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

Abstract

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.

Just 2% of SARS-CoV-2-positive individuals carry 90% of the virus circulating in communities

“…Although it remains to be seen exactly how transmission probability relates to viral load, a strong implication is that these individuals who are viral super-carriers may also be super-spreaders. Higher viral loads have been shown to increase the probability of transmission to others in China [38], in Spain [39], and between pairs of roommates on our University campus (Bjorkman et al, in prep). A higher rate of spread by viral super-carriers would be consistent with recent contact tracing analyses suggesting that 80-90% of infections are spread by 10-20% of infected individuals [14][15][16][17].…”

Upper respiratory tract SARS-CoV-2 RNA loads in symptomatic and asymptomatic children and adults

“…There is a consistent direct correlation between magnitude of SARS-CoV-2 RNA load in the upper respiratory tract (URT) and probability of recovering live virus in cell culture, in both adults and children [2][3][4][5]; hence, viral load in URT may be used as a proxy for contagiousness. Supporting this assumption, transmission risk was recently shown to be strongly associated with initial SARS-CoV-2 RNA levels of index cases [6]. There is scarce information on how SARS-CoV-2 RNA load in UTR compares between children and adults [7][8][9][10][11], whether viral load in pediatric subjects differ across ages [9,10], and whether dissimilarities in the dynamics of SARS-CoV-2 shedding in URT exist between symptomatic and asymptomatic children [12][13][14].…”

The rise of SARS-CoV-2 variant B.1.1.7 in Israel intensifies the role of surveillance and vaccination in elderly

“…001 ) (Figure 1D). Collectively, as viral load drives transmission 15,16 these data underscores the important role of random surveillance testing in nursing homes.…”