PLoS One: incidence of anogenital warts after the introduction of a quadrivalent HPV vaccine program somewhere

Background: Anogenital warts (AGW) are caused by human papillomavirus (human papillomavirus), 90% of which are caused
by human papillomavirus subtypes 6 and 11.
The quadrivalent human papillomavirus (qHPV) vaccine targets human papillomavirus subtypes 6 and 11, as well as 16 and 18, which cause 70% of cervical cancers
.
The incidence of population-level AGW in Manitoba, Canada, before and after the introduction of the qHPV vaccine, has been reported in many studies and several meta-analyses
.
However, the literature is highly heterogeneous, both in terms of source data and AGW determination
.
When the methods differ, the results of the study cannot be directly compared
.
Some studies looked at the entire population of a country or region, some looked at customers of specific health insurance companies, and others were limited to patients at
specific clinics.
The AGW case definitions in these studies are variable, ranging from the use of specific AGW diagnostic codes, prescriptions for AGW medications, diagnostic codes or prescriptions, clinical diagnoses, to patient self-assessment
.
In addition, patterns of sexual behavior may have changed after the introduction of the qHPV program, but these changes or changes in other sexually transmitted infections (STIs) are not usually examined
at the population level.
With the introduction of the universal qHPV vaccination program, doctors' perceptions and attitudes towards AGW and its treatment may also have changed
.
All of these factors may have influenced the decline in incidence reported by AGW, but the publication of incidence trends in different jurisdictions now allows for an assessment
of these non-vaccine factors.

Objective: The incidence of anogenital warts (AGW) decreased
after the introduction of quadrivalent human papillomavirus (qHPV) vaccine in multiple regions.
We looked at how comparing the incidence of AGW with different outcomes affects the interpretation
of the qHPV vaccination schedule.
To do this, we repeated multiple study designs
within one jurisdiction (Manitoba).

Methods: We measured the incidence
of AGW, AGW-related prescriptions, chlamydia, and gonorrhea (the latter two being false outcomes) between 2001 and 2017 using several clinical and administrative health databases in Manitoba.
We then compared the incidence of the 1997–1998 birth cohort (the first cohort eligible for the publicly funded qHPV vaccination schedule) with the 1995–1996 birth cohort using incidence
ratios (IRRs).

Results: Following the introduction of the female-specific qHPV vaccination program, the incidence of AGW decreased by 72% (95% CI 54-83%) in Manitoba girls aged 16-18 years and by 51% (14-72%)
in boys.
Trends in AGW-related prescriptions differ from those in AGW diagnostics because these prescriptions peaked
shortly after the introduction of publicly funded qHPV vaccine programs.
For girls aged 16-18 years, the incidence of chlamydia and gonorrhea also decreased by 12% (5-18%) and 16% (1-30%)
, respectively.

Figure 1 Age-standardized incidence rates of related diseases by year and sex (index to school-based qHPV vaccination programme launched in 2008)

Figure 2 Age-standardized incidence rates of related diseases by year, age group and sex (indexed to the school-based qHPV vaccination program introduced in 2008)

Table Incidence rates of certain diseases in the two-year cohort (1997-1998 birth cohort versus 1995-1996 birth cohort) before and after school-based qHPV vaccination (95% confidence interval)

Conclusion: The publicly funded school-based qHPV vaccine program reduced the incidence of AGW among young women in Manitoba by three-quarters
.
Following the introduction of a publicly funded qHPV vaccination program, AGW-related prescriptions are not a good representation of medically involved AGW
.
The different sexual habits of adolescents are at best a small part of
the decline in the incidence of AGW.

Righolt CH, Willows K, Kliewer EV, et al.
Incidence of anogenital warts after the introduction of the quadrivalent HPV vaccine program in Manitoba, Canada PLoS One 2022; 17(4)