A Model of Long-Term Decline in the Transmissibility of an Infectious Disease: Implications for the Incidence of Hepatitis A

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Abstract
BackgroundThe epidemiology of hepatitis A in countries across the world is changing due to improvements in hygiene and living conditions which reduce the transmissibility of the infection.MethodsA mathematical model Is formulated to describe the changes in incidence of a directiy transmitted infection produced by a long term decline in its transmissibility. The basic reproduction number, the parameter describing transmissibility, is considered as a function of time. The relationship between the basic reproduction number and the force of infection is derived.ResultsTheoretical examples demonstrate that a decline in transmissibility results in an initial decline in the force of Infection, but that this may be followed by a substantial resurgence. Resurgences may be possible after several decades of declining incidence, and are most marked following a rapid decline.ConclusionsCountries which have experienced a rapid decline in the incidence of hepatitis A may be at risk of a resurgence. More detailed mathematical models, informed with data from regular age-stratified serologlcal surveys, should provide the basis for decisions on vaccination policy.

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