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A clinical trial of a new vaccine that could prevent herpes simplex type 2 infections in women has run into problems drawing volunteers for the tests, with only 1,400 of a needed 7,500 women having signed up so far, the Chicago Sun-Times reports. "I think we underestimated how hard it would be," Judy Falloon, medical monitor for the trial and a physician with the National Institutes of Health, told the Sun-Times. The problems with recruitment of trial participants is both in convincing volunteers that they will not get herpes from the vaccine and in finding women ages 18 to 30 who are not infected with either herpes simplex type 1, which causes cold sores and fever blisters, or herpes simplex type 2, which causes genital herpes. Type 1 affects 50% to 80% of Americans, scientists say. So far, more than 5,000 women have been ruled out for participation in the trial because they carry the virus. Health officials estimate that 20% of Americans over age 12 have genital herpes. The vaccine was developed by GlaxoSmithKline and was shown in preliminary human tests to prevent herpes infection in 74% of women exposed to the virus. None of the 1,736 men studied received any protection from the vaccine. Researchers are still unclear as to why the vaccine doesn't work for men.