This suggests that scientists need to better understand how the foreskin affects the immune system's response to HIV, Buchbinder said.
Infection rates were also higher among those who had developed immunity to a virus that causes colds. This is important because scientists got the HIV vaccine to enter the body by piggybacking on a weakened bit of the same type of cold virus.
The vaccine can't cause HIV, and the weakened cold virus can't cause a cold, Buchbinder said.
But, there's another possibility, she said. The increase in infection rates among the two groups -- uncircumcised men and those who had immunity to the cold virus -- could be purely due to chance. Or some other factors could be at play.
The failure of the Merck vaccine "profoundly affected the HIV-vaccine development field," Dr. Merlin L. Robb, of the U.S. Military HIV Research Program, wrote in a commentary accompanying the study.
"We have learned that future HIV vaccines will need to generate stronger, broader or different types of immune responses in order to be effective," he said in an interview.
Learn more about HIV vaccine research from the U.S. Centers for Disease Control and Prevention.
SOURCES: Susan P. Buchbinder, M.D., director, HIV research section, San Francisco Department of Public Health; Merlin L. Robb, M.D., U.S. Military HIV Research Program, Rockville, Md.; Nov. 13, 2008, The Lancet, online
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