(This article was #26 in Discover Magazine’s top 100 stories of 2007.)

This past year, several countries and at least 24 states in the United States introduced laws to mandate vaccination against cervical cancer for preteen girls. Although the vaccine was initially hailed as a breakthrough, urgent proposals to make it mandatory quickly triggered a backlash. “In the long term,” says Susan Wood, a former director of the FDA Office of Women’s Health, “the rush to get this mandated immediately has done more harm to the issue.”

The FDA approved Merck’s Gardasil vaccine in 2006, after clinical trials showed that it protects against four strains of human papillomavirus (HPV), which together cause about 70 percent of cervical cancers and 90 percent of genital warts. HPV is the most common sexually transmitted virus among Americans (and cervical cancer is the second most common cancer in women worldwide). Because the vaccine doesn’t reverse existing exposure, the CDC recommends it be administered before the age of sexual activity—specifically to girls ages 11 to 12. As for women who already have HPV, two separate studies published in May in the New England Journal of Medicine reported that the vaccine’s effectiveness in preventing cervical lesions dropped to 20 percent or less.

Some conservative groups oppose targeting preteens, arguing that because the virus is sexually transmitted, the vaccine will encourage promiscuity. Meanwhile, bioethicists who are skeptical about compulsory vaccination laws note that all other mandated vaccines protect against diseases easily transmitted in schools. “In my opinion, there’s not a compelling ethical reason [to mandate],” says Richard Zimmerman, professor of family medicine at the University of Pittsburgh. “The ethics is the opposite: to strongly recommend, but not to mandate.”