Transmission of COVID-19 in 282 clusters in Catalonia, Spain: a cohort study

BackgroundThere remains limited data on what variables affect risk of transmission of SARS-CoV-2 and developing symptomatic Covid-19 and in particular the relationship to viral load (VL). We analysed data from linked index cases and their contacts to explore factors associated with transmission of SARS-CoV-2.MethodsPatients were recruited as part of a randomized control trial, conducted between March to April 2020, that aimed to assess if hydroxychloroquine reduced transmission of SARS-CoV-2. Non-hospitalised Covid-19 cases and their contacts were identified through the local surveillance system. VL, measured by quantitative PCR from a nasopharyngeal swab, was assessed at enrollment, at day 14, and whenever the participant reported Covid-19-like symptoms. Risk of transmission, developing symptomatic disease and incubation dynamics were evaluated using regression analysis.FindingsWe identified 314 cases, 282 of which had at least one contact (753 contacts in total). Ninety (33%) of 282 clusters had at least one transmission event. The secondary attack rate was 16% (125/753), with a variation from 12% to 24% for VL of the index case of <106, and >109copies/mL, respectively (OR per log10increase in VL 1.3 95%CI 1.1–1.6). Increased risk of transmission was also associated with household contact (OR 2.7; 1.4–5.06) and age of the contact (OR 1.02 per year; 1.01–1.04). The proportion of PCR positive contacts who developed symptomatic Covid-19 was 40.3% (181/449), with a variation from 25% to 60% for VL of the contact <107, and >109copies/mL (HR log10increase in VL 1.12; 95% CI 1.05 – 1.2). Time to onset of symptomatic disease decreased from a median of 7 days (IQR 5–10) for individuals with an initial viral load <107to 6 days (4–8) and 5 days (3–8) for individuals with an initial viral load of 107–109and >109, respectively.InterpretationViral load of index cases is a leading driver of SARS-CoV-2 transmission. The risk of symptomatic Covid-19 is strongly associated with viral load of contacts at baseline and shortens the incubation time in a dose-dependent manner.FundingCrowdfunding campaign YoMeCorono (http://www.yomecorono.com/), and Generalitat de Catalunya. Support for laboratory equipment from Foundation Dormeur.Research in contextEvidence before this studyIn September 2020, we searched PubMed database for articles reporting on factors influencing transmission and the risk of developing symptomatic disease. Search terms included “Covid-19”, “SARS-CoV-2”, “transmission”, “incubation time”, and “risk”, with no language restrictions. By 20thSeptember, various authors had reported on retrospective analyses of clusters of index cases and their corresponding contacts, as well as series of patients who developed symptomatic Covid-19 disease after PCR positive result. Besides describing the secondary attack rate, various authors identified risk factors for transmission associated with the place and duration of exposure and the lack of use of personal protective equipment. A single study suggested that symptomatic individuals might be more likely to transmit than asymptomatic cases but we found no clear evidence regarding the influence of viral load of the index case on transmission risk. Similarly, although various retrospective series of patients with positive PCR results had reported incubation times elsewhere, the characteristics of index case and contacts that may influence the risk of developing symptomatic Covid-19 and the time to this event had been barely addressed.Added value of this studyWe analyzed data from a large cluster-randomized clinical trial on post-exposure therapy for Covid-19 that provide new information on SARS-CoV-2 transmission dynamics. Several design components add value to this dataset. Notably, quantitative PCR was available for the index cases to estimate risk of transmission. Furthermore, quantitative PCR was also performed on asymptomatic contacts at the time of enrollment allowing to investigate the dynamics of symptomatic disease onset among them. We found that the viral load of the index case was the leading determinant of the risk of SARS-CoV-2 PCR positivity among contacts. Among contacts who were SARS-CoV-2 PCR positive at baseline, viral load significantly influenced the risk of developing the symptomatic disease in a dose-dependent manner. This influence also became apparent in the incubation time, which shortened with increasing baseline viral loads.Implication of all the available evidenceOur results provide important insights into the knowledge regarding the risk of SARS-CoV-2 transmission and Covid-19 development. The fact that the transmission risk is primarily driven by the viral load of index cases, more than other factors such as their symptoms or age, suggests that all cases should be considered potential transmitters irrespective of their presentation and encourages assessing viral load in cases with a larger number of close contacts. Similarly, our results regarding the risk and expected time to developing symptomatic Covid-19 encourage risk stratification of newly diagnosed SARS-CoV-2 infections based on the initial viral load.