—from Epidemiological Bulletin, Vol. 21 No. 4, December 2000—

Interrupting Measles Transmission in the Americas

Despite the availability of an effective vaccine, measles continues to be one of the leading causes of childhood morbidity and mortality in many regions of the world, representing 30% of the estimated 3 million deaths due to vaccine-preventable diseases in the world every year. In the Region of the Americas, the interruption of measles virus transmission was set as a target for the year 2000 by the ministers of health at the XXIV meeting of the Pan American Sanitary Conference in 1994. While several different genotypes of wild measles virus are circulating worldwide, virologic surveillance conducted since 1997 has suggested that only the D6 genotype is circulating in the Americas, indicating continued endemic circulation. As summarized below, most countries have already achieved the goal of interrupting indigenous measles transmission, which represents great progress in the fight against the disease.

In 1990, during the last major epidemic in the Region, 250,000 cases of measles were reported. In 1996, an all-time low of 2,109 confirmed cases were reported in the Region. Nevertheless, there was a resurgence in 1997, with 52,284 confirmed cases reported from Brazil. The epidemic continued throughout 1998, with 14,332 confirmed cases reported from 17 (35%) of the 48 countries in the Region, 71% of which from Argentina. Over the past two years, there has been no reported measles case in all of Central America and Cuba, in most of the Caribbean and in the majority of South American countries. In 1999, the 3,091 confirmed cases were reported from 11 countries, a reduction of 78% compared to 1998 and 94% compared to 1997. Ongoing endemic transmission occurred only in 4 countries - Bolivia (1,441 cases), Brazil (797 cases), Argentina (313 cases) and the Dominican Republic (274 cases). Also during 1999, Canada, Chile, Costa Rica, Mexico, Peru, Uruguay and the United States reported measles importations, but the spread was limited by the high vaccination coverage.

As of 28 October 2000, only 1,148 confirmed measles cases were reported in the Region, and the disease was affecting only 30 ( <1%) of the approximately 12,000 municipalities in the Region. Countries with high measles vaccination coverage presenting imported measles cases, such as Canada (194 cases as of 28 October 2000), the United States (71), Peru (1) and Costa Rica (1), have also succeeded in keeping measles transmission at low levels. Other countries that reported cases include Argentina, Brazil, Bolivia, the Dominican Republic and Haiti (see box 1).

The case of the Dominican Republic and Haiti is of special concern, as endemic transmission continues in spite of nationwide vaccination campaigns, mainly because measles coverage in the campaigns did not reach 95%, leaving pockets of susceptible populations. Reasons for the lack of coverage included deficient supervision and monitoring of house-to-house vaccination, delayed case investigations that prevented rapid assessment of the situation in areas with poor coverage and severe logistical obstacles. These outbreaks highlight the difficulty of stopping the spread of measles in areas with low vaccination coverage. Maintaining a high level of vaccine-induced immunity is the most effective approach for the control of measles. In this regard, PAHO’s recommendations include the appropriate and timely implementation of the following strategies to achieve, maintain, and monitor the interruption of endemic measles transmission in the region:

1) Obtaining >95% routine coverage with measles-containing vaccine in all municipalities. Countries should validate coverage regularly through house-to-house monitoring and/or comparing the number of measles vaccine doses administered to the number of first doses of DTP or the number of doses of Bacille Calmette-Guerin vaccine;

2) Performing follow-up campaigns at least every 4 years and achieving >95% vaccination coverage in all municipalities. Supervisors should verify the vaccination coverage daily during the campaign through house-to-house monitoring;

3) Vaccinating and monitoring coverage among groups at high risk for acquiring or transmitting the disease (i.e., health-care workers, migrant workers, groups philosophically opposed to vaccination, military recruits, and other young adults of rural origin);

4) Conducting reliable, routine surveillance for disease and actively validating data by looking for disease during all house-to-house vaccinations, regular visits to schools and health-care centers by each district’s supervisor, including monthly visits to high-risk areas (those where coverage is low, that do not submit weekly reports, with limited access to health services, where tourism or immigration are high, or that have had cases during the preceding weeks); and

5) Investigating all outbreaks, including
a) conducting household visits within 48 hours of identifying a suspected case and investigating all contacts and settings where case-patients were during both their exposure periods (7—18 days preceding rash onset) and their infectious periods (from the first respiratory symptoms until 4 days after rash onset);
b) collecting blood and either throat or nasopharyngeal swabs or urine specimens at the first contact with the suspected case-patients, sending them to the country’s measles reference laboratory within 5 days of taking them and analyzing the serum specimen, and reporting results within 4 days after the laboratory received the specimen;
c) identifying the epidemiological links of confirmed cases and evaluating the risk factors involved in every outbreak; and
d) verifying the absence of measles exportations/importations between countries within the region, including determining the viral genotypes to identify endemic or imported viruses.

Overall, the countries of the Region of the Americas have made important progress in interrupting measles transmission, by dedicating their efforts to both vaccination and intensified disease surveillance. These efforts to significantly reduce the burden of measles were recently recognized by the Technical Advisory Group on Vaccine Preventable Diseases (TAG) of the Pan American Health Organization (PAHO) during its XIVth Meeting at Foz do Iguaçu, Paraná, Brazil, on 2-5 October 2000.

For updated information on measles surveillance in the Americas, please consult PAHO’s Measles Weekly Bulletin.

Source: PAHO. Division of Vaccines and Immunization (HVP)

To view the case definition of measles from Vol. 20, No. 3 (September 1999) of the Epidemiological Bulletin, select this link.

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Epidemiological Bulletin , Vol. 21 No. 4, December 2000