Primary infections of human herpesvirus-7 and herpesvirus-6: a comparative, longitudinal study up to 6 years of age.

In order to understand the infection status of human herpesvirus 7 (HHV-7) in Taiwan and clarify the serological cross-reactivity between HHV-7 and HHV-6, a longitudinal study was carried out in 52 infants from whom 9 sera were available from birth to 6 years of age. All sera were tested for antibodies against HHV-6 and HHV-7 by indirect immunofluorescence assay. HHV-7 infection was not seen before 6 months of age and gradually emerged thereafter, reaching a cumulative rate of 28.8%, 67.3%, 73.1%, 78.8%, 82.7%, and 86.5% by the ages of 1, 2, 3, 4, 5 and 6 years, respectively. Primary HHV-6 infection induced a reactive HHV-7 antibody in 10/28 (35.7%) cases with a titre of no more than 10. Twenty-eight children (53.8%) were infected by HHV-6 earlier than HHV-7, while eight (15.4%) were infected by HHV-7 earlier than HHV-6. In summary, HHV-7 infected children later than HHV-6. A one-way cross-reaction of HHV-7 serology by HHV-6 was revealed and a titre of 20 is appropriate for defining HHV-7 infection.