identified [21].

Measles-related deaths are estimated to have fallen by 75 percent from 2000 to 2007 [40]. Southeast Asia Region — The WHO-designated Southeast Asia Region improved measles vaccine coverage between 2000 to 2008, which was associated with a 46 percent reduction in measles deaths [20]. Nonetheless this Region had approximately 126,000 measles deaths in 2008, representing the majority of all measles deaths in the world for that year (approximately 77 percent).

This Region includes India, identified as the only WHO-designated “priority country” that has not implemented the WHO measles-control strategies [12,20]. There have been calls for India to improve measles vaccination [12,20,41]. (See 'Epidemiology and global eradication' above.)

TRANSMISSION — Measles is highly contagious; the attack rate in a susceptible individual exposed to measles is 75 percent [42]. The incubation period for measles is 6 to 19 days (median 13 days) [43]. Subclinical illness is unusual. The period of contagiousness is estimated to be from 5 days before the appearance of rash to 4 days afterwards. 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 [44]. (See"Clinical presentation and diagnosis of measles".)

Infectious droplets from the respiratory secretions of a patient with measles have been reported capable of remaining airborne for several hours [45]. Thus, person-to-person contact may not be necessary to transmit measles; the illness may be transmitted in hospitals and physicians' offices. Measles transmission between airplane passengers during flight has also been described [46]. Large measles outbreaks can occur because of high virus transmissibility in areas of crowding such as schools and densely populated communities.

The peak incidence of measles in temperate areas occurs in the late winter and early spring. However, cases occur throughout the year and in some regions no seasonal incidence is apparent.

EFFECT OF IMMUNITY — Natural measles infection is thought to confer life-long immunity.

Immunity due to measles vaccination is also highly protective against clinical measles infection, as illustrated by the marked increase in risk among unvaccinated individuals during measles outbreaks. During an outbreak in the Netherlands, for example, unvaccinated individuals were 224 times more likely to become infected than vaccinated individuals [33].

Groups at risk for measles include children too young to be vaccinated, individuals who were never vaccinated (including those who declined vaccination for religious or philosophical reasons), individuals who have not received a second dose of measles vaccine, and individuals for whom the vaccine failed to elicit a protective immune response. Travel in the developing world or contact with ill persons arriving from the developing world increases the risk of exposure to measles. In developing countries, a younger age of measles infection has been noted among children born to HIV seropositive mothers than children born to HIV seronegative mothers. In these settings the titer of transplacentally-acquired measles antibody may be reduced. (See "Clinical presentation and diagnosis of measles", section on 'Immunocompromised patients'.)

EPIDEMIOLOGY OF MEASLES COMPLICATIONS — Complication rates associated with measles infection are variable. In the 1990 United States outbreak, the following complication rates were observed [47]:

· Overall complication rate — 22.7 percent

· Diarrhea — 9.4 percent

· Otitis media — 6.6 percent

· Pneumonia — 6.5 percent

· Encephalitis — 0.1 percent

· Death — 0.3 percent

Higher measles complication rates have been observed in developing countries [48-53]