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

<sec><st>Background</st> The 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. </sec><sec><st>Methods</st> A 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. </sec><sec><st>Results</st> Theoretical 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. </sec><sec><st>Conclusions</st> Countries 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. </sec>