Only 1 patient had an HAI antibody titer ≥10 (titer 20) 1 day after illness onset. The HAI geometric mean titer increased 5 days after symptom onset compared with that 1 day after symptom onset and continued to increase until it reached a peak level of 137.9 at 37 days after symptom onset (25.5-fold increase). Peak HAI antibody titers ≥40 and ≥4-fold increases were observed in 27 (96.4%) patients.