Measles Epidemiology

Pre-Vaccination era

In the decade prior to the licensure of live measles vaccine in 1963, an average of 549,000 measles cases and 495 measles deaths were reported annually. However, almost every American was affected by measles during their lifetime; it is estimated that 3-4 million measles cases occurred each year. Following implementation of the one dose measles vaccine program, there was significant reduction in the reported incidence in the United States by 1988 resulting in decline in measles-related hospitalizations and deaths.

During 1989-1991, a resurgence of measles occurred when over 55,000 cases and 123 deaths were reported. The epidemiology during the resurgence was characterized mainly by cases in unvaccinated preschool-age children who had not been vaccinated on time with one dose of measles vaccine. In addition, outbreaks were reported among highly vaccinated school-age children who received one dose of measles-containing vaccine. In 1989, a second-dose vaccination schedule was recommended by the Advisory Committee on Immunization Practices (ACIP), the American Academy of Pediatrics (AAP) and the American Academy of Family Physicians (AAFP). In 1998, the ACIP recommended that States ensure second dose coverage of children in all grades by 2001. Following the resurgence, improved implementation of the timely administration of the first dose of MMR vaccine and increased implementation of two doses among school-age children led to a dramatic decline in measles cases.

In 2000, endemic measles was declared “eliminated¹” from the United States.

Post-Vaccination era

During 2001-2011, 911 measles cases were reported. The median number of measles cases reported per year was 62 (range: 37-220 cases/year). Measles incidence has continuously remained below one case per million since 1997. The majority of measles cases were unvaccinated (65%) or had unknown vaccination status (20%). Of the 911 reported measles cases, 372 (40%) were importations (measles exposure outside of the United Sates), on average 34 importations/year, 239 (26%) were epidemiologically linked to these importations, 190 (21%) either had virologic evidence of importation or had been linked to those cases with virologic evidence of importation, and 110 (12%) had unknown source. Unknown source cases represent cases where epidemiologic- or virologic-link to an imported case was not detected.

The highest incidence of measles cases in recent years occurred in 2008 (0.48 cases/million) and 2011 (0.72 cases/million). The epidemiology of measles in 2008 was characterized by

(1) a high proportion (95%) of cases among U.S. residents who were unvaccinated or who had unknown vaccination status, most of whom were U.S. school-age children whose parents had religious or philosophical objections to vaccination, and

(2) More spread from imported cases than other years. In 2011, 220 measles cases were reported, the highest number of reported measles cases since 1996; 80 (36%) were importations, 144 (65%) were unvaccinated, and 47 (21%) had unknown vaccination status. Most of the importations were the result of unvaccinated U.S. travelers who had traveled to measles endemic countries, mainly Western Europe and India.

Although measles elimination has been achieved in the United States, importation of measles will continue to occur as measles remains endemic in many other parts of the world. Thus, current measles epidemiology in the United States is determined by characteristics of the imported case and their susceptible contacts.

Measles outbreaks in the United States in the post-vaccination era

From 2001 through 2011, 63 outbreaks of measles were reported; they were small with a median of six cases (range: 34). The outbreaks mostly involved individuals who were exposed to imported measles cases or were infected during a resulting chain of transmission and who were either unvaccinated or had unknown vaccine status. Lack of adherence to existing recommendations for measles prevention among groups at high risk (for example, individuals who travel internationally), can spread measles to susceptible populations, including infants too young to be vaccinated and groups who routinely oppose vaccination. However, the size of the outbreaks was limited due to immediate control measures, high population immunity, and high measles vaccine effectiveness.

Maintenance of Elimination

Although endemic measles was declared eliminated in the United States in 2000, pockets of unvaccinated populations can pose a risk to maintaining elimination. To maintain elimination, rapid detection of cases is necessary so that appropriate control measures can be quickly implemented to prevent imported strains of measles virus from establishing endemic chains of transmission. The key challenges to maintaining the elimination of measles from the United States are:

·         vaccinating children at age 12-15 months with a first dose of MMR vaccine,

·         ensuring that school-age children receive a second dose of MMR vaccine,

·         vaccinating high-risk groups, such as health care personnel and international travelers including infants aged 6 to 11 months,

·         maintaining measles awareness among health care personnel and the public, and

·         working with US Government agencies and international agencies, including World Health Organization (WHO), on global measles mortality reduction and elimination goals.

Source:

https://www.cdc.gov/vaccines/pubs/surv-manual/chpt07-measles.html