(This article was #27 in Discover Magazine’s top 100 stories of 2006.)

The cervical cancer vaccine—the second vaccine after the hepatitis B vaccine to target a sexually transmitted disease—debuted this year. In June the Food and Drug Administration approved the vaccine, distributed under the brand name Gardasil, for girls and women ages 9 to 26. The European Union and Australia have also approved the vaccine, and a similar product to be manufactured by GlaxoSmithKline is in the works.

The vaccines work by inducing antibodies to the human papillomavirus, or HPV, which can cause genital warts and cervical cancer. In large clinical trials, the vaccines were more than 99 percent effective in preventing HPV infection. Like most vaccines, however, they are most effective among people who have not yet been exposed to the virus. An American government advisory panel therefore recommended that Gardasil be given routinely to 11- and 12-year-old girls—and in some cases to girls as young as 9.

Every year cervical cancer kills more than 230,000 women worldwide, about 80 percent of them in developing countries. In the United States, the disease claims the lives of roughly 4,000 women each year. Based on a mathematical model, GlaxoSmithKline claims that immunizing every 12-year-old girl with the vaccine would reduce U.S. cases and deaths from cervical cancer by 70 percent. But some conservative groups have opposed the vaccine, saying it might promote sexual activity.

Poor countries also face a more practical obstacle. The full course of the vaccine—three shots over a six-month period—costs about $360. “The biggest issue will be price,” says John Schiller, a senior investigator at the National Cancer Institute, who did some of the early work that led to the vaccine’s development. “It’s the most expensive vaccine we have.”