The U.S. Food and Drug Administration has approved a vaccine to protect millions of women against cervical cancer. The vaccine, called Gardasil, is being hailed as a major medical breakthrough.

Deputy Health and Human Services Secretary Alex Azar announces the approval of Gardasil at HHS headquarters in Washington

Human papilloma virus, or HPV, is among the most common sexually transmitted diseases in the world, typically affecting women in their late teens and mid-20's. The infection does not cause a problem in 70 to 90 percent of the women.

But in the remainder, HPV causes cellular changes to the cervix, or entrance to the womb, that can lead to cancer. The disease is the second leading cause of cancer death among women worldwide, accounting for more than 300,000 deaths per year.

An HPV vaccine keeps the infection in check.

In clinical trials, the vaccine, which was approved by the Food and Drug Administration and will be marketed as Gardasil by the drug maker Merck, was shown to be 100 percent effective in protecting women infected with HPV from developing cancer.

Diane Harper of Dartmouth University's Gynecologic Cancer Prevention Group in New Hampshire headed up the clinical trials in the development of the vaccine.

Harper says the drug is targeted at the two most destructive types, HPV 16 and 18, and appears also to protect women from two lesser HPV viral strains.

"So with a single vaccine that's directed to two types, it looks like we can actually get protection to four types, which would allow us then to be able to hopefully protect against 80 percent of all cervical cancers, actually a little more than 80 percent," said Diane Harper.

The FDA approved the vaccine for women and children between the ages of nine and 26.

Giving the vaccine to children as young as nine has caused some controversy in the United States because some people do not believe a vaccine that prevents a sexually transmitted disease belongs in pediatric medicine.

But Richard Haupt, of Merck's Vaccine Division, says studies show that the vaccine is most effective when given to women years before they become sexually active.

"The younger age group is actually very critical to be vaccinated because you're going to get them and you are going to get them before they are exposed to these very important HPV types," said Richard Haupt.

Another pharmaceutical company, GlaxoSmithKine, is working on its own cervical cancer vaccine and is expected to seek regulatory approval soon.