Transmission of SARS-CoV-2 from pre and asymptomatic infected individuals. A systematic review
Authors: Jefferson Tom , Spencer Elizabeth A. , Brassey Jon , Onakpoya Igho J. , Rosca Elena C. , Plüddemann Annette , Evans David H. , Conly John M. , Heneghan Carl J. 
Published: July 2021

AbstractBackgroundThe transmission role of SARS-Cov-2 infected persons who develop symptoms post testing (pre symptomatics) or not at all throughout the course of positivity (asymptomatics) is unknown. We carried out a systematic review of available evidence to determine whether they were infectious or not and if so for how long and their probable contribution to the pandemic spread of SARS-CoV-2.MethodsWe searched LitCovid, medRxiv, Google Scholar and the WHO Covid-19 databases and reference lists of included studies. Search terms were COVID-19, SARS-CoV-2, transmission, asymptomatic, presymptomatic and appropriate synonyms. Searches were carried out to 31 March 2021. We included studies on people exposed to SARS CoV-2 within 2-14 days (incubation time) of close contact or suspected community or institutional exposure to index asymptomatic (at the time of observation) infected individuals, as defined in the study. We included studies with a proven or hypothesised chain of transmission with secondary case infected based on fulfilling a confirmed or probable case definition and confirmation of infectiousness and transmission outcome based either on serial PCR cycle threshold readings or viral culture or gene sequencing or any combination thereof and adequate follow up. We assessed the reliability of eliciting symptom and signs compatible with contemporary knowledge and extracted documentation of the likelihood of transmission, presence of replicating virus and/or documentation of phylodynamics (genetic sequence lineage) and/or adequate follow-up and reporting of symptoms and signs. We wrote to all included studies corresponding authors to request further details and assessed likelihood of transmission using adapted causality criteria.ResultsWe included 18 studies from a variety of settings. Because of the current lack of standardized methodology and clear reporting criteria there was substantial methodological variation in transmission studies. Asymptomatic prevalence at the time of initial testing varied from 12.5% to 100% and of these 6% to 100% were pre-symptomatic cases, depending on the setting and the methods of case ascertainment and the population. Nursing/care home facilities reported high rates of presymptomatic: 50% - 100% (n=3 studies). Fifteen studies were classified as high risk and three studies at moderate risk of symptom ascertainment bias. In practice, this assessment means that high-risk studies may be less likely to distinguish between pre-symptomatic and asymptomatic cases. Six of the asymptomatic studies and four presymptomatic studies reported growing infectious virus although the data was too sparse to determine duration of infectiousness. Three studies were judged as providing possible and three of probable/likely evidence of asymptomatic transmission of SARs-CoV-2. Five studies provided evidence of possible and two of probable/likely presymptomatic transmission of SARs-CoV-2. Author response rate was 100%.ConclusionsReliable studies included here provide probable evidence of transmission of SARS-CoV-2 from presymptomatic and asymptomatic individuals. Single point in time estimates and binary PCR testing alone cannot provide reliable information on symptom status and information on infectivity. The number of studies and asymptomatic and presymptomatic cases eligible for inclusion was low, with more data and international standardisation of methods needed to further reduce uncertainty.