In documents posted online, the Food and Drug Administration said Cervarix - Glaxo's vaccine against human papilloma virus or HPV - successfully blocked the two leading strains of the virus nearly 93 percent of the time.
But even as the British drugmaker moves closer to competing in the U.S., Merck is poised to begin marketing Gardasil to boys and men. In a separate review, the FDA said that vaccine prevented genital warts in males 90 percent of the time.
The agency will ask a panel of vaccine experts next week to weigh in on both vaccines. The FDA is not required to follow the group's advice, though it usually does.
While Merck has suggested the approval for boys could double the potential market for Gardasil, Leerink Swann analyst Seamus Fernandez said the benefit likely will be minimal.
Genital warts caused by HPV usually clear up by themselves, and the cancers caused by the virus are extremely rare in men.
"If it's not preventing something serious like cervical cancer and there are questions about safety, I think parents' acceptance of the vaccine in young boys might be less urgent than for their girls," said Fernandez.
Glaxo has won a number of government contracts for Cervarix in Europe, but its U.S. launch was delayed in 2007 when the FDA said it needed more data about the vaccine. Earlier studies of Cervarix showed a higher number of muscular and neurological problems among patients who used the vaccine compared with an alternate treatment.
The FDA said Friday that outside experts have now determined Cervarix did not cause those problems.
"The conclusion in the case of each of these efforts was that the data are not sufficient to establish a link," the agency said in its review.
Glaxo wants its vaccine approved for girls and women ages 10 to 25. The FDA is expected to make a decision on the vaccine later this year, but analysts say Glaxo may still have trouble making inroads into the U.S. market.
Gardasil and Cervarix both defend against HPV strains 16 and 18, which cause about 70 percent of cervical cancer cases. But Merck's vaccine also defends against two other HPV types that cause 90 percent of genital warts, something Cervarix does not target.
Gardasil became an early success story for Merck after its 2006 launch, achieving sales that are rare for a vaccine. The company has sold about 50 million doses worldwide, with more than $1.4 billion in revenue last year.
But sales have been slowing amid questions about the longevity of the vaccine's effect and its price tag of nearly $400. In the most recent quarter, Gardasil sales fell to $268 million, down 18 percent from the prior year period.
A government-funded study last year found that the HPV vaccine is cost-effective for young teens, but not for women in their 20s. And labeling for the vaccine notes that the "duration of protection of Gardasil has not been established." Company trials have only tracked immunity out to five years after receiving the three-injection regimen.
Whitehouse, N.J.-based Merck will make the case on Sept. 9 for approving the vaccine for boys and men ages 9 to 26.
But the company is expected to face push-back next week from public health activists who already question the vaccine's benefit for its main target market: women.
Some argue that $375 is too much to pay to protect against genital warts and diseases like penile cancer, which affect less than 1 percent of American men.
"It's a lot of money for a little bit of extra protection from a disease that is not a big problem in this country," said Dr. Diana Zuckerman of the Research Center for Women and Families in Washington. Zuckerman, who is scheduled to speak at next week's meeting, says the additional use will do more to boost Merck's sales than control HPV-related diseases.
"This is great for the company. The more people use this vaccine, the more money the company makes," Zuckerman said.
Cancer of the penis and anus are extremely rare, with less than 2,000 cases reported annually. But Merck argues the cost of treating those cases is often underestimated.
"When I talk to people I often say the burden of these diseases in men is understated and unrecognized," said Dr. Richard Haupt, Merck's head researcher for Gardasil. "Gardasil is highly efficacious and we should see an impact on men's health because of that."
Each year about 6 million people in the U.S. contract HPV, which usually causes no symptoms and goes away within two years. While 40 strains of the virus are spread through sexual contact, only about 15 cause cancer in men and women.
Last year nearly 4,000 women died of cervical cancer in the U.S., less than 1 percent of all cancer related deaths.
"Protection against cervical cancer is a very important health need for girls and young women, and we believe Cervarix has an important role to play in addressing this," said Glaxo spokeswoman Lisa Behrens.