The new vaccination that promises to protect women against cervical cancer actually targets a virus that when present is known to cause this form of cancer.
There are hundreds of different strains of the human papillomavirus (HPV), about 30 of which are transmitted by sexual activity. Of these about 10 are linked to the development of cervical cancer in infected women.
HPV is one of the most common sexually transmitted infections, with 80 per cent of sexually active women likely to have encountered the virus by age 50. Aside from cancer the virus also causes genital warts.
In most cases there are no symptoms and the body clears the infection. Certain strains of the virus are more dangerous, however, and cause changes to cervical cells that can lead to cancer. The disease kills up to 300,000 women worldwide each year.
The new vaccine, Gardasil, works against four of the most important HPV strains. Two of these are responsible for about 70 per cent of cervical cancer cases. It also blocks infection by two other strains responsible for 90 per cent of genital wart cases.
As is common with many vaccinations, the vaccine is given in three doses to provide the highest level of immunity against HPV. It is being licensed in the US for use in girls and young women aged from nine to 26, in an effort to provide immunity before sexual exposure to HPV can take place.
The vaccine does not help once infection has taken hold.
The vaccine does not block all cancer-causing HPV strains so women will continue to benefit from cancer screening. The treatment is also too new to know how long it will provide protection or whether booster vaccinations might be required.
Tests are under way to determine whether older women might benefit and also men who can carry and transmit HPV and can also suffer from cancers associated with HPV infection.
(Additional reportage AP, Reuters)