Merck & Co., looking for a boost to fight off its Vioxx-related losses, could get it from the first big advance against cervical cancer in a half-century.

The drugmaker is awaiting approval soon for Gardasil, a vaccine which blocks infection by the four human papilloma virus types that cause most cervical cancers and genital warts.

The Whitehouse Station, N.J., company has been touting Gardasil as its next big product and is already running ads about HPV.

Merck needs a hit, with lawsuits over its withdrawn painkiller Vioxx topping 11,500 and its biggest seller, the anti-cholesterol drug Zocor, facing generic competition shortly.

On Thursday, a panel of Food and Drug Administration advisers will discuss recommending the vaccine against HPV, the most common sexually transmitted disease, for females 9 to 26, the first groups tested. The FDA's decision is expected by June 8.

Merck has already begun laying the groundwork, though. Its "Tell Someone" TV and magazine ad campaign, launched in late April, ironically uses what's called "viral marketing," urging women to tell others about HPV, with magazine pull-out postcards about it in English and Spanish. Merck plans a similar campaign, plus materials for doctors' offices, once the vaccine is approved.

"This is exceedingly important for Merck," said Steve Brozak, health care analyst at the WBB Securities. "This does carry blockbuster potential," with perhaps $1 billion in eventual annual revenues.

Doctors say the vaccine is even more important for public health.

Human papilloma virus is the No. 2 cancer in women, killing nearly 300,000 a year, most in developing countries where women don't get the regular Pap smears that detect precancerous lesions and early cancer. Cervical cancer each year kills about 3,500 American women, mostly in their 40s and 50s.

"This would be the first significant advance since the introduction of the Pap smear" in the 1950s, said Dr. Bobbie Gostout, an obstetrician-gynecologist at the Mayo Clinic.

She said the five-year U.S. survival rate for women with cervical cancer has only risen from 70 percent in 1975 to 73 percent in 2000. The immune system clears the virus in most women, but the rest have an 800-fold higher risk of developing cervical cancer or precancerous lesions. About 80 percent of young women are infected with HPV five years after becoming sexually active, and it can cause cancers in men.

Dr. Thomas Wright, a gynecological pathologist at Columbia University, said the vaccine's biggest impact initially will be in preventing genital warts and sparing roughly a million U.S. women a year the stress and cost of abnormal Pap smears and followups, including removal of precancerous lesions.

Experts estimate up to two-thirds of cervical cancers could be prevented if the vaccine is widely used — but not for years because cervical cancer can take two decades to develop.

In late June, the national Advisory Committee on Immunization Practices will decide whether to endorse routine vaccination. Its HPV vaccine workgroup is recommending that for girls 11 and 12, and the committee will consider recommendations for females 13 to 26, said Dr. Lauri Markowitz, who heads the workgroup's Centers for Disease Control and Prevention contingent.

She said vaccinating older women and males, who can pass the virus to their sex partners, will be considered if current studies in those groups lead the FDA to approve the vaccine for them.

Testing shows Gardasil is nearly 100 percent effective and works for at least five years, said Dr. Eliav Barr, Merck's head of biological clinical research. He said some of the first women and girls who got the vaccine will be followed for years to see if booster shots are needed.

The vaccine targets four of the roughly 40 types of human papilloma virus: HPV 16 and 18, which cause about 70 percent of cervical cancer cases, and HPV 6 and 11, which cause 90 percent of genital warts.

Rival GlaxoSmithKline is testing a vaccine against types 16 and 18, Cervarix, on females aged 10 to 55, and plans to seek U.S. approval by year's end. The British drugmaker also is evaluating targeting other HPV types.

Some research indicates parents and doctors generally support giving kids the vaccine, but how young remains an issue. Early opposition from conservative groups has dissipated. They now say they oppose requiring the vaccine for school attendance, and that abstinence before marriage should be stressed.

With Merck's Gardasil requiring three shots over six months, the estimated $300 to $500 total cost likely will be a bigger issue.

"This is going to be an enormous challenge," unless government and commercial insurance plans cover the shots, said Cynthia Dailard, a senior policy analyst at the nonprofit research group Guttmacher Institute.