Girls as young as 12 years old are commonly infected with various forms of herpes viruses, including the one that causes genital herpes, a study of urban U.S. adolescents shows.

Researchers say the findings support the belief that a vaccine now under study for herpes simplex virus 2 (HSV-2), the virus behind genital herpes, will need to be given to young adolescents to have the greatest benefit.

The study, which followed 174 girls treated at an urban health clinic, found that seven percent of the 12- to 15-year-old girls had HSV2 at the start. Three years later, 14 percent were infected with the virus, with an infection rate of just over four percent per year among girls who were sexually active.

In addition, the large majority of girls - 81 percent by the study’s end - carried cytomegalovirus (CMV), a form of herpesvirus that, like HSV-2 and other members of that viral family, remains in the body after infection, usually in a dormant state.

CMV infection, which is transmitted through bodily fluids, causes no harm in most adults and children. But if a woman is pregnant when she acquires the infection it can sometimes cause serious birth defects, including hearing or vision loss and mental retardation.

A vaccine is being developed for CMV as well, study co-author Dr. Susan L. Rosenthal pointed out, and if it becomes available, young adolescent girls are again likely to be a prime target for vaccination.

Rosenthal, a professor of pediatrics at the University of Texas Medical Branch in Galveston, and her colleagues report the findings in the journal Clinical Infectious Diseases.

Past research has shown that HSV-2, CMV and HSV-1 - the herpesvirus that causes most cold sores - are highly common infections, in teenagers as well as adults. The new study adds to what’s known by revealing the rate of new infections in a group of young adolescent girls, according to Rosenthal and her colleagues.

It also looks at the question of whether infection with HSV1 offers some protection against HSV-2, Rosenthal told Reuters Health. The study found that girls who carried HSV-1, which half did by the study’s end, were less likely than the rest to become infected with HSV-2.

This is of interest, in part, Rosenthal explained, because a vaccine now in clinical trials was shown in earlier-stage research to protect women from HSV-2 infection, but only if they’re were not infected with HSV-1.

If HSV-1 confers a “natural immunity” to HSV-2, this could help explain the vaccine’s lack of benefit in women who carry the cold-sore virus, according to Rosenthal. Further research into the question is needed, she and her colleagues conclude in the report.

The bottom-line from the current findings, Rosenthal said, is that the three herpesvirus infections “are common, and young adolescents get them.”

This, according to the researcher, supports the notion that if and when vaccines become available, they’ll need to be given at a fairly young age to offer the most widespread protection.

The study was funded by the National Institutes of Health. Rosenthal and colleague Dr. Lawrence R. Stanberry, the study’s lead author, have in the past received research funding from GlaxoSmithKline, maker of the genital herpes vaccine now in clinical trials.

SOURCE: Clinical Infectious Diseases, November 15, 2004.