[Epidemiological Study of the Transmission Chain of a Severe Acute Respiratory Syndrome Outbreak]

Objective: To investigate the process of SARS transmission and to evaluate infectivity of SARS patients in different periods of disease development.

Methods: A case of SARS outbreak beginning from a male, 74-year-old patient in Beijing. Two hundred and seventy five primary, secondary and tertiary contacts, including family members relatives, colleagues, and others exposed to him, were traced by field investigation and telephone interview using a standardized questionnaire. The transmission process and infectivity of disease were analyzed with transmission chain diagram and contact history diagram.

Results: Among the 275 contacts of the index case, 41 fell ill of SARS with an attack rate of 14.9% and 5 deaths with a fatality rate of 12.2%. The transmission chain of this outbreak could be clearly delineated, and all the 41 cases had close contacts with their infectors, SARS cases of the previous generation within the symptomatic period thereof. All of the 114 contacts that contacted their infectors, SARS cases of the previous generation during the incubation period thereof remained healthy. The incubation period of 27 cases that had contacted their infectors only once was 1 - 10 days with a median of 3 days. The highest risk of being infected was caring the patients unprotectedly, followed by living together with patient, visiting patient, and handling patient without protection.

Conclusion: All the secondary cases have the history of close contact with their symptomatic infectors of previous generation. It cannot be proved at all that SARS is infective during the incubation period. The infectivity of SARS is related to the degree of closeness of contact with the patient.