Plasma samples were obtained from patients during the acute phase of their illness. Data on demographic characteristics, premorbid conditions, clinical features, and routine hematological and biochemical laboratory test findings (i.e. full blood count, renal and liver function tests, C-reactive protein) were also collected. All symptomatic patients were isolated at CDC/TTSH until the febrile illness resolved and a negative CHIKV RT-PCR test was obtained. During the hospital stay, daily monitoring of body temperature, vital parameters, and blood counts were carried out. A patient was defined as having severe illness, if he had either a maximum temperature of more than 38.5uC, or a maximum pulse rate of more than 100 beats/minute, or a nadir platelet count of less than 100610 9 /L. Laboratory results were expressed as mean6SD.