Discussion
This is the first study to look at the impact of the HPV vaccine program on in-patient treatment of genital warts in Australia. We found that there was a large (85.3%) decline in the number of in-patient treatments for genital warts in the youngest (15–24 year old) women after the HPV vaccination program began in 2007; the decline in the 25–34 year old age-group was more modest and there was no significant decline in the numbers of in-patient treatments in older (35–44 year old) women. There were also moderate declines in the numbers of treatments for penile warts in 15–24 and 25–34 year old men and for anal warts in 35–44 year old men.
The major strength of this analysis is the use of nation-wide population-based data and a long follow-up time, providing a complete picture of the private in-patient treatment of genital and anal warts in Australia. Use of a national register eliminates the problem of recall bias from self-reported information. The major limitation of this study is that Medicare data only represents treatments in private hospitals and does not include treatments in publicly funded hospitals; thus the data are not representative of all in-patient treatments. Another limitation of the study is that the hospital data extracted may contain repeat in-patient treatments for the same individual but numbers would be limited and we do not anticipate this pattern to have changed during the study period. Lastly, as sexual behaviour data were not available we were unable to adjust for any change in sexual risk behaviour – a decline for example could have contributed to decreasing trends observed. However, national surveys show that sexual risk taking behaviour has increased in adolescents and gay men in the vaccine period compared to pre-vaccine period and these changes have been associated with an increase in the prevalence of chlamydia in both populations.
A number of factors could potentially influence the trends observed. Firstly, socio-economic status of a person or a family could theoretically affect both going to private hospitals and the uptake of vaccine. However, we don't believe the vaccine uptake would vary considerably by socio-economic status as the HPV vaccination program in Australia provides vaccines free of charge to all young women, and boasts high coverage rates (>80% in school-girls). Second, a decline in the proportion of the population with insurance coverage over time could explain the trends observed. However, data from the Australian Private Health Insurance Administration Council shows that there was a significant increase in the proportion of population which have hospital treatment insurance coverage; from 45.4% in Dec 2000 to 46.3% in Dec 2011 (p < 0.001). Third, any changes in healthcare-seeking behaviour or clinical practices could also influence the declining trends observed. It is possible that an increase in the use of self-applied topical treatment of genital warts over time may have contributed to the decline in the surgical treatments seen in our analysis. However, topical treatments were available throughout the study period and the price did not change substantially over time. Furthermore, if there was a decline due to increase in the use of topical treatments we would expect it to have been seen in all age-groups, thus the decline observed only in young people cannot be explained by changes in the topical treatment patterns. Lastly, the national incidence of genital warts has been reported to be 2.2 per 1,000 persons (2.1 in males and 2.3 in females) in the pre-vaccine period, thus the few hundred cases of genital warts treated in the private hospitals is a relatively small proportion compared to ~45,000 cases of genital warts in 2006 nationally. However, this is an important proportion of clients as mainly chronic and/or severe cases are likely to be referred to a hospital for in-patient treatment (surgery under anaesthesia is not recommended as a first-line therapy for the treatment of genital warts and cannot be performed in a general practice or sexual health clinic setting).
The results from our study validate findings from a national sentinel surveillance system at sexual health services and confirm that the numbers of cases of genital warts are declining in young women since the vaccine roll out (the 15–24 year old women in our study were all eligible for free HPV vaccination between 2007 and 2009). We believe that the smaller decline in 25–34 year old women is because only a proportion of these women were vaccinated as part of the initial vaccine catch-up program. Men were not eligible for the free HPV vaccination and thus the decline in penile warts in men after 2007 is likely to reflect herd immunity, as observed in the sentinel surveillance system. Our study also showed that a higher number of women underwent inpatient treatment compared to men. That could be because penile warts are easier to treat in the out-patient setting (and to self-treat), compared to vulval/vaginal warts.
We also reported on the numbers of treatment of anal warts in men as a comparison. There was no decline in the number of treatments for anal warts in the younger men after 2007. The moderate decline in older men in the vaccine period could also reflect herd immunity as some men undergoing treatment for anal warts were likely be heterosexual or bisexual. In one study 18.2% of men with anal warts were heterosexuals. A recent review of anal sexual practices in heterosexuals highlighted the paucity of data in this area including sexual practices such as anal digital stimulation of men by their female partner. Ongoing monitoring of the impact of the vaccine in men is warranted as the Australian Government has extended the national HPV vaccination program to include vaccination for boys in 2013.