ELISA plates were coated with 50 ng of purified virus or 100 ng of rE in carbonate buffer at pH 9.6 for 2 hrs at room temperature and incubated with blocking buffer (0.05% TBS-T containing 3% skim milk or 3% normal goat serum) at 37 o C for 1 hr. Human immune sera or hMAbs serially diluted in blocking buffer were added for 1 hr at 37uC followed by alkaline phosphataseconjugated goat anti-human IgG (Sigma) for 1 hr at 37uC. Finally, p-nitrophenyl phosphate substrate (Sigma) was added to each well and the reaction was allowed to develop for 15 minutes before recording optical density at 405 nm on a spectrophotometer.