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This week, the New England Journal of Medicine is publishing two studies on the new Human Papilloma Virus vaccine. The FDA approved the vaccine last year. It protects against the viruses that cause 70 percent of cervical cancer.

The new studies show that the vaccine is highly effective. But NPR's Patty Neighmond reports some health officials say there are still many unanswered questions about the vaccine.

PATRICIA NEIGHMOND: More than 17,000 women between the ages of 16 and 26 were involved in the studies. Half were vaccinated against HPV. Half were not. After three years, those who were vaccinated saw a 17 percent reduction in pre-cancerous cervical lesions.

That's promising says Dr. George Sawaya, an epidemiologist and OB/GYN at University of California, San Francisco. But, Sawaya points out that 17 percent fewer lesions translates into a small reduction in the risk of disease.

Dr. GEORGE SAWAYA (Epidemiologist and OB/GYN, University of California, San Francisco): The overall efficacy of the vaccine on pre-cancer rates is modest, reducing risk of pre-cancer from 1.5 percent to 1.3 percent. It's a fairly small change in risk.

NEIGHMOND: Laura Koutsky is an epidemiologist at the University of Washington in Seattle. She explains the 17 percent reduction in lesions was among all women in the study, including those who had already been sexually active and so were likely to have been exposed to HPV.

Looking only at women who had never had sex, and therefore were not exposed to HPV, Koutsky says, the vaccine was extraordinarily protective.

Dr. LAURA KOUTSKY (Epidemiologist, University of Washington in Seattle): When you looked at the group who had not been exposed, the vaccine efficacy was over 95 percent, so is 98 to 100 percent for preventing the HPV-related lesions.

NEIGHMOND: OB/GYN George Sawaya says he wants to see much longer-term studies to answer a number of questions. For example, how long does the vaccine's immunity last?

Dr. SAWAYA: We don't know if this vaccine needs booster shots, for example. If a girl is vaccinated at age 11, it's unclear whether or not she will still be protected if she becomes sexually exposed at the age of 20, for example.

NEIGHMOND: Sawaya says studies in 11- and 12-year-old girls need to be conducted because that's the group this vaccine is recommended for. While the studies published today showed no side effects, Sawaya says longer-term studies are needed to make certain there aren't safety concerns.

In this study, Sawaya did see one issue that raised a question.

Dr. SAWAYA: There was an occurrence of a rare vulvar cancer in one vaccinated woman in these studies. Again, it's quite rare but is thought to be associated with HPV. So this requires further investigation, and further study and a cautious approach.

NEIGHMOND: Cautious because, in this study, there was one rare cancer among 2,500 women when that type of cancer typically occurs in one in every 100,000 women.

Connie Trimble is also an OB/GYN. She specializes in cervical cancer at Johns Hopkins University. Trimble says the vaccine itself is not dangerous.

Dr. CONNIE TRIMBLE (OB/GYN, Johns Hopkins University) It mimics the virus but it's empty. It's not what it makes it go or anything. It's not the DNA. It's like a Trojan horse, but it's empty. So once your immune system recognizes something, the idea is the second time it sees it, it reacts very quickly, like: Oh, I know that. And can clear it or eliminate it.

NEIGHMOND: Since the vaccine does not protect against all HPV strains that can cause cancer, Trimble says it's still important for women over 21, and those who are sexually active, to be screened for cancer with a yearly pap smear.

Patty Neighmond, NPR News.

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