A vaccine to prevent cervical cancer, triggered by the human papillomavirus (HPV), has helped reduce the number of teenage girls developing abnormalities in their cervix by as much as 50 percent in a study in Australia, researchers reported on Friday.

Some strains of HPV are known to cause abnormal lesions in the cervix, which may turn cancerous later on. Vaccines are aimed mainly at girls between the ages of nine and 12 as they are regarded as most effective when given before the onset of sexual activity.

The researchers compared pap smear test results of girls after they received Merck & Co's Gardasil vaccine in a national, public-funded vaccination program in 2007 and 2008 with test results of earlier batches of girls who were never vaccinated.

Proportionately fewer of the vaccinated girls (0.42 percent) were found with high-grade cervical abnormalities compared to unvaccinated girls (0.8 percent), said the researchers, who published their study results in The Lancet journal.

"This data ... shows a reduction in the number of very young woman with high grade cervical lesions diagnosed since the vaccine program started," said Julia Brotherton, an epidemiologist with the Victorian Cytology Service Registries and lead author of the paper.

"In conjunction with the data from our colleagues in the sexual health field, who have already demonstrated a significant reduction in the occurrence of genital warts since the vaccine program started, we are optimistic that this is an indication that the vaccine program is already beginning to have an impact."

The vaccine appeared, however, to have much less impact on older women.

The study, which took place in Victoria state in Australia, was conducted independently without any corporate funding.

Australia introduced an HPV vaccination program to fight HPV strains 6, 11, 16 and 18 for all women aged 12-26 years between 2007 and 2009.

Experts not involved in the study called for more research to confirm that the reduction in cervical abnormalities in the youngest age group was indeed due to HPV vaccination.

"The study looked at overall trends, and we cannot be absolutely certain that the drop in cervical abnormalities in the youngest age group was due to HPV vaccination," said Associate Professor Karen Canfell, an epidemiologist with the Cancer Council of New South Wales.

"The group in which the effects were observed were younger than the age group in whom screening is normally recommended, so although the results are suggestive, there are some issues in interpreting the findings."

Apart from Merck, GlaxoSmithKline also produces an HPV vaccine called Cervarix.