Conclusive demonstration of cancer prevention benefits of HPV vaccination will only be possible in the relatively long term; good indicators of its probable effectiveness are, of course, already available. These include: reduced prevalence of the HPV types that have been vaccinated against; the decline in diagnoses of – or prescription for – genital warts; and even reductions in HPV specific neoplasia. Conclusive evidence of the first kind has been emerging in Australia, where there has been a quasi-elimination of the vaccination types (Chow & Fairley/2/STIs; Garland & Jayasinghe/STIs): of the second, in Australia, where presentations of warts have been reduced among the those aged 21 yrs and under by over 90% (Chow & Fairley/STIs ; Ali & Donovan (STIs)), and New Zealand, where there has been a corresponding fall in prescriptions (Wilson & Baker/STIs): of the third, in Finland, where the FUTURE trial showed in the intervention arm the absence of lesions CIN3 and ICC lesions present in the control arm (Paavonen/3/STIs).
Assessment of the impact of HPV vaccination will be harder in health systems where only a relatively more partial coverage is achieved – as in the US (single dose: 57%, triple dose: 38%, according to a 2013 national survey). Yet Markowitz & Unger – in their recently published study on the basis of data from the CDC based National Health And Nutrition Examination Survey (NHANES) – show evidence of a decline in quadrivalent (4v) HPV types (HPV-6, -11, -16, -18) that is larger than the authors would have predicted, given the inadequacy of coverage. The reason, the authors suggest, may be the greater than expected efficacy of a single dose of the vaccine. Data for individual HPV type prevalence in females aged 14-34 years from the four-year period 2009-12 were compared with data for prevalence in the same group from the four year pre-vaccination period 2003-2006. For purposes of analysis the total 14-34 year interval was divided into five-year tranches. The key findings of the study are that 4vYPV type prevalence has declined from 11.5% to 4.3% (64%) in the 14-19 yr group, and from 18.5% to 12.1% (34%) in the 20-24 yr group. Equivalent data comparing the pre-vaccination period with the first four years after vaccine introduction (years 2006-2009) are given in an earlier study by the same authors (Markowitz & Unger/STIs).
The study also pursues a number of secondary goals. The most important of these is to use data emanating from sexually active 14-24 yr olds, vaccinated and unvaccinated, to estimate the efficacy of the vaccine (one dose or more), and the evidence for herd immunity. But it also investigates the possibility of cross protection from 4vHPV vaccine against other (i.e. non-4vHPV) types in the study population at large – especially cross protection against types HPV-31, -33 and -45, for which there has emerged some evidence in past studies. So far as the sexually active 14-24 yr participants were concerned, 4vHPV type prevalence was found to be 2.1% in vaccinated, as against 16.9% in unvaccinated. There was no statistically significant evidence for her immunity. As regards cross protection against HPV types in the participants at large – there was, once against, no statistically significant evidence.