BACKGROUNDTo find out the timing of serologic responses after illness onset and distribution of IgG antibody to SARS-CoV in SARS cases of transmission chain or non-transmission chain.

METHODSThe IgG and IgM antibodies to SARS-CoV were tested by indirect ELISA in serum samples from 301 clinically diagnosed SARS cases.

RESULTSTotally 158 SARS cases were involved in 15 chains of transmission. The positive rates of SARS-CoV IgG in those chains were 85.70%-100.00% and the overall rate was 94.30% (149/158). The chain of transmission could spread to four generations, but the SARS cases were reduced with increase of generations. There was no significant difference among positive rates of SARS-CoV IgG for generations, Chi square=5.11, P greater than 0.05. The positive rate of SARS-CoV IgG in cases who were not in chain of transmission was 12.59%(18/143) which was statistically significantly different from that of cases in chain of transmission, Chi square=199.64, P less than 0.001. During days 0-7,8-14,15-21,22-30 after onset, the cumulated positive rate of SARS-CoV IgG was 16.67%, 40.00%, 70.00% and 93.10%, respectively, then was kept at the level above 90% and lasted for 217 days. The cumulated positive rate of SARS-CoV IgM during days 0-7 after onset was the same to that of IgG. During days 8-14, 55.17% of cases had seroconversion for IgM which reached a peak (86.96%) during days 21-30. Then the rate rapidly declined.

CONCLUSIONMore than 94% of cases with SARS could produce IgG antibody when they were infected by SARS-CoV. Detecting SARS-CoV IgG could provide a diagnostic evidence for case confirmation. SARS-CoV IgG appeared as early as 7 days after onset and reached the peak at about weeks 4. Then the high rate of antibody was maintained for more than 6 months.