Laboratory assays and field dengue vaccine evaluation at Ratchaburi province, Thailand: a preliminary result.

Laboratory-based epidemiological studies have been initiated in a cohort of school-aged children in Ratchaburi province, Thailand in preparation for imminent dengue vaccine field trials. In these studies, levels of neutralising antibodies were determined by the plaque reduction neutralisation test (PRNT) while virological confirmation was performed using RT-PCR or mosquito inoculation. Serological confirmation was performed using IgM and IgG ELISA. The incidence rate of dengue in the Ratchaburi cohort was 1635 per 100,000 during 2005-2006. Among 3547 patients in this cohort, 331 were classified with febrile disease and 58 with dengue. All four dengue serotypes were observed in Ratchaburi province. Efforts to identify the infecting serotype in symptomatic patients, based solely upon the neutralising antibody response, were complicated by cross- neutralising antibodies. In addition, 20 symptomatic and 49 asymptomatic cases were identified among 897 subjects investigated during 2005-2006, a ratio of 1:2.5. Serological analysis of asymptomatic dengue infections demonstrated boosting of immune responses due to subclinical infections with dengue or Japanese encephalitis. These results demonstrate that laboratory-confirmed dengue disease in areas of high transmission can be established. Epidemiological data of this kind are critical to dengue vaccine efficacy trials.