ATLANTA - The controversial HPV shot given to girls should also be given to boys, in part to help prevent the spread of the virus through sex, a U.S. government medical panel said Tuesday.

The pricey vaccine, which protects females against cervical cancer, hasn't been popular. And doctors admit it will be a tough sell to parents of 11- and 12-year-old males, too.

For males, the vaccine is licensed to prevent genital warts and anal cancer. Experts say another key benefit of routinely vaccinating boys could be preventing the spread of the human papillomavirus to others through sex -- making up somewhat for the disappointing vaccination rate in girls.

The Advisory Committee on Immunization Practices made the recommendation Tuesday in a unanimous vote. Federal health officials usually adopt what the panel says and ask doctors and patients to follow the recommendations.

The vaccine has been licensed for use in boys for two years, but Tuesday's vote was the first to strongly recommend routine vaccination. Officials acknowledged the disappointing rate in girls encouraged them to take a new, hard look.

Last year, just 49 per cent of adolescent girls had received at least the first of the recommended three HPV shots. Only a third had all three doses by last year.

"Pretty terrible," said Dr. Anne Schuchat, a U.S. Centers for Disease Control and Prevention administrator who oversees the agency's immunization programs.

She attributed the low rates for girls to confusion or misunderstanding by parents that they can wait until their daughter becomes sexually active. It only works if the shots are given before a girl begins having sex.

The vaccine is approved for use in males and females aged nine to 26; but it is usually given to 11- and 12-year-olds when they are scheduled to get other vaccines.

The committee also recommended the vaccination for males aged 13 to 21 who have not been vaccinated previously or have not completed the three-dose series.

In Canada, the National Advisory Committee on Immunization, or NACI, recommends that girls and young women be vaccinated against HPV. That recommendation has not been extended to include males, but NACI says on its website that it is reviewing the current advice on HPV immunization.

The HPV vaccine Gardasil is approved in Canada for both females and males aged nine to 26. A second HPV vaccine, Cervarix, was recently approved for use in Canada for females aged 10 to 25, but has not yet been approved for males, NACI said.

All provinces cover the cost of vaccination for girls in certain grades -- usually around the time of middle school. Others who want to be inoculated but are not covered by public plans will have to pay $400 to $500 for the three doses or rely on coverage from private health insurance, says the Society of Obstetricians and Gynaecologists of Canada website.

The U.S. panel's vote Tuesday follows recent studies indicating that the HPV vaccine prevents anal cancer in males. A study that focused on gay men found it to be 75 per cent effective. But while anal cancer has been increasing, it's still a fairly rare cancer in males, with only about 7,000 cases in the U.S. each year that are tied to the strains of viruses targeted in the HPV vaccine. In contrast, about 15,000 vaccine-preventable cervical cancers occur annually in the U.S.

Some feel it's unlikely that most families will agree to get their sons vaccinated primarily to protect girls. The threat of genital warts hasn't been persuasive yet, either: some data suggest that less than 1.5 per cent of adolescent males have been inoculated.

The vaccine's use against anal cancer may not be much of a selling point, said Dr. Ranit Mishori, a family practice doctor in Washington, D.C., and an assistant professor at the Georgetown University School of Medicine.

Some parents may say: "'Why are you vaccinating my son against anal cancer? He's not gay! He's not ever going to be gay!' I can see that will come up," said Mishori, who supports the committee's recommendation.

While there are two vaccines against HPV, Tuesday's vote applies only to Merck's Gardasil, which costs US$130 a dose.

An estimated 50 per cent to 80 per cent of men and women are infected with HPV in their lifetimes, although most clear the infection without developing symptoms or illness, according to the CDC.