ArticlesHas the 2005 measles mortality reduction goal been achieved? A natural history modelling study
Introduction
Measles was the single most lethal infectious agent before the licensure in 1963, and subsequent widespread use, of live attenuated measles vaccine. In the early 1960s, as many as 135 million cases of measles and over 6 million measles-related deaths are estimated to have occurred yearly.1 The immunosuppressive nature of measles reduces patients' defences against complications such as pneumonia, diarrhoea, and acute encephalitis. Pneumonia, either a primary viral pneumonia or a bacterial superinfection, is a contributing factor in about 60% of measles-related deaths.2, 3 The introduction of routine measles vaccination in most developing countries during the 1980s as part of the Expanded Programme on Immunization had a major effect on global measles mortality. By 1987, WHO estimated that the number of deaths from measles worldwide had been reduced to 1·9 million.4
Global measles vaccination activities can be characterised into three broad phases. The first phase involved the introduction of routine vaccination against measles in almost every country in the world through the Expanded Programme on Immunization, beginning in 1974,5 and the UNICEF-led initiative for Universal Childhood Immunization by 1990.6 In this phase the recommendation was for one dose of measles vaccine to be administered at or shortly after 9 months of age to at least 80% of children in every country. During the second phase from 1990 to 1999, routine measles vaccination levelled off in the 70–80% coverage range7 and many industrialised countries introduced a second routine dose, usually at or around the time of school entry, to protect children who did not respond to the first dose.8 Also during this period, the Pan American Health Organization (PAHO) implemented a strategy that included a second opportunity for measles immunisation for all children to stop endemic measles transmission in the Americas.9
The third phase began around 2000 with the realisation that despite the availability of a safe, effective, and relatively inexpensive measles vaccine for over 40 years, measles remained a leading cause of childhood mortality, especially for children living in developing countries.10 To address this problem, WHO and UNICEF began to target 45 priority countries (panel), together accounting for more than 90% of estimated global measles deaths, to implement a comprehensive strategy for accelerated and sustained reduction in mortality due to measles. The strategy emphasised the PAHO approach to provide all children with a second opportunity for measles immunisation.11 At present 47 countries are targeted for measles mortality reduction, because Yemen and Timor Leste have been added to the list of priority countries.
The WHO/UNICEF comprehensive strategy for measles mortality reduction has four components: achieving and maintaining high coverage (>90%) for routine measles immunisation in every district; ensuring that all children receive a second opportunity for measles immunisation; effective surveillance for cases of measles, including monitoring of immunization coverage; and assuring appropriate clinical management of patients with measles, particularly the provision of vitamin A.10, 11
Achieving high immunisation coverage for all birth cohorts is the foundation of the strategy for accelerated and sustained measles mortality reduction. Because about 15% of infants who receive measles vaccine at 9 months of age do not develop lasting immunity, even high coverage with a single-dose vaccination policy will result in a substantial proportion of children who remain susceptible to the disease.9 Since measles is highly infectious, the risk of an outbreak increases over time through an accumulation of susceptible children in the population. The ongoing strengthening of routine immunisation services at the district level alone will not result in a rapid reduction in deaths from measles. To obtain a timely reduction of measles deaths, a critical component of the strategy is to provide all children with a second opportunity for measles immunisation. This approach aims to protect children who did not previously receive measles vaccine, as well as those who were vaccinated but failed to develop an immune response.
The second opportunity for measles immunisation can be delivered either through a routine two-dose schedule (in which immunisation services achieve and sustain high coverage), or through periodic supplementary immunisation activities where routine coverage is low to moderate. Supplementary immunisation activities are mass vaccination campaigns that target all children in a defined age group and wide geographical area regardless of previous disease or vaccination history. They use a range of additional strategies (eg, outreach to remote areas, door-to-door canvassing, additional clinic hours, mobile vaccination teams) that reach children who do not routinely access health services and thereby achieve very high vaccination coverage. Catch-up campaigns are one-time only events generally targeting children aged 9 months to 14 years with a goal of rapidly increasing population immunity among pre-school and school-age children.12 The specific target age group depends on the age-specific susceptibility in the population.
To maintain high population immunity in pre-school-age children over time, follow-up campaigns, generally targeting all children aged 9 months to 4 years, are periodically done every 3–5 years. The interval between follow-up campaigns is a function of routine immunisation coverage (the higher the routine coverage, the longer the interval between campaigns). By contrast, in countries that have achieved and maintained high routine vaccination coverage, the second opportunity for measles immunisation can also be provided through implementation of a routine two-dose measles vaccination schedule. This appproach usually involves administration of a second dose of measles vaccine at age 12–18 months of age or at school entry.13
In May, 2003, the World Health Assembly endorsed a resolution urging member states to achieve the goal adopted by the UN General Assembly Special Session on Children (2002) to halve the number of deaths due to measles by the end of 2005, compared with 1999 estimates.14, 15 We report the achievement of this goal, and outline remaining challenges to reduce mortality further and prospects for the eventual global eradication of measles.
Section snippets
Measuring vaccination coverage
By July of each year, all Member States of WHO and UNICEF are requested to submit information on routine measles vaccination coverage, supplementary measles immunisation activities, and reported measles cases from the previous year to WHO and UNICEF. WHO/UNICEF estimates of national routine coverage16 with one dose of measles vaccine are based on a review of coverage data from administrative records, surveys, national reports, and consultation with local and regional experts. Coverage achieved
Results
During the 1980s, worldwide coverage of routine measles vaccination increased to about 70%, and then levelled off during the 1990s. Between 1999 and 2005, coverage of routine immunisation increased from 71% to 77%, with substantial variation across geographical regions (table 1). Moreover, we noted a marked increase in the proportion of countries providing children with a second opportunity for measles immunisation either through a routine two-dose schedule or a nationwide supplementary
Discussion
Intensified large-scale vaccination efforts, particularly in priority countries with the highest burden of measles, have substantially decreased reported incidence of measles and the estimated number of deaths from measles worldwide. Although difficult to quantify, the widespread administration of vitamin A through supplementary immunisation activities against polio and measles and through routine services has also probably contributed to the reduction of measles mortality. Based on modelled
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