Older women can benefit just as much as younger women from Merck's Gardasil vaccine against cervical cancer, researchers in Colombia reported on Monday.
They found that women ages 24 to 45 who had no history of cancer-causing genital warts or cervical disease were much less likely to become infected with the wart virus if they got the vaccine than women who got placebo jabs.
The study, published in the Lancet medical journal, points to a potentially lucrative new market for the vaccine against the human papilloma virus or HPV.
Merck, which paid for the Colombian study, has also shown that Gardasil is 90 percent effective in preventing sexually transmitted warts in men.
GlaxoSmithKline's rival vaccine Cervarix protects against two HPV strains and is used in Europe.
Dr. Nubia Munoz of the National Institute of Cancer in Bogota and colleagues tested 1,900 women who got the recommended series of three Gardasil shots and 1,900 who got sham injections.
After two years, four women who got the vaccine developed an HPV infection or cervical disease, compared to 41 in the placebo group, they wrote — which translates to an efficacy of more than 90 percent.
HPV is the most common sexually transmitted disease in the world. About 20 million Americans are infected with it, according to the U.S. Centers for Disease Control and Prevention.
It is the main cause of cervical cancer, which kills 3,870 women a year in the United States and 300,000 globally.
It can also cause other types of cancer, including anal and penis cancer, as well as mouth and neck cancer. The CDC estimates that HPV caused 25,000 cases of cancer a year in the United States between 1998 and 2003.
Many women in industrialized countries get regular exams of the cervix, called a Pap smear, to check for precancerous changes caused by the virus.
Gardasil is designed to protect against HPV types 16 and 18, which are known to cause about 70 percent of all cases of cervical cancer. It also is designed to protect against HPV strains 6 and 11, which cause genital warts.
Gardasil is approved in the United States for use in girls and women ages 9 to 26, but Merck is seeking to expand its use to older women. The vaccine does not protect anyone who has already been infected with one of the strains of HPV.
Vaccinating women over age 26 has not been approved by the U.S. Food and Drug Administration and is not included in U.S. CDC guidelines.
Munoz's team noted that older women may, however, be at risk.
"Changes in sexual behavior during the past 30 years, characterized by rising age at first marriage and an increase in divorce rates, have led to more widespread premarital sexual intercourse and acquisition of new sexual partners around middle age, respectively," they wrote.
"Published work suggests that in the USA, nearly 40 percent of men and women have married and divorced by 55 years of age, and that more than 25 percent of these people have remarried at least once," they added.
"As the potential for HPV infection and disease exists in women in their third, fourth, and fifth decades of life, these women could benefit from prophylactic HPV vaccination."
A mathematical model published in October showed that vaccinating older women against cervical cancer could cut rates in half for women through age 45.