The findings represent the longest follow-up yet of an experimental human papillomavirus (HPV) vaccine, which has the potential to dramatically lower deaths from cervical cancer.
A spokesman for GlaxoSmithKline tells WebMD that the company plans to seek FDA approval for its HPV vaccine -- Cervarix -- by the end of the year.
"With fast-track status, if all goes well, we could have [approval] within six months," says Phillippe Monteyne, who heads global vaccine development for the company.
The study included 800 women who were vaccinated with either three doses of Cervarix or a placebo.
In November 2004, researchers reported that the vaccine was 100% effective in preventing persistent infection with the two HPV strains that cause about 70% of cervical cancers -- HPV-16 and HPV-18.
In a follow-up, published in the latest issue of The Lancet, they reported that the women given the vaccine continued to be protected four and a half years after receiving the last dose of the vaccine.
The women were also protected against two other HPV strains that have also been associated with cervical cancer -- HPV 45 and HPV 31. This cross-protection occurred even though the vaccine was not designed to target these strains.
About half a million cases of cervical cancer are diagnosed worldwide each year, and an estimated 280,000 women -- mostly in developing countries -- die from the disease.
Researchers estimate that if all eligible women got the vaccine, it would reduce the worldwide incidence of cervical cancer by almost 70%.
"This is a very exciting time," says Diane M. Harper, MD, MPH, who led the study team. "We haven't had an advance like this in cancer care or in women's health for 50 years. This has tremendous potential."
There are still many unanswered questions, most notably who will get the vaccine, when they will be vaccinated, and which vaccine they will get.
But Harper says her findings suggest that the GlaxoSmithKline vaccine may protect women against cervical cancer longer than its competitor.
Harper directs the Gynecologic Cancer Prevention Research Group at Dartmouth Medical School's Norris Cotton Cancer Center. The study was funded by GlaxoSmithKline, but she says she has also done research on the Merck vaccine. GlaxoSmithKline and Merck are WebMD sponsors.
"I'm not trying to pick a fight and say one vaccine is better than the other," she says. "These are both good vaccines, but it is clear that there are real differences between them."
If, as expected, the FDA approves the Merck vaccine in early June, the CDC's Advisory Committee on Immunization Practices is expected to weigh in on who should be vaccinated at its meeting later that month.
It may recommend that preteen girls be given the vaccine, or it may recommend the vaccine for girls in their teens.
HPV infection rates peak among women in their late teens and very early 20s, Harper says.
"Stay tuned for June," American Cancer Society Director of Breast and Gynecological Cancer Debbie Saslow, PhD, tells WebMD. "There are a lot of groups interested in having a voice in how this vaccine is used."
Some conservative groups have been vocally opposed to the vaccine, arguing that it could undermine efforts to discourage sex among teens.
Saslow says other conservative groups have expressed support for the vaccine, although they don't want it to be among the immunizations that schools require.
The Developing World
Immunization strategies for nonindustrialized countries, where 80% of cervical cancer deaths occur, remain to be determined.
In the U.S, the vaccine is expected to cost between $200 and $300 for a three-dose series, although the companies have not said how much they will charge, according to Saslow.
Projections are that without the vaccine, cervical cancer deaths in the developing world will rise dramatically in the coming decades.
The Bill and Melinda Gates Foundation has pledged $50 million for the prevention of cervical cancer in the coming decades, but it is not clear how big a role vaccination will play in its efforts.