Predictions of the emergence of vaccine-resistant hepatitis B in The Gambia using a mathematical model

Vaccine escape variants of hepatitis B virus (HBV) have been identified wor ld-wide. A mathematical model of HBV transmission is used to investigate th e potential pattern of emergence of such variants. Attention is focused on The Gambia as a country with high quality epidemiological data, universal i nfant immunization and in which escape mutants after childhood infections h ave been observed. We predict that a variant cannot become dominant for at least 20 years from the start of vaccination, even when using a vaccine whi ch affords no cross protection. The dominant factor responsible for this lo ng time scale is the low rate of infectious contacts between infected and s usceptible individuals (we estimate the basic reproduction number of hepati tis B in The Gambia to be 1.7). A variant strain that achieves high prevale nce will also take many years to control, and it is questionable whether em ergence will be identifiable by sere-surveillance until of high prevalence. The sensitivity of the model predictions to epidemiological and demographi c factors is explored.