Modelling forces of infection for measles, mumps and rubella.

Serological data from 8870 persons collected prior to the introduction of measles, mumps and rubella (MMR) vaccine in the UK are used to describe the rate at which individuals acquire infection by these diseases at different ages. A parsimonious model is developed and fitted under various interpretations of the data, particularly concerning the probability of lifelong susceptibility to infection. It is shown that, while the force of infection curves are relatively robust in their general features, they exhibit considerable sensitivity in matters of important detail. This is true in particular of the values taken by the force of infection in older age groups. As a result, estimates of the average age at infection are highly sensitive to these interpretations. This in turn may limit the accuracy of predictions from mathematical models based on these parameters, in particular regarding the level of immunization required for eradication of disease.