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[MONTEVIDEO] The human papillomavirus (HPV) vaccine does not reduce infection in women that already have the disease, reveals a new study.

The research was published this week (15 August) in the Journal of the American Medical Association (JAMA).

Previous studies have shown that the HPV-16/18 vaccine — which protects against the types of HPV that most often cause cervical cancer — prevents infection and the development of cervical precancer and cancer (see Vaccines can fight South's top fatal cancer in women).

But the new study shows that it does not reduce the presence of the virus in already-infected individuals and should not be used to treat the infection, say researchers.

Allan Hildesheim, from the US-based National Cancer Institute, and colleagues conducted a study in Costa Rica with 2,189 women aged 18–25 who where positive for HPV.

Hildesheim told SciDev.Net that they chose Costa Rica because the rate of HPV infection among sexually experienced women is high.

Over six months, 1,088 participants were randomly assigned to receive three doses of HPV vaccine and the remaining 1,101 received a control hepatitis A vaccine.

The researchers found no evidence that HPV vaccination significantly altered rates of HPV clearance.

After six months, average clearance rates were 33.4 per cent in the women who received the HPV vaccine and 31.6 per cent in the control group.

After 12 months, the rates were 48.8 per cent and 49.8 per cent, respectively.

In an accompanying JAMA editorial, Lauri E. Markowitz, of the US-based Centers for Disease Control and Prevention, concluded that the study and other epidemiologic data supported recommendations for routine immunisation at 11–12 years of age.

"Because the vaccine has no therapeutic efficacy, the greatest effect will be realised if the vaccine is administered before sexual debut, prior to exposure to HPV," writes Markowitz.

Link to full paper in JAMA

Reference: JAMA 298, 743 (2007)

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