Academic journal article Bulletin of the World Health Organization

Can Measles Be Eradicated Globally?

Academic journal article Bulletin of the World Health Organization

Can Measles Be Eradicated Globally?

Article excerpt

Voir page 137 le resume en francais. En la pagina 137 figura un resumen en espanol.

Introduction

Measles is one of the most infectious diseases. Before measles vaccine was introduced, nearly everyone contracted the disease at some point in childhood. Humans are the only reservoir of measles, although other primates, such as monkeys, can also be infected by deliberate inoculation in the laboratory. The most infectious phase is the prodromal, before other symptoms appear, such as lever and exanthema. Communicability diminishes rapidly after exanthema appears (1).

An attenuated live measles virus vaccine was authorized for use in the USA in 1963 and was already widely disseminated in several parts of the world by the late 1970s. This vaccine gives protection for more than 20 years, but the immunity it confers is believed to last for life (2). Its effectiveness is 90-95%. Because maternal antibodies interfere with those produced by the vaccine, the effectiveness of the vaccine increases after the first six months of age, peaking at 95-98% between 12 and 15 months of age (3). By the late 1980s, most countries had incorporated measles vaccine into their routine immunization programmes, and coverage with this vaccine has increased considerably. By 1990, the reported global coverage of children by two years of age was about 70%.

Data from WHO indicate that measles causes 10% of deaths worldwide among under-5-year-olds. Globally, about 40 million cases of measles and 800 000 resulting deaths still occur every year, over half of them in Africa alone. Eradicating measles would therefore play an important role in improving children's survival.

Can measles be eradicated globally? Answering this requires reviewing the experiences with measles eradication in the WHO Region of the Americas. This article therefore briefly describes the strategies being implemented in the Americas to interrupt indigenous measles transmission and the results achieved so far.

The strategy

In 1994, on the same day that the International Commission for the Certification of Poliomyelitis Eradication declared the Region of the Americas polio free, the 24th Pan American Sanitary Conference established the goal of eradicating measles from the western hemisphere (4). The rationale for the strategy used to eradicate measles was based on the epidemiology of the disease before and after introduction of the vaccine. In the pre-vaccine era, measles epidemics occurred every few years as soon as a pool of susceptible children provided by every birth cohort was available to fuel transmission once the virus had been introduced into a given population. Introducing the vaccine and subsequently increasing vaccination coverage during the post-vaccine era elongated the periods between epidemics; in some instances this reached several years. For example, the maximum interepidemic period was 9 years in Chile and 12 years in the USA.

Furthermore, in the pre-vaccine era, some very young children contracted measles and nearly all children did so by age five years. When the vaccine was introduced and coverage increased, the age-specific incidence increased among older children, and even young adults and some older adults contracted measles (5).

Another important factor is that the vaccine effectiveness is not 100%. Many children therefore remain susceptible because they never received the vaccine, and a small proportion of those vaccinated were primary vaccination failures and also remain susceptible. The result is that susceptible children accumulate over a few years, even with a very good immunization programme. Thus, vaccine coverage is not equivalent to population immunity.

With dais background, the Pan American Health Organization (PAHO) recommended a strategy for its Member States that called for high vaccination coverage of the susceptible population at all times and effective surveillance to detect measles transmission and respond accordingly. …

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