HPV vaccination programmes have been implemented in many countries, but gender- and age-specificities vary strongly. Preadolescent girls are always included into HPV vaccination programmes, but the lower and upper ages of admissibility are variable, as were the number of birth cohorts considered for catch-up vaccination at the time of vaccine introduction. Countries are gradually extending vaccine eligibility to preadolescent boys, sometimes without underlying cost-effectiveness evaluations. Selectively expanding HPV vaccination to at-risk adults (e.g. men-who-have-sex-with-men (MSM) or non/partially vaccinated women attending cervical screening) is advocated as an alternative to improve HPV prevention efforts.
We consider general principles for informing strategy about gender- and age-specificity of HPV vaccination programmes. Many of these principles are derived from analyses based on mathematical models designed to inform decision-making. Some principles relate to structural issues whereas others are regulated by model parameters.
Vaccinating girls prior to sexual debut is unequivocally considered cost-effective, but findings regarding catch-up campaigns strongly depend on local cost and sexual network assumptions. Analyses of boys’ vaccination are more elaborate because these are often considered conditional on girls’ vaccination. Gender-neutral vaccination evaluations may hinge on concealed entities such as herd immunity from girls’ vaccination, etiologic fractions of non-cervical cancers considered, attribution of residual disease burden to MSM, and realistic costs of vaccine procurement and delivery. Consequently, the estimates of vaccinating boys in addition to girls are highly variable between settings and assessments. The efficacy of selective adult vaccination has been demonstrated in clinical trials, but relatively few estimates exist of the absolute health gains from vaccinating MSM or screen-eligible women. Their population impact likely depends on factors (e.g. latency, reactivation, HIV coinfection) that still may require elucidation in mathematical models.
The consistent estimates on cost-effectiveness of girls’ vaccination contrast with those for boys, which is partly due to the conditioning of gender-neutral on girls-only vaccination. The health economics of selective adult vaccination are worth exploring in model-based assessments.