A new economic analysis shows that the HPV vaccine, which protects against the viruses that cause most cervical cancers and genital warts, could be a good financial investment in public health if given to those who have the most to gain: preadolescent girls and women up to age 21.

Authors of the study, in today's "New England Journal of Medicine," measured the Gardasil vaccine's value by calculating the cost of giving one person an extra healthy year of life and balancing the expense of vaccinations with the benefits of avoiding cancer, death and related expenses.

The analysis supports some of the Centers for Disease Control and Prevention's guidelines, including its recommendation to routinely vaccinate 11- and 12-year-old girls. Gardasil, made by Merck, is the best bargain if girls are vaccinated before they're exposed to HPV, or human papillo-mavirus, which is spread through sexual contact, says co-author Jane Kim of the Harvard School of Public Health.

More than half of girls have been exposed to HPV by the time they finish high school, says Carol Baker, professor of pediatrics at Baylor College of Medicine and a member of the CDC's Advisory Committee on Immunization Practices.

Because some women could still be protected, it makes economic sense to offer "catch-up" shots to women up to age 21, but not to older women, Kim says. The vaccine was approved in 2006 for women up to age 26.

About 11,000 Americans develop cervical cancer each year and nearly 4,000 die from it, according to the American Cancer Society. The CDC, which recommends catch-up shots through age 26, has no plans to revise its guidelines, says Dale Morse, head of the advisory committee. The CDC considers not just a vaccine's cost, but also its safety and potential to prevent disease.

If the vaccine's immunity fades over time, cervical cancer screening — which has been shown to save lives — could prove to be more cost effective, says Charlotte Haug of the "Journal of the Norwegian Medical Association."

About 40 percent of 11- and 12-year-olds have received the $375 shot, according to Merck. Because the vaccine is new, doctors plan to continue following patients to see how long their immunity lasts. Kim's study was funded by the CDC, the cancer society, the National Cancer Institutes and the Bill and Melinda Gates Foundation.