Objective To evaluate the infectivity of severe acute respiratory syndrome(SARS) during its incubation period by investigating chains of transmission and individuals isolated for medical observation with a view to providing scientific evidence for updating protocols of medical isolation.Methods Individuals related with the two SARS chains of transmission in Beijing in 2003 and a group of individuals isolated for medical observation in Haidian district of Beijing during the SARS outbreak were selected as subjects of study.Contactors with SARS patients and those with symptom development following the contacts were investigated via questionnaire.Serum samples were collected from super transmitters and tested for SARS-CoV antibody by neutralization test and enzyme linked immunosorbent assay(ELISA) .Results A total of 1 112 contactors were investigated in three surveys.Of them,669 had a history of close contact with symptomatic SARS patients,101 developed symptoms with a rate of 15.1%,363 had a history of close contact with patients in their incubation period,none of whom developed symptoms(0%) .Serum samples were collected from 32 highly-exposed individuals,of whom 13 developing SARS symptoms after contact had serum samples positive for SARS-CoV antibody.Samples collected from the asymptomatic contactors were all negative for SARS-CoV antibody.Conclusion SARS cases are infectious only during their symptomatic period and are non-infectious during the incubation period.Isolation for medical observation should be placed for individuals who are in close contact with symptomatic SARS patients.The results of our study are of decisive significance for the Ministry of Health to the definition of SARS close contactor. Objective To evaluate the infectivity of severe acute respiratory syndrome (SARS) during investigating chains of transmission and individuals isolated for medical observation with a view to provide scientific evidence for updating protocols of medical isolation. Methods Individuals related with the two SARS chains of transmission in Beijing in 2003 and a group of individuals isolated for medical observation in Haidian district of Beijing during the SARS outbreak were selected as subjects of study .Contactors with SARS patients and those with symptom development the same the investigated were investigated via questionnaire.Serum samples were collected from super transmitters and tested for SARS-CoV antibodies by neutralization test and enzyme linked immunosorbent assay (ELISA). Results A total of 1 112 contactors were investigated in three surveys. Of them, 669 had a history of close contact with symptomatic SARS patients, 101 developed symptoms with a rate of 15.1%, 363 h ad a history of close contact with patients in their incubation period, none of whom developed symptoms (0%). Serum samples were collected from 32 highly-exposed individuals, of whom 13 developing SARS symptoms after contact had serum samples positive for SARS-CoV antibody.Samples collected from the asymptomatic contactors were all negative for SARS-CoV antibody.Conclusion SARS cases are infectious only during their symptomatic period and are non-infectious during the incubation period. Isolation for medical observation should be placed for individuals who are in close contact with symptomatic SARS patients. results of our study are of decisive significance for the Ministry of Health to the definition of SARS close contactor.