M. Marks, Pere Millat-Martínez, D. Ouchi, Chrissy h. Roberts, Andrea Alemany, M. Corbacho-Monné, María Ubals, A. Tobias, C. Tebé, E. Ballana, M. Vall-Mayans, Camila G-Beiras, N. Prat, J. Ara, B. Clotet, O. Mitjà
medRxiv, 2020
Background There remains limited data on what variables affect the risk of transmission of SARS-CoV-2 and developing symptomatic Covid-19 and in particular the relationship to viral load (VL). Methods We analysed data collected in a trial of hydroxychloroquine post-exposure prophylaxis. Covid-19 cases and their contacts were identified through the local epidemiological surveillance system. VL, estimated by quantitative PCR, was assessed at enrollment, at day 14, and whenever the participant reported Covid-19-like symptoms. Risk of transmission, risk of developing symptomatic disease and incubation dynamics were evaluated using random-effects regression analysis. Findings We 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 >109 copies/mL, respectively (OR per log10 increase in VL 1.3 95%CI 1.1 to 1.6). Increased risk of transmission was also associated with household contact (OR 2.7; 1.4 to 5.06) and age of the contact (OR 1.02; 1.01 to 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 >109 copies/mL (HR log10 increase in VL 1.12; 95% CI 1.05 to 1.2). Time to onset of symptomatic disease decreased from a median of 7 days (IQR 5 to 10) for individuals with an initial viral load <107 to 6 days (4 to 8) and 5 days (3 to 8) for individuals with an initial viral load of 107 to 109 and >109, respectively. Interpretation We show that the viral load of the index case is a leading driver of SARS-CoV-2 transmission. The risk of symptomatic Covid-19 is strongly associated with viral load of the contact at baseline, which shortens the incubation time in a dose-dependent manner.
Cited by 78 publications.
Field of study: Medicine