All close contacts were traced and quarantined for 7 days after the most recent exposure to the index casepatient. All index case-patients detected between May 16 (the fi rst case, the date of confi rmation) and September 15, 2009 (before widespread transmission in Beijing), and their close contacts were included in this study. We excluded cluster or outbreak cases for which close contacts could not be determined clearly by epidemiologic investigation (the transmission chain was obscure).