Q: Scientists have announced an experimental vaccine that, in trials, proved 100 percent effective in protecting women from most cervical cancers. How does it work?

A: Cervical cancer—which kills some 3,700 American women a year—is caused by human papillomavirus (HPV), an extremely common sexually transmitted infection. The vaccine blocks the two types of HPV that most often develop into cancer.

Q: Will it work for all women?

A: The vaccine, which may get FDA approval by late 2006, could be helpful for any female who is or plans to be sexually active. The three-shot series may also prevent hard-to-treat genital warts.

Q: When is the best time to be vaccinated?

A: Optimally, before becoming sexually active—even in adolescence—since 80 percent of women will in time be infected with HPV. Although most show no symptoms and develop a natural immunity, some 10,400 women a year are diagnosed with cervical cancer.

Q: What if I have already been infected with HPV?

A: The vaccine will not reverse infections you already have, but it may stop new ones, says Dr. Eliav Barr, who heads the vaccine research for Merck & Co.

Q: How long does the shot last?

A: Unclear. In tests, women remained protected for 3½ years. Scientists will follow up to see if a booster is needed.

Q: Does this mean the end of Pap smears?

A: No. Strains of HPV that this vaccine does not prevent still account for 30 percent of cervical cancers, so women will need to get screened at least once every three years.