All Canadian provinces and territories offer school-based HPV vaccination programs, but it might be too early to see the results. (Harry Cabluck/Associated Press)

The incidence of genital warts in Australia went down quickly and significantly after a vaccine to prevent a sexually transmitted infection was introduced there, a new study suggests.

Genital warts are the most common sexually transmitted infection. It's estimated up to 75,000 Canadian women aged 15 to 49 currently have visible genital warts, according to Statistics Canada.

Starting in April 2007, Australia made the Gardasil vaccine available to all girls in schools. In July 2007, it expanded the program to women under the age of 27 outside of schools. The vaccine protects against four types of the human papillomavirus or HPV, which can cause the most common forms of genital warts and cervical cancer.

Before the vaccine was introduced, 15 per cent of women treated at the Melbourne Sexual Health Centre had genital warts, but the rate has now declined to six per cent, said Dr. Christopher Fairley, director of the centre, which is Australia's largest sexual health clinic.

"We saw a very dramatic decline of more than half in the proportion of women attending the centre with warts," said Fairley, lead author of the study in Thursday's online issue of the British journal Sexually Transmitted Infections.

Potential benefit for men

Since 2008, the researchers also found an average quarterly decline of five per cent in the incidence of genital warts in heterosexual men but not homosexual men.

The decline also did not occur among women over age 28, the team reported.

It's too soon to see any effects of school-based programs to vaccinate girls against HPV in Canada, said Ronda Phillips, executive director of the Halifax Sexual Health Centre.

"They just started the program a couple of years ago. It might be quite some time before we see any reduced incidence of genital warts."

The study's authors concluded, "Our data suggest that a relatively rapid and marked reduction in the population prevalence of genital warts among vaccinated women may be achievable through an HPV vaccination programme targeting women. Our data also supports some potential benefit being conferred to men."

Australia is seeing the decline now because that 70 per cent of women under age 28 have been vaccinated, by far the highest coverage in the world, Fairley said.

Questions continue to swirl around HPV vaccination, which works best if given to females before they become sexually active. Some would prefer to promote abstinence and safe sex.

There are also questions about the cost effectiveness of Gardasil, which costs about $404 for the required three doses.

The federal government announced in its March 2007 budget that $300 million over three years will be available to the provinces and territories in support of a national HPV vaccination program.

No funding was obtained for the study. Some of the study's authors either own shares in the manufacturer of Gardasil or have received honoraria from it or other drug makers.