An outbreak of novel coronavirus (SARS-CoV-2) was identified in China in December 2019, and has spread rapidly to more than 200 countries and areas in four months. A few studies have reported that transmissibility exists during the late incubation period based on one single infection cluster caused by SARS-CoV-2. Here based on 178 SARS-CoV-2 clusters confirmed in Zhejiang Province, we analyzed the epidemic link between all 212 secondary cases with their previous cases, and found 49 secondary

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... es (from 26 clusters), which were 23.11% (49/212) of the total secondary cases infected from previous cases during the latter's incubation period. The median days from the last exposure of secondary cases to the onset of previous cases was 2.0 days (IQR: 1.00~5.00 days, 90th percentile: 9.00 days) .This study has shown transmission of the SARS-CoV-2 during the incubation period and indicated that some cases might be infectious soon after they were exposed to a prior transmitter. The results highlight the importance of extending the contact group for medical observation and isolation to those in contact with the index case nine (90th percentile) or more days before the latter's illness onset, when medical resources were sufficient.