People who become sexually active at a young age are at increased risk of contracting herpes simplex virus type 1 (HSV- 1), according to the results of a new study from the United Kingdom.

HSV-1 causes fever blisters on the mouth or face; HSV-2 typically affects the genital area. However, HSV-1 can also affect the genitals and does not confer immunity to genital herpes. In past decades, HSV-1 infection was quite common and is estimated to have infected most people during the 1940s, according to the report published in the current issue of Sexually Transmitted Infections, "Herpes Simplex Virus Type 1 Infection: A Sexually Transmitted Infection of Adolescence?" (2002;78:346-348). By the 1970s, HSV-1 infection had dropped to about 50 percent of adults; socioeconomic improvements are credited. British experts estimate that one in four children between ages 10 and 14 now carry the virus.

The proportion of genital herpes due to HSV-1 rather than HSV-2 is on the rise in the United Kingdom, particularly among young people, but the extent to which the virus is acquired sexually is not clear, according to Dr. Frances M. Cowan of the Royal Free and University College Medical School in London and colleagues.

Cowan and colleagues surveyed 869 people attending a genitourinary medicine clinic and 1,494 blood donors about their sexual histories and tested them for HSV-1. Slightly more than 60 percent of those attending the clinic had antibodies for HSV-1, signifying that they had been exposed to the virus, compared with roughly 46 percent of those who donated blood.

Based on analysis of the surveys, Cowan and colleagues write that "age of first intercourse is the strongest predictor of infection" with HSV-1. The researchers found that among the people who visited the clinic, those who had sex for the first time at age 20 were over 60 percent less likely to have HSV-1 antibodies than those who reported their first sexual intercourse at 15. Among those who donated blood, those who had sex at 20 were 36 percent less likely to carry the virus than those who had sex at 15.

"The association with early age at first intercourse is striking and may reflect the particular sexual practices of people initiating sex in this age group," according to the researchers.

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