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Transmission interval estimates suggest pre-symptomatic spread of COVID-19

Our take —

This study was available as a pre-print, and thus not yet peer reviewed. Using data from two distinct outbreaks, where close contacts of confirmed cases were monitored and quarantined, and low risk contacts were placed under active surveillance in Singapore and Tianjin, China, this modeling study estimated the serial interval to be shorter than the incubation period suggesting pre-symptomatic transmission with infection occurring 2.55 (Singapore) and 2.89 (Tianjin) days before symptom onset in the primary case. This paper provided early evidence of pre-symptomatic transmission of SARS-CoV-2. If corroborated, isolation of detected cases alone is unlikely to curb the spread of the pandemic.

Study design


Study population and setting

In this study authors estimated the incubation period and serial interval distribution of SARS-CoV-2 using data from transmission clusters in Singapore (93 cases) and Tianjin, China (135 cases), reported in January and February 2020. Using these estimates, authors calculated the basic reproductive number, R0, and the extent of pre-symptomatic transmission.

Summary of Main Findings

The authors estimated the mean incubation period as 7.1 days (95% CI: 6.1, 8.3) in Singapore and 9 days (95% CI: 7.9, 10.2) in Tianjin. The mean incubation period was shorter for cases occurring earlier in the epidemic in both locations. Using the first 4 cases in each cluster, the mean serial interval was estimated to be 4.6 days (95%CI: 2.7, 6.4) in Singapore and 4.2 days (3.4, 5.0) in Tianjin. Estimates of the serial interval (the time between symptom onset of the primary case to symptom onset in the secondary case), and incubation period (the time between infection and symptom onset) suggest there is pre-symptomatic transmission, with shorter serial intervals compared to incubation period estimates. Infection occurred, on average, 2.9 days (Tianjin) and 2.6 days (Singapore) before symptom onset. The estimated R0‘s in both settings were estimated to be 2.

Study Strengths

Serial interval and incubation period directly estimated using information from COVID-19 transmission clusters from two distinct locations (Singapore and Tianjin, China). Parameter estimates were robust in sensitivity analyses and inferences were congruent in both locations.


The timing of exposure and presumed infectors are uncertain, and the analysis does not does not account for uncertainty in the date of symptom onset within the clusters. This impacts our confidence in both the serial interval estimates and the timing of pre-symptomatic transmission because the serial interval is defined by symptom onset in case pairs. Any changes to estimates in the serial interval would influence estimates of how much transmission occurs before an individual showed symptoms.

Value added

Early studies estimate COVID-19 transmission parameters using prior information from SARS. In this study, the incubation period and serial interval are directly estimated using information from COVID-19 transmission clusters.