Dengue is the most important arboviral disease affecting humans. Although mortality is generally low, dengue generates a significant burden for health services in endemic countries. Pathogenesis is poorly understood. Secondary dengue is one important risk factor for severe disease, but current algorithms to differentiate primary from secondary infections are of limited utility. Viral burden is also thought to be a major factor influencing disease severity, but existing knowledge of the magnitude and kinetics of plasma viremia in association with different dengue serotypes, primary versus secondary immune status, and/or clinical markers of disease severity, is limited and inconsistent. I first set out to establish novel algorithms that permit differentiation of primary from secondary dengue at any day of illness with high accuracy. Using serial samples from 249 well-characterized patients, categorized as having primary or secondary infections on the basis of PRNTs after six months, I developed a number of models; the algorithm based on the Panbio Indirect IgG performed best (accuracy, 85%), although the models based on the in-house IgG and in-house IgM/IgG ratio were also good (accuracy, 84%). I subsequently applied the IgM/IgG ratio algorithm to define the immune status of 884 patients previously enrolled into early dengue studies. Using serial daily plasma samples I describe detailed plasma viremia responses for 581 DENV1, 152 DENV2 and 151 DENV3 infections. In general DENV1 infections resulted in higher plasma viremia than did DENV2 or DENV3 infections. Overall viremia was higher in primary than secondary DENV1 and DENV3 infections, although similar in DENV2 primary and secondary infections. In addition, higher viremia on Day3 of illness was significantly associated with several disease severity markers: lower platelet nadir, higher haemoconcentration and an increased risk of developing shock and bleeding. Finally I evaluated the effect of early prednisolone therapy on plasma viremia and immune responses, using serial samples from a clinical trial assessing the safety of early oral corticosteroid therapy. Prednisolone did not augment plasma viremia or NS1 antigenaemia, confirming the safety of using the drug during active viral replication. However, the effects of the drug on a range of immune correlates was also limited, consistent with prednisolone having negligible measurable benefits in the clinical trial on which this work was based
