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Estimating pre-symptomatic transmission of COVID-19: a secondary analysis using published data

By Miriam Casey, John Griffin, Conor G McAloon, Andrew W Byrne, Jamie M Madden, David McEvoy, Aine B Collins, Kevin Hunt, Ann Barber, Francis Butler, Elizabeth A Lane, Kirsty O Brien, Patrick Wall, Kieran A Walsh, Simon J. More

Posted 11 May 2020
medRxiv DOI: 10.1101/2020.05.08.20094870

Objective: To estimate the proportion of pre-symptomatic transmission of SARS-CoV-2 infection that can occur and timing of transmission relative to symptom onset. Setting/design: Secondary analysis of international published data. Data sources: Meta-analysis of COVID-19 incubation period and a rapid systematic review of serial interval and generation time, which are published separately. Participants: Studies were selected for analysis if they had transparent methods and data sources and they provided enough information to simulate full distributions of serial interval or generation time. Twenty-three estimates of serial interval and five of generation time from 17 publications were included. Methods: Simulations were generated of incubation period and of serial interval or generation time. From these, transmission times relative to symptom onset were calculated and the proportion of pre-symptomatic transmission was estimated. Outcome measures: Transmission time of SARS-CoV-2 relative to symptom onset and proportion of pre-symptomatic transmission. Results: Transmission time ranged from a mean of 2.91 (95% CI: 3.18-2.64) days before symptom onset to 1.20 (0.86-1.55) days after symptom onset. Unweighted pooling of estimates of transmission time based on serial interval resulted in a mean of 0.60 days before symptom onset (3.01 days before to 1.81 days after). Proportion of pre-symptomatic transmission ranged from 42.8% (39.8%-45.9%) to 80.6% (78.1%-83.0%). The proportion of pre-symptomatic transmission from pooled estimates was 56.4% (34.9%-78.0%). Conclusions: Whilst contact rates between symptomatic infectious and susceptible people are likely to influence the proportion of pre-symptomatic transmission, there is substantial potential for pre-symptomatic transmission of SARS-CoV-2 in a range of different contexts. Our work suggests that transmission is most likely in the day before symptom onset whereas estimates suggesting most pre-symptomatic transmission highlighted mean transmission times almost three days before symptom onset. This highlights the need for rapid case detection, contact tracing and quarantine.

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