Merck says Gardasil is effective for at least five years. It says the vaccine works best in those who are not yet sexually active.

The company is also testing Gardasil in boys and men.

The vaccine is given in three injections over a six-month period. Gardasil is expected to cost three hundred to five hundred dollars. There are concerns that the price could limit its use. Also, some conservative groups have expressed concern that the vaccine could lead young people to become more sexually active.

Later this year, the drug maker GlaxoSmithKline plans to seek approval of a cervical cancer vaccine called Cervarix.

This VOA Special English Health Report was written by Cynthia Kirk. I'm Barbara Klein.