Evidence for transmission of COVID-19 prior to symptom onset

TLDR

The study aimed to quantify pre‑symptomatic transmission by analyzing contact‑tracing data from Singapore and Tianjin clusters. Incubation periods and serial intervals were estimated from the data to assess transmission timing. Incubation periods averaged 4.9 days in Singapore and 7.5 days in Tianjin, while serial intervals averaged 4.2 days in both locations, indicating that pre‑symptomatic transmission accounts for roughly 40–50 % of cases in Singapore and 60–80 % in Tianjin, underscoring the need for public health measures even among asymptomatic individuals.

Abstract

We collated contact tracing data from COVID-19 clusters in Singapore and Tianjin, China and estimated the extent of pre-symptomatic transmission by estimating incubation periods and serial intervals. The mean incubation periods accounting for intermediate cases were 4.91 days (95%CI 4.35, 5.69) and 7.54 (95%CI 6.76, 8.56) days for Singapore and Tianjin, respectively. The mean serial interval was 4.17 (95%CI 2.44, 5.89) and 4.31 (95%CI 2.91, 5.72) days (Singapore, Tianjin). The serial intervals are shorter than incubation periods, suggesting that pre-symptomatic transmission may occur in a large proportion of transmission events (0.4–0.5 in Singapore and 0.6–0.8 in Tianjin, in our analysis with intermediate cases, and more without intermediates). Given the evidence for pre-symptomatic transmission, it is vital that even individuals who appear healthy abide by public health measures to control COVID-19.