Contact tracing period and epidemiological characteristics of an outbreak of the SARS-CoV-2 Delta variant in Guangzhou

•Time for tracing close contacts may need expanding to 4 days before symptom onset•Low Ct values were associated with a higher risk of SARS-CoV-2 transmission•Presymptomatic transmission was common in the epidemic of Delta variant An outbreak of the SARS-CoV-2 Delta variant occurred in Guangzhou in 2021. This study aimed to identify the transmission dynamics and epidemiological characteristics of the Delta variant outbreak to formulate an effective prevention strategy. A total of 13102 close contacts and 69 index cases were collected. The incubation period, serial interval, and time interval from the exposure of close contacts to the symptom onset of cases were estimated. Transmission risks based on the exposure time and various characteristics were also assessed. The mean time from exposure to symptom onset among non-household presymptomatic transmission was 3.83 ± 2.29 days, the incubation period was 5 days, and the serial interval was 3 days. The secondary attack rate was high within 4 days before onset and 4–10 days after symptom onset. Compared with other contact types, household contact had a higher transmission risk. The transmission risk increased with the number and frequency of contact with index cases. Cycle threshold (Ct) values were associated with lower transmission risk (adjusted odds ratio [OR] 0.93 [95% CI 0.88–0.99] for ORF 1ab gene; adjusted OR 0.91 [95% CI 0.86–0.97] for N gene). The contact tracing period may need to be extended to 4 days before symptom onset. The low Ct value of index cases, the high number and frequency of contact with index cases, and household contacts were associated with a higher transmission risk of SARS-CoV-2 Delta.