TORONTO -- Provincial and territorial HPV vaccination programs should be expanded to cover boys, the Society of Obstetricians and Gynecologists of Canada said Thursday.

The professional organization's CEO, Dr. Jennifer Blake, wrote to ministers of health across the country urging them to follow the lead of Prince Edward Island, which will be opening up its program to both genders.

"P.E.I. has been the first province to actually take the initiative and extend the program to boys. And so it really makes it much harder to justify not doing this right across the country," Blake said in an interview.

All provinces and territories offer the vaccine to girls, though at different ages. The Northwest Territories starts in Grade 4. In most provinces, the vaccine is offered in Grade 6. Ontario's program is the latest, waiting until girls are in Grade 8 before offering them the vaccine.

There are two approved vaccines on the market to protect against human papillomaviruses, which cause cervical cancer, head and neck cancers, anal and penile cancers and genital warts.

Gardasil, by Merck Canada, protects against four HPV strains, HPV 16 and 18, which are the major causes of cervical cancers, and HPV 6 and 11, which are responsible for about 90 per cent of genital warts. Cervarix, by GlaxoSmithKline, protects against HPV 16 and 18.

Both are approved for use in girls and young women, but only Gardasil is approved for use in boys and young men in Canada.

Blake said extending the vaccination programs to boys would both protect them against genital warts and cancers caused by HPV viruses and should cut the risk girls will become infected though sexual contact. She suggested it is difficult to justify not offering boys the same protection as girls get.

Canada's National Advisory Committee on Immunization, which reviews the scientific literature on vaccines and makes recommendations on their use, has recommended vaccinating boys.

But provinces and territories have been slow to follow the advice, given the high cost of the vaccines, which are administered in three doses. A full course of HPV vaccine costs between $400 and $500 if purchased outside the provincial programs, where the shots are provided for free.

Blake said the upfront outlay for provinces and territories would be more than recouped by reduced use of the health-care system later.

"Just in reduction of genital warts, the vaccine will more than pay for itself," she said, explaining that people who develop genital warts often end up making repeated trips to a doctor for treatments that stretch out over months.