Study finds symptomatic COVID cases are 8.16 times more likely to spread symptomatic infection

Contact tracing data from Karnataka between March 9 and July 21, 2020, was analysed for the research

The findings of a study by a group of researchers has found that symptomatic COVID patients are 8.16 times more likely to cause symptomatic infection in their contacts when compared to asymptomatic cases.

The study titled “Contact tracing of COVID-19 in Karnataka, India: Superspreading and determinants of infectiousness and symptomatic infection” was published on July 11 in PLOS One, a  peer-reviewed open access scientific journal.

The study 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. 

“We analysed contact tracing data from Karnataka between March 9 and July 21, 2020. Metrics of transmission including the reproduction number (R), overdispersion, secondary attack rate (SAR) and serial interval (defined as the interval between symptom onset of the index case and the secondary case in a transmission chain) were estimated. We found that symptomatic cases were 8.16 times more likely to cause symptomatic infection in their contacts,” Giridhar R. Babu, the corresponding author of the study, told The Hindu on Friday.

“Up to July 21, 2020, 111 index cases that crossed the super-spreading threshold of ≥8 secondary cases were found. Among 956 confirmed traced cases, 8.7% of index cases had 14.4% of contacts but caused 80% of all secondary cases. Among 16,715 contacts, overall SAR was 3.6% and symptomatic cases were more infectious than asymptomatic cases,” according to the study.

“As compared to infectors aged 19-44 years, children were less infectious. Infectors who were confirmed ≥4 days after symptom onset were associated with higher infectiousness. Serial interval had a mean of 5.4 days, and case fatality rate was 2.5%, which increased with age,” stated the study.

Dr. Babu, who is part of the State’s COVID-19 Technical Advisory Committee (TAC) and also heads Lifecourse Epidemiology at Indian Institute of Public Health in Bengaluru, said although the study was published late, the results were used in Karnataka in prioritising testing and isolation for symptomatic cases.