A Seroprevalence Survey of Dengue Virus Infection in Healthy Singapore University Undergraduates by Enzyme Immunoassay and Plaque Reduction Neutralization Test

A seroepidemiological survey was conducted from 1998 to 2000 to determine the seroprevalence of dengue virus infection in a cohort of healthy Singapore university undergraduates aged 19-26 years using a commercial dengue indirect IgG enzyme immunosorbent assay (ELISA) kit (PanBio). Out of 184 volunteers, 41 (22%) tested seropositive, suggesting that 78% of this young cohort would be susceptible to primary dengue infection. This relatively low dengue seroprevalence in this age group appeared to be predictive of the increasing dengue incidence in Singapore for the period 2001-2004, which witnessed a mean incidence of 5141 cases per year. In addition, the dengue incidence rate in Singapore for 2004 was the highest in the age group of 15-24 years. The general distribution pattern of the seropositive subjects matched well with the geographical distribution of confirmed dengue cases in Singapore for 1998. In order to ascertain the level of protective immunity against the virulent DENV-2 in this young adult population, 38 ELISA-seropositive and 18 ELISA-seronegative sera were subjected to dengue plaque reduction neutralization test (PRNT). Approximately 60% of ELISA-positive samples but none of the ELISA-negative sera showed DENV-2 neutralizing antibody activity. Thus, this implies that only a small proportion of the Singapore young adult population in 1998-2000 had protective antibodies against DENV-2. Intriguingly, DENV-2 was the predominant circulating serotype in Singapore in 19901991, 1993, 1998 and 2001-2003. There was no correlation between the ELISA PanBio index and PRNT titre. However, a negative correlation between the percentage of reduction of plaque-forming units (PFU) against serum dilution factor was observed, with a Pearson correlation coefficient of 0.479 (P<0.01). Interestingly, several samples subjected to PRNT exhibited increased percentages of PFU, thereby alluding to the presence of infection-enhancing antibodies to DENV-2.