Measles active and passive immunity in a worldwide perspective.

'The simplest of all virus disease is measles' said Kenneth Maxy 40 years ago in a chapter on epidemiology. I hope that the data set out here provide the reader with a sufficiently complete and clear picture of the factors that determine measles epidemiology, that he or she will agree with Maxy's prescient words. Measles is an antigenically complex virus, but few components of the immune response to this virus are epidemiologically relevant. The relevant components are durable for a lifetime. They can be conveniently measured by serological tests, and the results of these tests correlate well with measles immunity. The tests show that measles is an extremely infectious disease, and that very high antibody prevalence rates are needed for herd protection. The currently available measles vaccine is capable of yielding adequate antibody prevalence rates for herd immunity, but to achieve this, immunization procedural flaws and faulty records must be kept to very low levels. The greatest obstacle to worldwide control of measles is a failure of vaccination programs to produce adequate herd immunity levels in less-developed countries. There, vaccine must be given promptly after passive immunity wanes, because the level of endemicity is so high. It is difficult to determine just what age is optimal, because it varies from one country to another. Premature vaccination not only fails to immunize, but also interferes with subsequent re-immunization. Because we now know this, further direct tests of vaccine effectiveness in very young children are ethically undesirable, and methods that use determination of passively acquired antibody are to be preferred. The levels of antibody that mothers have to pass to their children vary considerably. These differences are important in comparisons of South Asian countries with others, but not elsewhere. Differences in efficiency of transport of antibody across the placenta also play a role, but usually a minor one. Most important seems to be variation in antibody durability in the infant. Where families are poor, the children acquire many infections at an early age, and passively acquired antibody is swept out. These children who are least able to withstand the effects of measles infection, are hit at the earliest age. To provide protection for them, the vaccine must be given at a carefully determined age, specific for each community. Only when this is done can we hope to reduce measles worldwide to a sufficiently low level that it will be removed as a threat to persons in the United States, or anywhere else.