Positive IgG (3-4 weeks after exposure) Asymptomatic group: 81.1% (30/37) Symptomatic group: 83.8% (31/37)

Positive IgM Asymptomatic group: 62.2% (23/37) Symptomatic group: 78.4% (29/37)

• IgG levels in the symptomatic group (median S/CO, 20.5; IQR, 5.8–38.2) were significantly higher than those in the asymptomatic group (median S/CO, 3.4; IQR, 1.6–10.7) in the acute phase (P = 0.005).

Median percentage of decrease: 8.3% (range, 0.5–22.8%) for neutralizing serum antibodies in the asymptomatic group, 11.7% (range, 2.3–41.1%) in the symptomatic group

40.0% (12/30) of asymptomatic individuals, but only 12.9% (4/31) of symptomatic individuals, became seronegative for IgG