Towards eradication of measles virus: global progress and strategy evaluation

Despite an increase in global measles vaccine coverage from under 20% in 1980 to around 80% in 1990, measles remains a major cause of morbidity and mortality world-wide. This paper addresses a number of issues relating to efforts to control measles, namely, (i) at the global level, how might we assess the impact of measles vaccination on the incidence of infection and associated disease, and, (ii), at the strategic level, how can we utilise an understanding of the transmission dynamics of childhood viral infections to aid the design of optimal immunisation programmes? Based on WHO vaccine coverage data, and organising countries according to similarities in demographic and epidemiological parameters, an age-structured model of measles transmission is used to capture the non-linear dynamics of infection and mass vaccination and to generate projections of the impact of measles immunization world-wide. The results provide a crude indication of the percentage reduction in measles cases by year 2000 (compared with no immunization) and suggest an approximately 70% reduction in cases over all ages, and 77% reduction in cases under 5 years (where there is the greatest risk of case fatality); these suggest that WHO targets for 1995 are unlikely to be achieved. In the second part of the paper, examples are given to illustrate the usefulness of a modelling approach in evaluating measles immunization policy. The introduction of MMR vaccine in the UK in 1988 has resulted in measles incidence falling to an all time low and attention has turned to the requirements of elimination. A realistic age structured model, validated using extensive serological data, is used to compare the merits of single or two dose strategies. Based upon recent estimates of vaccine efficacy (90%) and coverage (92% by end of second year of life) it is suggested that a two-dose policy with a pre-school second dose given irrespective of vaccine history is required to prevent the build up of susceptibles to epidemic proportions in the longer term. In a second example, prompted by the success of the campaign approach to polio and measles elimination in Central and South America, simple models are used to explore and quantify the process by which pulse vaccination programmes (i.e. repeated application across a wide age range) act to control transmission.