WEDNESDAY, Aug. 20 (HealthDay News) -- Vaccinating all 12-year-old girls against the virus that causes cervical cancer, along with "catch-up" immunizations of women under the age of 21 and revised screening guidelines, would be cost-effective ways to combat the disease, a new study by Harvard researchers suggests.

But this conclusion, published in the Aug. 21 issue of the New England Journal of Medicine, is based on computer modeling rather than "real world" data, leading some experts, including the authors of a related editorial, to question the advisability of implementing widespread vaccination programs.

"For this generation of teenagers, we're not going to have data for the next 15 or 20 years on how effective it [a cervical cancer vaccine] is, what it changes about the dynamics of cervical cancer, how long the vaccine lasts, whether you can then extrapolate this to decreasing screening guidelines," said Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. "We need more long-term data from this generation of girls getting vaccinates now [to] make decisions on a population basis."

The study data may also not be relevant to individual patients -- for example, a 28-year-old woman who has not yet become sexually active, Wu said.

Human papillomavirus (HPV) is a sexually transmitted virus that causes cervical cancer. Three-quarters of U.S. women will be exposed to HPV at some point in their lifetime and, at any one time, one-quarter have been infected.

Two strains of HPV -- HPV-16 and HPV-18 -- cause roughly 70 percent of cervical cancer cases worldwide. HPV also causes 90 percent of anal cancers, 40 percent of vulvar and vaginal cancers, and 3 percent of oral cancers.

The first vaccine against the virus, Gardasil, was approved in 2006. The U.S. Centers for Disease Control and Prevention currently recommends that 11- and 12-year-old girls be targeted for this vaccine, as most girls of this age are not yet sexually active, have not yet been exposed to HPV, and will therefore achieve maximum protection.

The authors of the new study used computer models to compare vaccinating preadolescent girls (12 years old) with vaccinating older girls and women (up to ages 18, 21 or 26) in "catch-up" programs. Among other things, the models assumed that the vaccine would confer complete immunity against cervical cancer.

"Our main findings were that preadolescent vaccinating targeting 12-year-old girls is fairly attractive," explained study leader Jane Kim, an assistant professor of health decision science at the Harvard School of Public Health. "We found consistently that catch-up vaccination up to the age of 18 was cost-effective, that extending catch-up vaccinations in women to the age of 21 was cost effective when we made very generous assumptions about vaccines' properties, and that, generally speaking, the catch-up up to age 26 was not cost-effective."

The cost-effectiveness of vaccinating 12-year-old girls was $43,600 per quality-adjusted life-year, or QALY, (under $50,000 is generally considered cost-effective). This increased to $97,300 per QALY when the vaccine was extended to girls up to age 18.

The model also found that cervical cancer rates would decrease and that screening for the cancer could begin later and occur at less-frequent intervals.

But even the study authors stressed that these conclusions are tentative.

"There's a lot of uncertainty in the data, and we had to make a lot of assumptions about the vaccine's properties," Kim explained. "We made very optimistic assumptions, and we haven't looked at the vaccine long enough to know how these assumptions will bear out."

More information

The CDC has more on HPV vaccination.