In addition, even when risk factors are present, physicians may not identify the risk and need for vaccination in a large proportion of such cases. Finally, targeted programs can usually identify candidates for vaccination only after the risk behaviour has begun. Thus, the failure of a selective vaccination strategy alone to eliminate HBV infection could be predicted. The prevalence of unrecognized acute and chronic infection in children in Canada is unknown but believed to be low except in readily identified risk groups. Since risk cannot be predicted for a substantial proportion of cases, universal coverage is required.
Protection appears to be of long duration even if antibody levels decline or become undetectable.
Whether young adolescents would respond to lower doses is being investigated. The need for booster doses after receiving a primary series has not been established. Protection against clinical disease and the carrier state has been documented to persist for at least 10 years. Whether booster doses may be required after 15 to 25 years following primary vaccination will not be known for many years. This theoretical concern is particularly important if an infant vaccination program is chosen but could also be a consideration for vaccination of older groups.