Government scientists have created a potential vaccine against a virus that causes cervical cancer, a promising development against a cancer that each year kills 200,000 women worldwide, including 5,000 Americans.

The experimental vaccine just passed its first human safety test but has years more testing ahead to prove if it does protect women against cancer.

Still, "the prospects for this vaccine are remarkably promising," said Dr. Harald zur Hausen, a cervical cancer expert in Heidelberg, Germany. He reviewed the research in Tuesday's Journal of the National Cancer Institute.

Until a vaccine becomes reality, a widely available virus test is highly effective at telling which of some 2 million American women with inconclusive Pap smears each year need further cancer exams - and who can relax, says a second study in Tuesday's journal.

Digene Corp.'s Capture test is "a very good option," NCI chief researcher Dr. Diane Solomon said.

Not everyone needs virus testing, Solomon stressed. Most American women with cervical cancer didn't get yearly Pap smears, which are remarkably successful at catching precancerous cells in time to prevent cancer.

"Women have to understand the importance of regular Pap testing as the first step," she said.

But when that exam isn't conclusive, Digene's test may help those women decide if they need a biopsy - or, by ruling out viral infection, provide "excellent reassurance" that they're healthy and should just continue regular Pap testing.

Human papillomavirus, or HPV, is a sexually transmitted virus that infects some 40 million Americans. There are over 80 strains, the vast majority symptomless and harmless. But some strains cause cervical cancer, including HPV-16, considered the riskiest and the one the experimental vaccine targets.

Cervical cancer strikes 400,000 women worldwide every year, including almost 13,000 Americans. The global toll is much higher because women in developing countries cannot afford those $25 Pap smears, so doctors there are hoping anxiously for a vaccine.

Paps aren't perfect - about 2 million of the 50 million performed each year in the United States are inconclusive. The vast majority of those women prove to be healthy, but doctors struggle over how to catch the precancer cases without subjecting too many healthy women to unnecessary biopsies or repeat Paps.

That's where HPV testing comes in. The government approved Digene's $50 test in 1999 as an aid in the diagnosis dilemma and about 15 percent of inconclusive Pap patients get it, but should more?

Tuesday's studies shed light on both fronts:

NCI researcher Dr. Douglas Lowy and colleagues created a vaccine against HPV-16 that proved so successful at preventing infection in animals that they tested it in 72 healthy people. This Phase 1 safety testing found no serious side effects; the main complaint was mild injection-site pain.

Better, most participants' blood developed 40 times more virus-fighting antibodies after vaccination than do people naturally infected with HPV. Those are "sky-high levels," and "the higher the immune response, the more likely it is that you will get protection," Lowy explained.

To prove if the vaccine prevents HPV-16 infection and consequently reduces cancer, NCI researchers plan to begin studying thousands of women next year in Costa Rica, where cervical cancer is far more prevalent. The study will take up to eight years - and even if it succeeds, doctors must develop vaccines against other cancer-causing HPV strains, too, Lowy cautioned.

For today, Solomon's study of 3,488 women shows HPV testing is an option that can help women worried about an inconclusive Pap. She compared how well Digene's HPV test or a repeat Pap predicted which women needed a biopsy.

Among those ultimately diagnosed with precancer, Digene's HPV test was 96 percent accurate, compared with a repeat Pap's 85 percent accuracy.

More good news: Women who tested infection-free had a 99.5 percent probability of being healthy, meaning the 45 percent of women studied who were HPV-negative safely avoided a biopsy.

Still, "there's no one option that's best in all circumstances for all women," cautioned Solomon, who advises women to discuss the alternatives with their doctors.