MEASLES AND MODELS

Although measles is a mild disease for most people, it can result in severe complications and death. The Expanded Program on^lmmunization (EPI) of the World Health Organization (WHO) estimates that each year approximately 1.5 million children die of measles around the world. Pneumonia and diarrhoea cause most of the deaths, but other important complications include bronchitis, croup laryngitis, otitis, stomatitis, proteinlosing enteropathy, deafness and blindness. In developing countries up to 10% of infected children often die acutely with the highest death-to-case ratio occurring in infants. In those who survive recovery is slow. Affected children have marked weight loss from diarrhoea and the inability to eat or drink because of severe stomatitis and general malaise. Catch-up weight gain can take longer than three months, and the ilness can precipitate marasmus or kwashiorkor in those whose nutritional status is marginal at the onset of infection. Severely malnourished children suffer more complications and are more likely to die. Even in the developed world, the health impact of measles can be substantial. Between 1970 and 1975 there was an average of 6000 hospitalizations in the United States for measles annually. In Denmark, approximately 9% of cases developed otitis, 7% respiratory complications and 0.3% encephalitis.

Because the essential epidemiological characteristics of measles are well known and clearly defined, measles has been of continuing interest to epidemic theorists. Live attenuated measles virus vaccine

was introduced in 1963 and its ability to provide protection in a high proportion of those vaccinated was rapidly demonstrated. The first successful measles elimination programme started in 1966 as part of a programme of smallpox eradication and measles control in 21 countries of West and Central Africa and provided a useful opportunity to merge theoretical and implementation considerations. From surveillance data generated in Africa the late professor George McDonald developed mathematical models simulating the transmission of measles. His models suggested two approaches that might interrupt measles transmission in West Africa: Annual mass vacci­nation reaching at least 90% of susceptible children: or, an ongoing programme in which 75% of all susceptible children were vaccinated as they became susceptible at approximately six months of age. Absence of the necessary infrastructure necessitated the choice of mass campaigns. Using this approach, measles transmission was interrupted for varying periods: in the Gambia, a zero incidence of indigenous measles was achieved and maintained for more than two years.

Mathematical models also played a role in the measles elimination programme which began in the US in 1966. No precise estimate was made of the immunization levels needed to interrupt transmission, the effort in the US focused on achieving high immunity levels through the routine immunization of infants at one year of age and of immunizing at school entry children not previously immunized. However, in 1966 funding was directed to the development of programmes to prevent .rubella and as a result the effort was incomplete.