What if you could protect your child from a potentially life-threatening disease with a simple vaccination, but administering those shots might encourage her to engage in behavior that, statistically speaking, would be far more likely to cause her grave harm?

Parents of young girls face precisely this dilemma in deciding whether to immunize their pre-teen daughters with the new vaccine to prevent human papillomavirus (HPV), a sexually transmitted disease that can cause cervical cancer. Now, to complicate matters further, 20 states are considering mandating such vaccinations, even insisting girls can't attend school unless they've been inoculated; and one state -- Texas -- already requires the shots (effective September 2008).

Gardasil, an anti-HPV vaccine produced by Merck & Co., was recently approved by the Food and Drug Administration, and the company is aggressively marketing it through television ads and lobbying state legislatures to mandate vaccinations of all young girls. Gov. Rick Perry of Texas, who signed an executive order requiring sixth-grade girls to receive the shots, is under fire because his former chief of staff is now a lobbyist for Merck.

The problem is the vaccine must be given before a girl is sexually active to guarantee immunity from the most common forms of the virus (there are 40 types of genital HPV, and Gardasil only protects against four, though two of these cause about 70 percent of cervical cancer cases in the U.S.). But the question is: What age is appropriate, especially since the vaccine may only provide protection for five years and no booster currently exists (though one is in the works)?

The makers of Gardasil and their advocates in state legislatures are operating on the assumption that all girls will become sexually active in their teen years. According to the Centers for Disease Control and Prevention, about two thirds of girls have had sex by the time they are seniors in high school -- which means at least one third haven't done so. But if the school starts immunizing girls as young as 9, as the makers of Gardasil recommend, doesn't it send a very strong message that the school expects those girls will be sexually active before they hit 14, since the drug won't necessarily be effective after that age?

And the message may be even more provocative if the parents make the choice voluntarily. It may say to the girl, "Mommy and Daddy think you'll probably lose your virginity before your 14th birthday, so we are going to make sure you're vaccinated against HPV when you're 9 years old."

There is no question that parental expectations play a role in teenage sexual behavior. An annual poll taken by the National Campaign to Prevent Teen Pregnancy, for example, shows that 87 percent of teens think it would be easier to postpone sex and avoid teen pregnancy if they had open, honest discussions with their parents. And a whopping 92 percent say they think "society should provide them with a strong message to not have sex until they are at least out of high school."

Of course, "society" sends exactly the opposite messages in advertising, television, film and popular music.

What makes the campaign to vaccinate pre-pubescent girls against HPV all the more puzzling is what we know about current cervical cancer rates. Cervical cancer, while a killer worldwide, has been reduced dramatically in the U.S. by screening women through the use of a simple test, the Pap smear. Since 1955, cervical cancer rates are down 74 percent.

Fewer than one in 10,000 women over 18 will get cervical cancer each year, and only about one in 30,000 will die from it, mostly those who failed to be tested on a regular basis and, therefore, didn't discover they had the disease until its later stages. Moreover, of those who do end up with cervical cancer, the majority contracted HPV, which usually takes 10-20 years to cause cancer, in their 20s or 30s.

Parents ought to be able to make decisions about when to vaccinate their daughters against HPV on their own, without pressure from schools or lawmakers. Responsible parents might even use the occasion to open a frank discussion with their daughters about the dangers of all sexually transmitted diseases and the value of delaying sexual activity.