In addition, the risk of getting herpes was reduced 7 percent "every additional 25 percent of the time that condoms were used during anal or vaginal sex," the researchers wrote.
The risk of getting herpes increased with the frequency of unprotected sex, and there was no significant difference between men and women in the effectiveness of condoms in preventing herpes transmission, they add.
Martin said using a condom not only reduces the odds of getting herpes, but of other STDs as well. "If you don't know the STD status of your partner, a condom is always a good idea," she said.
Dr. Jeffrey D. Klausner, director of STD Prevention and Control Services at the San Francisco Department of Public Health, said the study provides more evidence that condoms work.
"We know condoms can prevent the spread of sexually transmitted infections like HIV, herpes, warts, hepatitis, gonorrhea, chlamydia and syphilis, but it's always been hard to show that in research studies," Klausner said.
"If condoms can hold air and water, I never understood why folks thought they would not prevent the spread of germs, which are much, much larger than air or water molecules," he said.
Klausner said that the study provides scientific evidence that condoms work and should help in efforts to get condoms into the hands of sexually active teenagers and adults.
The U.S. Centers for Disease Control and Prevention has more on herpes.
SOURCES: Emily T. Martin, postdoctoral fellow, M.P.H., Ph.D., Seattle Children's Hospital Research Institute; Jeffrey D. Klausner, M.D., M.P.H., director, STD Prevention and Control Services, Department of Public Health, San Francisco; July 13, 2009, Archives of Internal Medicine
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