Passively acquired antibody to flavivirus in Thai infants

Our studies show that most healthy Thai children have circulating antibodies to flaviviruses in early infancy; presumably these antibodies are acquired transplacentally, then gradually disappear until age 12-14 months, when antibodies to flaviviruses are no longer detectable. The unusual broad reactive antibody response of a rather high titer noted in a single 14-month baby is likely to be postnatally acquired. Variations between children of the same age group of the antibody levels and the breadth of reactivity of these passively acquired antibodies are probably related to the maternal antibody status. The decay kinetics of passively acquired antibody to flaviviruses appear to be in the same pattern as other transplacentally acquired antibodies. In Bangkok, the association of DHF with secondary or repeated dengue infection is a well known phenomenon but occasionally DHF is observed in children with primary dengue, particularly in infants under the age of one year. The age distribution curve of DHF cases seen in Bangkok shows two modal ages, one at 4 to 11 months of age and the other at 7 years of age. Between these two modal age peaks there appears to be a nadir at around one year, the time by which passively acquired antibodies to flaviviruses, as shown in our studies, can no longer be detected. This is strikingly in contrast to other viral diseases of childhood as it appears to be the 'silent period' for DHF. From these epidemiological observations, it seems that passively acquired antibodies to flaviviruses with their broad reactivity may not render these infants immune or completely immune to DHF. On the contrary, these antibodies may enhance dengue infections (and hemorrhagic fever).