"The vaccine also appears safe."

Gardasil protects against the four strains of the human papilloma virus (HPV) that cause most cases of cervical cancer.

One team of international researchers, led by Dr. Laura Koutsky of the University of Washington, studied 12,167 women aged 15 to 26 in 13 countries. Half got three doses of the vaccine and half got placebo shots.

The vaccine prevented pre-cancerous lesions in 98 percent of the women who had never been infected with the HPV-16 and HPV-18 strains over three years, Koutsky's team reported.

Dr. Suzanne Garland of the Royal Women's Hospital in Melbourne, Australia, and colleagues tested 5,455 women aged 16 to 24 and found the vaccine was 100 percent effective against the lesions that can develop into cervical, vulvar, vaginal and anal cancers.

Dr. Maura Gillison of Johns Hopkins University in Baltimore and colleagues studied 100 patients with oral or throat cancer and compared them to 200 healthy people and found those who had six or more oral sex partners had a high risk of the cancer.

They found evidence of HPV-16 in 72 percent of the tumors.

CONTROVERSIAL VACCINE

The U.S. Centers for Disease Control and Prevention has recommended the vaccine for 30 million women and girls aged 11 to 26.

But state proposals for mandatory vaccination have met with resistance, even though all allow parents to exclude their child.

"Such a slight burden on parents can hardly justify backing away from the most effective means of protecting a generation of women, and in particular, poor and disadvantaged women, from the scourge of cervical cancer," Alta Charo, a bioethicist at the University of Wisconsin, Madison, wrote in a commentary.

The vaccine does not work after a woman is already infected, and HPV is very common. By age 14 to 19, one quarter of U.S. teens are already infected with at least one strain.

"Delaying vaccination may mean that many women will miss an opportunity for long-lasting protection," Sawaya and Smith-McCune wrote.

Cervical cancer is the second most common type of tumor in women and the leading cause of cancer deaths in some countries. Such tumors kill about 300,000 women worldwide each year, mostly in developing countries.

The lesions that lead to cervical cancer can be detected with a Pap smear and removed before they become tumors, and experts stress that vaccinated women will still need to get the smears regularly.

The vaccine's cost - $360 for three doses over six months - is too high for women in poorer countries where HPV is the biggest threat, some experts have noted.

GlaxoSmithKline is also developing an HPV vaccine called Cervarix. A study released in April showed it protects women for more than 5.5 years.

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