We found that 1 index case-patient transmitted infection to 0.27 close contacts on average, i.e., reproduction number = 0.27. This fi nding suggests that among those quarantined index case-patients, the number of persons with secondary cases who could be traced through rigorous fi eld investigation was small and far less than the number needed for the sustainable transmission of infectious disease in the population (reproduction number >1). However, the fact that the pandemic eventually spread in Beijing indicates that contact and case tracing were far from complete, especially later in the summer and early fall of 2009. The strict control measures may have worked to some extent at the beginning but were outpaced by local transmission (16) ; the percentage of locally acquired infections ranged from <10% in June 2009 to >80% in September 2009 (data not shown).