A vaccine that protects women against a common sexually transmitted infection that leads to cervical cancer is now one step closer to reality.

The results of a major clinical trial of the vaccine show that it was 100 percent effective in preventing persistent infection with two strains of the human papillomavirus (HPV) that account for the majority of cervical cancer cases.

Nearly half a million cases of cervical cancer are diagnosed each year worldwide, and the disease accounts for about 230,000 deaths annually. Researchers say that if all eligible women got the vaccine, it would reduce the worldwide incidence of cervical cancer by nearly 70 percent.

“This is an incredible advance for women’s health, and it will change the way in which women’s preventive health and gynecologic exams will be planned and scheduled for the future,” says researcher Diane Harper, MD, MPH, director of the gynecologic cancer prevention research group at the Norris Cotton Cancer Center, in Lebanon, N.H.

Although many other cancer vaccines are currently being studied, experts say the current cervical cancer vaccine is the most effective and practical.

“As far as a cancer vaccine goes this is the best we have,” says Kevin Ault, MD, associate professor of obstetrics and gynecology at University of Iowa, who has been researching the HPV vaccine.

How HPV Vaccine May Prevent Cervical Cancer

In the same way that annual Pap smears for cervical cancer screening have dictated the routine of gynecologic exams for sexually active women in the last few decades, researchers say the HPV vaccine has the potential to switch the focus to cervical cancer prevention in pre-adolescent girls within the next decade.

In the 1930s, cervical cancer was the leading cause of cancer death in the U.S. Midway through the 20th Century Pap tests were introduced that screened for the presence of abnormal growths or lesions in the cervix that were likely to become cancerous.

But in the last 30 years, researchers have found that most cervical cancer cases are caused by infection with a small number of particularly high-risk strains of HPV, while most other strains are harmless.

That realization has quickly focused researchers’ efforts on developing a vaccine that could boost the body’s immunity to these high-risk HPV strains and, therefore, prevent cervical cancer.

“There is such as strong association between human papillomavirus and cervical cancer that the assumption would be that the great majority of cancers associated with these HPV strains could be prevented by vaccination with vaccines that target those strains,” say Christopher P. Crum, MD, director of women’s and perinatal pathology at Brigham and Women’s Hospital in Boston.

Cervical cancer vaccine researcher Ault says that while his grandmother may have lost many of her friends to cervical cancer in her era, and his wife benefited from screening with the Pap test in hers, he expects his daughters to benefit from this HPV vaccine.

“In four or five generations we’ve gone from the cervical cancer being the most common cause of cancer death in the United States for women to being a vaccine-preventable disease,” says Ault. “That’s the really remarkable story if you trace this research over the last 50-60 years.”

Latest HPV Vaccine Highly Effective

In this phase of the study on the HPV vaccine, researchers tested the effectiveness of the vaccine in preventing HPV infection in more than 1,100 women aged 15-25 years in North America and Brazil. The vaccine, known as Cervarix, is given in the form of a shot in the arm.

The women were divided into two groups, one that received three doses of the HPV vaccine (at zero, one, and six months) and another group that received three doses of a placebo. The results appear in the Nov. 13 issue of The Lancet.

The study showed that among women who received all three doses of the HPV vaccine, it was 100 percent effective at preventing persistent HPV infection with either HPV 16 or HPV 18 strain.

But the study also showed that the vaccine was also highly effective when women received only one dose or didn’t follow the recommended vaccination schedule. Among these women, the vaccine was 95 percent effective in preventing persistent HPV 16/18 infection, 93 percent effective in preventing abnormal Pap tests associated with these infections, and completely effective in protecting against cervical tissue abnormalities associated with HPV 16/18, which were considered precancerous lesions.

“I was very surprised at how effective it was,” says Harper, who is also associate professor of community and family medicine and obstetrics and gynecology at Dartmouth Medical School.

Harper says the vaccine appeared to work just as they had hoped by preventing the HPV infection and subsequently protecting against abnormal Pap tests and precancerous lesions on the cervix.

“If you prevent the HPV, you’re not going to get the infection, you’re not going to get the abnormal Pap smear, and you’re not going to get the [tissue abnormalities], and it did that,” says Harper. “It’s very exciting when that happens in science.”

No significant side effects were associated with the HPV vaccine other than minor irritation at the injection site.

Cervical Cancer Vaccine Nearing Reality

The next test for the vaccine is a large, phase III clinical trial, which is currently underway. This phase of the study will attempt to replicate these results in a larger number of women, about 15,000.

If that study goes well, experts anticipate that the vaccine may be ready to submit for FDA approval by 2006. However, preparing the vaccine for the market and implementing a widespread vaccination program will likely take much longer.

Initial vaccination efforts will likely target girls aged 10-12 years who have not become sexually active yet. Future efforts may target older women in areas where the cervical cancer is more widespread, such as sub-Saharan Africa and Latin America.

That’s because the vaccine only provides cervical cancer protection for women who have not been exposed to HPV, and it only protects against the two strains in the vaccine, HPV 16 and HPV 18.

“If they started vaccinating only young people before the start of sexual activity, it would be about three to four decades before you saw major changes in cervical cancer rates,” says Crum. “But the changes would nonetheless be significant, and this would have a major impact, particularly in developing countries where you don’t have the Pap smear.”

In an editorial that accompanies the study, Matti Lehtinen and Jorma Paavonen of the National Public Health Institute in Helsinki, Finland, say an HPV vaccine will “probably be the first licensed vaccine against a common sexually transmitted infection.”

They say long-term follow-up studies are needed to prove that HPV vaccination ultimately protects against cervical cancer, but these results show that preventive vaccination against the types of HPV most closely associated with cervical cancer will soon be available.

The study was funded by GlaxoSmithKline, which manufactures the Cervarix vaccine. GlaxoSmithKline is also a WebMD sponsor.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Harper, D. The Lancet, Nov. 13, 2004; vol 364: pp 1757-1765. Diane Harper, MD, MPH, director, gynecologic cancer prevention research group, Norris Cotton Cancer Center; associate professor of community and family medicine and obstetrics & gynecology, Dartmouth Medical School. Kevin Ault, MD, associate professor of obstetrics and gynecology, University of Iowa. Christopher P. Crum, MD director of women’s and perinatal pathology division, Brigham and Women’s Hospital, Boston. WebMD Medical News: Vaccine May Reduce Cervical Cancer Rates.