Current status of HBV vaccine escape variants - a mathematical model of their epidemiology

We present a deterministic model of the possible emergence of a vaccine esc ape variant of hepatitis B virus (HBV). The model identifies the key unknow ns determining this process: the protection afforded by she current vaccine s against particular HBV variants; the infectiousness of these variants; an d the current prevalence of individuals infectious with the variants (each factor relative to wild-type). By malting pessimistic assumptions about the se unknowns we show that even a highly infectious variant, under a vaccine programme that affords no protection against the variant, would still take decades to emerge. Thus the current low prevalence of variants is not evide nce for the cross-reactivity of the current vaccines or for a lack of infec tiousness in the variants. As any vaccine failure will be inapparent for de cades it may be sensible to recommend vaccine modifications now rather than later.