HPV types 6 & 11, included in the quadrivalent HPV vaccine (qHPV), are found in the majority of the condyloma cases. Sweden has offered subsidized qHPV vaccination to 13-17 year old girls between 2007-2011 after which an organized school-based vaccination program for girls ages 10-12 and catchup for girls ages 13-18 begun. Due to the short incubation time of condyloma, the effect of vaccination can be studied shortly after qHPV introduction. The aim of this study was to assess the incidence of condyloma after the introduction of qHPV in Sweden.
Trends in condyloma incidence over time was studied with an ecological study design. The study population included all men and women at ages 10-44 during the study period 2006-2012. First occurring condyloma cases were identified via the Swedish Patient register and Prescribed Drug register, where pharmaceuticals prescribed for treatment against condyloma was used as a proxy. Mean population estimates by sex, age, and calendar year were obtained from Statistics Sweden. Poisson regression was used to model the incidence rates and the corresponding 95% confidence intervals (CI) of condyloma stratified for age. Average annual percent changes (AAPC) were estimated for the post-vaccine availability period 2007-2012 by means of a broken line regression model using a Poisson distribution.
Decreases in incidence were observed in the post-vaccine availability period for women up to ages 34 with the greatest declines seen for girls ages 15-19 (AAPC=-11.1%, CI [-10.2;- 12.0]) and 20-24 (AAPC=-9.1%, CI [-8.3;- 9.8]) years old and with steeper declines in incidence from 2009 and onwards. Smaller reductions with no obvious breakpoints were seen for women ages 25-29 (AAPC=-7.6%, CI [-6.6;- 8.7]) and 30-34 (AAPC=-4.4%, CI [-2.9;-5.9]) years. Similar trends in incidence were seen in men, though smaller reductions in incidence were observed (AAPC ages 15-19=-3.9%, CI [-2.3;-5.4]; AAPC ages 20-24=-6.2%, CI [-5.4;-6.9]).
The observed decline in condyloma incidence in girls below age 25 following the introduction of qHPV in Sweden indicates anticipated effects. The observed decline among men and in women age 25 and above also points towards possible herd effects. Although an ecological study design cannot provide information on causality, the decline in condyloma among girls is well in line with observational studies comparing vaccinated with unvaccinated individuals. Future monitoring of the disease burden over time, as well as observational studies comparing vaccinated and unvaccinated individuals, is necessary to evaluate whether the qHPV program has the anticipated effect in the population.