Serological profiling in dengue patients identifies broad flavivirus reactivity associated with clinical severity

Background Dengue virus (DENV) is a major cause of viral haemorrhagic fever (VHF) in Southeast Asia, with rising cases in endemic countries like Vietnam. Cross-reactive antibodies from prior flavivirus exposures and secondary DENV infections are hypothesized to contribute to severe disease, but comprehensive serological profiling in clinical cohorts remains limited. Methods In this cross-sectional study, we performed serological profiling using IgG and IgM ELISAs against seven viral haemorrhagic fever-associated pathogens, including flaviviruses (DENV, JEV, ZIKV, WNV, and TBEV) as well as chikungunya virus and hantavirus, in 302 hospitalized dengue patients. Patients were categorised into dengue fever (DF) and dengue with warning signs or severe dengue (DWS/SD). We assessed antibody positivity, inter-virus correlations, and the association between anti-DENV IgG levels and disease severity using multivariate logistic regression. Results DWS/SD patients showed significantly higher rates of DENV and JEV IgM positivity, and broader IgG reactivity to multiple flaviviruses, including DENV, JEV, WNV, and TBEV, compared to DF patients (p < 0.001). IgG responses were highly cross-reactive (e.g., DENV-TBEV, r = 0.95), while IgM responses were more virus-specific. Anti-DENV IgG plasma levels were significantly higher in DWS/SD (median: 182 [range 11.9-239] RU/mL) compared to DF patients (141 [range 0.09-229] RU/mL) (p < 0.001). The association between IgG levels and disease severity remained significant (OR: 1.02; 95% CI: 1.01-1.02; p < 0.001) after adjusting for age and time from symptom onset. Conclusions Elevated anti-DENV IgG levels were associated with DWS/SD. As the study relied on ELISA-based serology without performing confirmatory neutralisation tests, the results should be interpreted as indicative of patterns of flavivirus seroreactivity rather than as evidence of virus-specific exposure.