An experimental cervical cancer vaccine made by GlaxoSmithKline Plc produced immune responses in all women ages 15 to 55 in a clinical study, the first evidence that a cervical cancer vaccine may work in women ages 26 and older, the company said on Monday.

The vaccine, called Cervarix, is designed to prevent infection with the human papilloma virus, or HPV, which causes most cervical cancer.

Researchers in the Phase 3 trial studied 666 women who received Cervarix injections and found 100 percent of them had developed antibodies to the two HPV types the vaccine targets, Glaxo said in a statement. The antibodies were detected seven months after the first of three doses and remained 12 months after the first shot.

Antibody levels were greater or equal to what was seen in females age 15 to 25 in other studies, which have shown 100 percent protection for 4.5 years against the two HPV strains and associated cervical lesions, Glaxo said.

The vaccine was “generally safe and well-tolerated,” Glaxo said.

“These are important data as older women remain at significant risk of acquiring infections with cancer-causing HPV types,” said Dr. Tino Schwarz, a professor at Stiftung Juliusspital Wuerzburg in Germany and the study’s lead author.

The findings were presented at a meeting of the American Society of Clinical Oncology in Atlanta.

Glaxo is lagging rival Merck & Co. Inc. in developing a vaccine for cervical cancer.

Merck is expected to win U.S. approval for its Gardasil experimental vaccine this week based on earlier studies showing it was effective in women and girls ages 9 to 26. Glaxo has said it plans to apply for U.S. approval later this year.

At the same meeting in Atlanta, researchers presented data showing Merck’s Gardasil was 100 percent effective at preventing pre-cancerous growths in the vagina or vulva caused by two HPV strains. It also was 81 percent effective at preventing the same conditions if they were caused by any HPV type, said Dr. Jorma Paavonen, the study’s lead author and chief physician at the University of Helsinki’s obstetrics and gynecology department.

Researchers based the conclusions on data from three clinical trials of more than 18,150 women given either Gardasil or a placebo.