Contact tracing of COVID-19 in Karnataka, India: Superspreading and determinants of infectiousness and symptomatic infection

Abstract

Background India has experienced the second largest outbreak of COVID-19 globally, yet there is a paucity of studies analysing contact tracing data in the region which can optimise public health interventions (PHI’s). Methods We analysed contact tracing data from Karnataka, India between 9 March and 21 July 2020. We estimated metrics of transmission including the reproduction number (R), overdispersion (k), secondary attack rate (SAR), and serial interval. R and k were jointly estimated using a Bayesian Markov Chain Monte Carlo approach. We studied determinants of risk of further transmission and risk of being symptomatic using Poisson regression models. Findings Up to 21 July 2020, we found 111 index cases that crossed the super-spreading threshold of ≥8 secondary cases. Among 956 confirmed traced cases, 8.7% of index cases had 14.4% of contacts but caused 80% of all secondary cases. Among 16715 contacts, overall SAR was 3.6% [95% CI, 3.4–3.9] and symptomatic cases were more infectious than asymptomatic cases (SAR 7.7% vs 2.0%; aRR 3.63 [3.04–4.34]). As compared to infectors aged 19–44 years, children were less infectious (aRR 0.21 [0.07–0.66] for 0–5 years and 0.47 [0.32–0.68] for 6–18 years). Infectors who were confirmed ≥4 days after symptom onset were associated with higher infectiousness (aRR 3.01 [2.11–4.31]). As compared to asymptomatic cases, symptomatic cases were 8.16 [3.29–20.24] times more likely to cause symptomatic infection in their secondary cases. Serial interval had a mean of 5.4 [4.4–6.4] days, and case fatality rate was 2.5% [2.4–2.7] which increased with age. Conclusion We found significant heterogeneity in the individual-level transmissibility of SARS-CoV-2 which could not be explained by the degree of heterogeneity in the underlying number of contacts. To strengthen contact tracing in over-dispersed outbreaks, testing and tracing delays should be minimised and retrospective contact tracing should be implemented. Targeted measures to reduce potential superspreading events should be implemented. Interventions aimed at children might have a relatively small impact on reducing transmission owing to their low symptomaticity and infectivity. We propose that symptomatic cases could cause a snowballing effect on clinical severity and infectiousness across transmission generations; further studies are needed to confirm this finding.

Limited role of children in transmission of SARS‐CoV‐2 virus in households—Immunological analysis of 26 familial clusters

“…This observation, although based on a small study group, strengthens the data on a limited role of children in the SARS-CoV-2 transmission compared with adults. Many recent studies suggest lower risk of transmission of SARS-CoV-2 infection from infected children in both household [8][9][10][11][12][13]23 and educational settings. [14][15][16]18,24 In addition, most countries did not observe any significant increase in SARS-CoV-2 infections after school reopening, with the exception of Israel, where school opening coincided with the opening of other facilities.…”

“…This assumption led to preventive measures such as school closures and isolation for a large number of children, which have had a significant impact on children's physical and mental health as well as their education. The evidence addressing the transmission of SARS‐CoV‐2 from infected children is still scarce, and the findings are controversial, with some studies suggesting children to be relevant spreaders of the virus 2–7 and others emphasizing their limited role in disease transmission 8–18 . Only few studies give direct evidence of children acting as index cases in their households 19 .…”

“…The evidence addressing the transmission of SARS‐CoV‐2 from infected children is still scarce, and the findings are controversial, with some studies suggesting children to be relevant spreaders of the virus 2 , 3 , 4 , 5 , 6 , 7 and others emphasizing their limited role in disease transmission. 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 Only few studies give direct evidence of children acting as index cases in their households. 19 While this could be underestimated due to asymptomatic courses of disease in children and in consequence less frequent testing, there is growing evidence that secondary attack rates from infected children are lower than from infected adults.…”

Looking under the lamp-post: quantifying the performance of contact tracing in the United States during the SARS-CoV-2 pandemic

“…We do not account for asymptomatic transmission, again making our estimates an upper bound. The percent of asymptomatic COVID-19 cases is estimated to be anywhere between 1.6% and 56.5% [90][91][92][93][94][95][96], with a these cases having a relative reduced infectiousness of 0 to 62% [90][91][92][93][94][95][96][97]. This would mean our estimated 1.65% of transmission pairs identified with PCR testing could be as low as 0.9%, assuming no asymptomatic index cases are identified.…”

“…We also have not accounted for superspreading, which has been estimated to be a significant feature in COVID-19 transmission [94][95][96]. This implies that missing a superspreading index case would have tremendous impact on downstream contact tracing efforts and that there is significant stochasticity [97]. We only consider the probability of identifying infected contacts, but it is ideal to also identify uninfected contacts accurately.…”