GlaxoSmithKline experimental vaccine Cervarix has proved more than 90 per cent effective in preventing precancerous lesions due to two types of a virus responsible for most cases of cervical cancer.

Interim results from the biggest cervical cancer vaccine trial to date, published in the Lancet medical journal, confirm previous promising data. Cervarix is a rival to Merck's similar product Gardasil.

Analysts expect both vaccines will be multibillion-dollar sellers. Gardasil, however, may have the edge, since it is already established in international markets.

So far, the only major market where Cervarix has a green light is Australia. Glaxo expects to launch it in Europe in the second half of this year but in the US -- the world's top market -- it will not go on sale before 2008.

Cervarix is designed to prevent types 16 and 18 of the sexually transmitted human papillomavirus (HPV), which are responsible for 70 percent of all cervical cancers. Gardasil targets two additional virus strains that cause genital warts, a less serious condition.

Results of the Phase III study involving 18,644 women aged 15 to 25 years showed Cervarix was 90.4 per cent effective against types 16 and 18, based on pre-specified analyses that only required detection of virus in the lesion.

In practice, however, researchers found most lesions contained multiple cancer-causing HPV types and when this was taken into account, Glaxo said the data showed Cervarix was actually 100 percent effective against strains 16 and 18.

Cervarix also provided significant cross-protection against infection caused by virus types 45, 31 and 52, which together account for another 12 per cent of cervical cancer cases.

'To have up to 100 per cent protection against two viruses that are responsible for more than 70 per cent of cervical cancers and to confirm some preliminary findings on cross-protection, as well as the safety profile, is extremely exciting news for us,' Philippe Monteyne, Glaxo's head of vaccine development, said in a conference call.

In an accompanying comment in the Lancet, Jessica Kahn of the University of Cincinnati College of Medicine and Robert Burk of the Albert Einstein College of Medicine in New York, said the results were encouraging but it was still early days to assess efficacy, since cervical cancer can evolve over several decades.

They also highlighted some increases in adverse reactions among vaccinated women, including injection site symptoms and some general symptoms, although there were no clinically meaningful differences in safety outcomes.

Many governments around the world have endorsed the idea of vaccinating young girls around the age of puberty, on the basis that it will save lives and healthcare costs in the years ahead.

The enthusiastic reception for HPV vaccination has helped fuel strong Gardasil sales, which totalled $365 million (183 million pounds) for Merck in the first quarter of 2007. Reuters