Herd protection against HPV is governed by the probability of infection transmission, the duration of the infection, and sexual activity pattern, which varies in different populations. As a result, overall effectiveness of HPV vaccination at a population level, that is the sum of vaccine efficacy and herd protection, is population specific and, within the same population, type specific. The heterogeneity of herd protection affects the vaccination program effectiveness in different populations and against different HPV types. We use the IARC transmission-dynamic model to illustrate the herd immunity effect of HPV vaccination against different HPV types and in populations with different HPV pre-vaccination prevalence.
We simulated A) HPV16 and HPV45 infections within the same population. We compared the impact of vaccination on type-specific prevalence and on cervical cancer prevention. For HPV45, we also assessed the effect of cross-protection from HPV16/18 vaccines. B) HPV16 in populations with different background pre-vaccination prevalence (range 1% to 8%). We compared the impact of vaccination on population-specific prevalence and calculated the coverage adequate to meet selected HPV control thresholds.
Prevalence reduction (PR) attributable to vaccination was larger for HPV45 than for HPV16, regardless coverage levels. The difference was wholly attributable to herd immunity. With 70% and 50% (or higher) coverage, assuming girls-only and gender-neutral vaccination respectively, the incidence of HPV16-related cancers decreased from 4.1 to <1 per 105 women, whereas HPV45-related cancers were virtually eliminated. Similar estimates were obtained when levels of cross-protection against HPV45 were assumed to be 50% (or higher) and vaccination coverage at least 70%. For any given coverage, HPV16 PR attributable to vaccination was larger in populations with lower pre-vaccination prevalence. This finding was consistent across populations with different sexual activity patterns.
Due to herd immunity, HPV16 is more difficult to control and eliminate than other less frequent/persistent types. Partial cross-protection may be sufficient to eliminate cervical cancer associated with HPV45 and, possibly other HR HPV types that may share with HPV45 lower duration of infectious period and less ability to produce cancer than HPV16 and 18. Furthermore, HPV16 is more difficult to control and eliminate from populations with higher pre-vaccination prevalence and minimal coverage thresholds for HPV control or elimination depend on the pre-vaccination prevalence and sexual activity patterns of each population.