But when Koutsky and her colleagues considered lesions caused by all strains of the virus, the vaccine reduced the risk by only 17%.
Because researchers had previously believed that 50% of all serious precancerous lesions were caused by types 16 and 18, this rate of protection seemed inexplicably low.
For The Record
Los Angeles Times Friday May 11, 2007 Home Edition Main News Part A Page 2 National Desk 1 inches; 37 words Type of Material: Correction
HPV vaccine: An article in Thursday's Section A about studies on human papillomavirus said that one by Johns Hopkins University researchers looked at 100 women with newly diagnosed throat cancers. The 100 patients were men and women.
Sawaya suggested in the editorial he co-wrote that the small size of the protection could be because other strains of HPV are filling the gap when types 16 and 18 are eliminated.
Dr. Douglas Lowy of the National Cancer Institute, who originally developed Gardasil, noted that even if the other types proliferate, there might not be a significant increase in cancers because the other strains are less carcinogenic.
If the other strains are, in fact, proliferating, USC's Kast said, "then the addition of other HPV types to the vaccine would be a logical option." Merck and GlaxoSmithKline are working on that.
Koutsky noted, however, that 17% was an improvement over the 13% figure that was presented to the FDA more than a year ago, which resulted in the administration's approval.
As time proceeds, she speculated, the gap between the protected and unprotected women would continue to grow.
Overall, the new results indicate that the vaccine is not living up to its initial prospects. The findings show that 129 women would have to be vaccinated to prevent one precancerous lesion.
When doctors give the vaccine to patients, they think that " 'I am protected against everything,' and that's just not true," said Dr. Diane M. Harper of Dartmouth University, who helped design a related Merck-funded HPV study in the journal.
She is still in favor of giving Gardasil to girls because it is safe and it "protects against the main HPV bad actors," but she argued that neither doctors nor women should be lulled into a false sense of security by the shots.
"I don't think this is the gun that is going to take cervical cancer off the map," she said.
Dr. Ursula Matulonis of the Dana-Farber Cancer Institute in Boston, who was not involved in the study, said the data supported the accepted position among doctors that the vaccine should be given to girls before they become sexually active.
In another related study in the journal, researchers from Johns Hopkins University in Baltimore looked at 100 women with newly diagnosed cancers of the back of the throat.
The researchers found HPV in nearly three-quarters of the tumors. They found a strong association between the infections and oral sex.
The finding suggests that the vaccine might also prevent these cancers, although that possibility has not been studied.