New data on the controversial HPV vaccine designed to prevent cervical cancer have raised questions about its efficacy, researchers are reporting Thursday, undercutting efforts in many states to make vaccination mandatory.
Although the Merck vaccine, called Gardasil, blocked nearly 100 percent of infections by the two HPV strains it targets, it reduced the incidence of cancer precursors by only 17 percent overall.
Part of the reason was that many of the teenage girls and young women in the three-year study already had been exposed to the virus, according to the report in the New England Journal of Medicine.
But the data also hinted that blocking the targeted strains may have opened an ecological niche that allows the flourishing of HPV strains previously considered to be minor players, partially offsetting the vaccine’s protection.
In an accompanying editorial, Dr. George Sawaya and Dr. Karen Smith-McCune of the University of California, San Francisco, called the benefits of the vaccine “modest” and said young women and their parents should take “a cautious approach” to vaccination.
“The effect is fairly small,” Sawaya said in an interview. “The recommendation for widespread vaccination of women after they become sexually active may need to be re-thought.”
The maker of the vaccine, Merck & Co., said the studies clearly showed that the vaccine prevents infections from the two HPV strains and reduces the number of pre-cancerous lesions caused by the them.
Immunologist Martin Kast of the University of Southern California’s Keck School of Medicine, who was not involved in the research, said he believed the study had not gone on long enough to prove the vaccine’s worth.
“In a three-year follow-up, it is very hard to reach statistical significance in a disease process that takes about a decade to fully develop,” he said.
Gardasil was approved by the Food and Drug Administration in June 2006 amid reports that it could largely prevent cervical cancer among vaccinated women.
The Centers for Disease Control quickly recommended that all women age 11 to 26 receive the vaccine, and the American Cancer Society seconded that recommendation.
Cervical cancer kills about 3,900 U.S. women each year, a number that has been reduced substantially by widespread Pap screening. In the rest of the world where Pap screening is too expensive, however, it kills at least 250,000 annually.
It is caused primarily by the human papillomavirus or HPV. About 20 strains of the virus are known.
