NEW YORK (Reuters Health) - The largest study to date of a genital herpes vaccine under development by GlaxoSmithKline indicates that it is safe, well-tolerated and immunogenic, researchers report in the May 1st issue of Clinical Infectious Diseases.

Lead investigator Dr. David I. Bernstein told Reuters Health that the antibody response to the herpes simplex virus (HSV) 2 glycoprotein D vaccine is at least as strong as that seen with natural infection.

However, he added, "since we don't know what protects you from getting genital herpes, it's still difficult to say what correlation there would be from the responses we measured in protection" with those in infection.

Dr. Bernstein of Cincinnati Children's Hospital Medical Center, Ohio and colleagues enrolled 7,460 men and women in the placebo-controlled study.

Among findings were that the vaccine produced local reactions including soreness. These reactions were no worse among people positive for HSV 1 or 2, and also did not increase after the second or third injection. This finding is important, Dr. Bernstein observed, because many people are HSV-1 positive.

And compared to natural infection, the researchers note, the vaccine produced higher HSV glycoprotein D antibodies when measured by enzyme-linked immunosorbent assay. By 13 months after immunization, antibody levels had decreased in people who received the vaccine but were still higher than those seen with natural HSV 2 infection.

"That's one of the big unanswered questions," Dr. Bernstein said, "we'll need to find out how long these antibodies last and whether they'll provide protection."

Ongoing trials, he and his colleagues conclude, will determine the effect of the vaccine "in adolescent and preadolescent female subjects, the target population for immunization." Clin Infect Dis 2005;40:1271-1281.

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