This week, the New England Journal of Medicine publishes two studies on the new Human Papilloma Virus vaccine. The FDA approved the vaccine last year. It protects against the viruses, which cause 70 percent of cervical cancer.
The studies show the vaccine is highly effective for women who've never been exposed to HPV. Nonetheless, some health experts say there are still many unanswered questions about the vaccine.
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 obstetrician-gynecologist at University of California, San Francisco. But, he points out, 17 percent fewer lesions translates into a small reduction in the risk of disease.
"Overall efficacy of the vaccine impact on pre-cancer rates is modest," Sawaya says. "It reduced the risk of pre-cancerous lesions from 1.5 percent to 1.3 percent. That's fairly small change in risk."
Laura Koutsky, an epidemiologist at the University of Washington in Seattle, chaired the committees that oversaw these studies. She explains that the 17 percent reduction in lesions was among all women in the study, including those who had already been sexually active and had probably already been exposed to HPV. The virus is transmitted during sex.
Looking only at women who had never had sex and also at women who were not exposed to HPV, Koutsky says, the vaccine was extraordinarily protective.
"When you looked at the group who had not been exposed, the vaccine efficacy was over 95, 98, even100 percent for preventing HPV-related lesions," Koutsky says.
Typically, about 40 percent of pre-cancerous cervical lesions turn into invasive cancer over a 10-year period.
These results come from three years of study. Sawaya says he wants to see much longer-term studies to answer a number of questions. For example, how long does the vaccine immunity last?
"We don't know if this vaccine needs booster shots, for example," Sawaya says. "If a girl is vaccinated at age 11, it's not clear whether she will still be protected if she becomes sexually exposed at the age of 20."
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 saw one safety problem that raised a question.
"There was an occurrence of a rare vulvar cancer in one vaccinated woman in these studies," Sawaya says. "It's quite rare but is thought to be associated with HPV. Therefore, that requires more investigation, more study and certainly a more cautious approach."
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. After the FDA approval of the vaccine, the federal Centers for Disease Control and Prevention recommended that all 11- and 12-year-old girls in the United States be vaccinated.
Dr. Connie Trimble is an obstetrician-gynecologist at Johns Hopkins University who specializes in cervical cancer. She says that the vaccine itself isn't dangerous.
"It mimics a virus but it's empty... It's like a Trojan Horse. It takes advantage of phenomenon called immunologic memory," Trimple explains. "So once your immune system recognizes something, the idea is that the second time it sees it, it reacts very quickly, sort of saying, 'Oh, I know that,' and can clear it or eliminate it."
Since the vaccine does not protect against all HPV strains that can cause cancer, Trimble says it's still important for women over 21, or those who are sexually active, to be screened for cancer with a yearly pap smear.