News

The HPV vaccine prevents infection with human papillomavirus, but it doesn't provide any benefit to women who have already been exposed to the common, sexually transmitted infection, according to new research.

The study, published in today's edition of the Journal of the American Medical Association, is important because it underscores that, to be effective, vaccination should occur before children become sexually active.

Earlier research suggested that, in addition to blocking transmission, the HPV vaccine could help women clear the infection and prevent the risk of cervical cancer, said Allan Hildesheim, a senior investigator at the U.S. National Cancer Institute in Bethesda, Md., and lead author of the study.

But Dr. Hildesheim said there is no evidence that HPV vaccination benefits women already infected.

"Our results demonstrate that in women positive for HPV DNA, HPV-16/18 vaccination does not accelerate clearance of the virus and should not be used for purposes of treating prevalent infections," he said.

HPV is the world's most common sexually transmitted infection. Virtually everyone who is sexually active is exposed to the virus; most clear it naturally but in some cases infection causes genital warts and cervical cancer.

HPV is also a leading cause of penile, vulvar, anal, oral, pharyngeal, and head and neck cancers. Two strains of HPV, 16 and 18, are responsible for most cases of cancer.

Two new vaccines, Gardasil by Merck Frosst Canada Ltd. and Cervarix by GlaxoSmithKline Inc., target HPV-16 and -18. To date, only Gardasil has been approved for use in Canada.

To determine if the vaccine could help women clear the virus, researchers conducted a clinical trial in Costa Rica.

A total of 2,189 women aged 18 to 25 who were positive for HPV participated. About half of them received the HPV-16/18 vaccine and the other half the hepatitis A vaccine.

At 12 months, 48.8 per cent of women who received the HPV vaccine had cleared the infection, compared to 49.8 per cent who had not received the vaccine.

Also in today's edition of JAMA, Lauri Markowitz, a medical epidemiologist and director of the HPV working group at the U.S. Centers for Disease Control and Prevention in Atlanta, said the research shows that the "vaccine has no therapeutic efficacy."

That, combined with the high likelihood of acquiring HPV infection shortly after the onset of sexual activity, demonstrates the vaccine must be administered in the early teens to be effective, she said.

The U.S. Advisory Committee on Immunization Practices recommends that girls be immunized at the age of 11 or 12. The Canadian National Advisory Committee on Immunization advises that girls be vaccinated between the ages of 9 and 13.

In Canada, the median age of first intercourse is 16.5 - meaning that half of teenagers have intercourse before that age.

The age at which teenagers have oral sex is believed to be even lower, and HPV can be transmitted orally.

In March, the federal government unveiled a $300-million program to promote HPV vaccination that allows provinces to dip into a fund on a per capita basis. To date, four provinces - Nova Scotia, PEI, Newfoundland and Labrador and Ontario - have said they will vaccinate girls this fall.

British Columbia and Quebec will probably do so in the fall of 2008, and other provinces and territories haven't yet decided.

The vaccines will be administered in school, ranging from Grade 6 to Grade 8 depending on the jurisdiction.