Achieving HPV herd immunity cost-effectively. When does it make sense to allocate resources preferentially to boys?

Recent empirical studies of HPV vaccination have provided evidence that marginal vaccination costs increase with coverage.  Let us take into account – they argue – not just the vaccine price, but the cost of education and outreach programmes that would be needed so as to reach the yet unvaccinated population.  If we do so, we are likely to find that raising the vaccination rate for pre-adolescent girls, let us say, from 40-41%, proves considerably more expensive than raising it from 20-21%.  This, in turn, raises the question whether – given the achievement of herd immunity is the ultimate goal – resources are necessarily best allocated when directed to the female rather than the male pre-adolescent population. Could it even be that – at certain levels of coverage and a certain rates of increase in marginal vaccination cost (for girls and for boys) – resources might be more effectively allocated to the vaccination of pre-adolescent boys? Ryser & Myers in a recent study seek to model the impact of marginal cost increase in order to answer this question.  In the case of US, at least – with rates of vaccination standing currently, for girls and boys, at, respectively 37% and 13.9% – they argue in cost benefit terms for re-directing resources to boys.  However, the question is no doubt relevant to other countries in which HPV has been introduced, but levels sufficient for herd immunity, have not yet been achieved.

Optimal allocation of new resources as between girls and boys for a given level of vaccination is indicated in the diagrams on p.40.  Marginal vaccination cost increase is estimated at a higher, a lower, and zero level.  What is striking is the radically different patterns of optimal allocation between girls and boys in those three scenarios.  At the very least, the results challenge the orthodoxy of the superior cost benefit of female vaccination. They also indicate the importance of further empirical research into marginal vaccination cost increase.

The limitations of the study are largely due to the assumption of a closed sexual network of 14-18 yr old heterosexual adolescents.  The attempt is made to factor in the impact of various complicating factors, such as the assortativity of vaccine uptake with sexual activity, the likely presence of additional relationships between females inside, and older males outside, the network, and asymmetric vaccination cost curves as between girls and boys.  But the most serious limitation was beyond the power of the study to address.  This is the restriction of the modelled network to heterosexual relations, and the exclusion from consideration of a large number of HPV-related conditions such as anal and oro-pharyngeal cancers which have higher incidence among MSM and HIV-infected individuals (Lawton & Asboe (STIs); English & Pourbohloul (STIs)).  One wonders, therefore, whether the case for male HPV vaccination is not a great deal stronger than might appear from this paper.  At all events, a case is made that, even when these conditions are excluded, a greater allocation of HPV vaccination resources to males may be justifiable – e.g. currently in the US.  For the impact of the recent extension of the HPV vaccination programme to males in Australia (the first country to have taken this step (2013)), see Korostil & Donovan (STIs).