MEASLES IMMUNIZATION STRATEGIES FOR AN EPIDEMIOLOGICALLY HETEROGENEOUS POPULATION - THE ISRAELI CASE-STUDY

Although the vaccine against measles has been routinely applied over a quarter of a century, measles is still an active disease in Israel. T he January 1991 outbreak caused high morbidity in infant and adolescen t populations and high mortality, especially among nomad Bedouins in t he southern region of the country. The Bedouins form a small fraction of the total Israeli population (ca. 2%), but it is thought that they may experience significantly higher rates of transmission than the maj ority group. In this work we use deterministic compartmental mathemati cal models to define the optimal immunization strategy for a populatio n consisting of a majority group characterized by low transmission rat es and a minority group characterized by high transmission rates; this study allows both for transmission differences between the two groups , and for possible differences in the average cost (or difficulty) in reaching individuals for vaccination. Our analysis shows that the opti mal vaccination policy for such a population involves different strate gies for the two groups: a smaller fraction is to be vaccinated in the minority group if transmission in this group is not much larger than in the majority group, whereas, if the difference in transmission is v ery large, a higher proportion is to be vaccinated in the minority gro up. The advantage of this non-uniform vaccination policy is that it in volves vaccination of a smaller fraction of the total population (and costs less, if there are differential costs between the groups), as co mpared with the proportion vaccinated under the conventional uniform v accination policy. The implications of our results for vaccination pol icies for other minority groups, or for other infectious diseases whic h are characterized by epidemiological heterogeneity, are as vet to be examined.