ELISA was performed according to Lopez et al. [33] and anti human IgG-or anti mouse IgG conjugated to alkaline phosphatase was used (Sigma, St Louis, MO) as second antibody. Individual human sera from 37, 42 and 39 adults donors from Burkina Faso, Tanzania and Colombia respectively were used at 1:200 dilution. Serum was considered positive if the optical density (OD) reading was higher than the mean OD value+3 standard deviation (SD) of the negative controls (individual serum samples from 8 to 11 naïve Swiss donors) or if the OD ratio of the mean of duplicate experimental values to the mean OD of the negative control was higher than 2. For mouse sera, the end point value was determined as the last dilution of the mean OD value+3 standard deviation (SD) of the negative control (non immune sera).