Epidemiology of SARS-CoV-2 Infection in Karnataka State, South India: Transmission Dynamics of Symptomatic vs. Asymptomatic Infections

Background: In this report, we describe the epidemiology of SARS-CoV-2 infection, specifically examining how the symptomatic persons drove the transmission in the state of Karnataka, India, during the lockdown phase. Methods: The study included all the cases reported from March 8 to May 31, 2020 in the state. Any person with history of international or domestic travel from high burden states, those presenting with Influenza-like or Severe Acute Respiratory Illness and high-risk contacts of COVID19 cases, who were SARS-CoV-2 RT-PCR positive were included. Detailed analysis based on contact tracing data available from line-list of the state surveillance unit was performed using cluster analysis software package. Findings: Amongst the 3404 COVID-19 positive cases, 3096 (91%) were asymptomatic while 308 (9%) were symptomatic. Majority of the asymptomatic cases were in the age range of 16-50 years while symptomatic cases were between 31-65 years. Most of those affected were males. Cluster analysis of 822 cases indicated that the secondary attack rate, size of the cluster (dispersion) and occurrence of overt clinical illness is significantly higher when the index case in a cluster was symptomatic compared to an asymptomatic. Interpretation: Our findings indicate that both asymptomatic and symptomatic SARS-CoV-2 cases transmit the infection; however, the main driving force behind the spread of infection within the state was significantly higher from symptomatic cases. This has major implications for policies related to testing. Active search for symptomatic cases, subjecting them to testing and treatment should be prioritized for containing the spread of COVID-19.  Funding: The study is funded by an Intermediate Fellowship of the Wellcome Trust-DBT India Alliance (Clinical and Public Health Research Fellowship) to Giridhara R Babu, Grant number: IA/CPHI/14/1/501499. Declaration of Interests: GRB and DAJ are employees of Indian Institute of Public Health, Bengaluru, Public Health Foundation of India and their employer had no say in the design of the study or the decision to publish. All other authors are employees of either state government (PD, PKBG) or NIMHANS (Central government). The employers had no role in the design of the study or the decision to publish. The authors declare that they do not have any other financial or non-financial relationships that could present a conflict of interest. Ethics Approval Statement: This is to certify that the study entitled Epidemiology of SARS-CoV-2 Infection in Karnataka State, South India: Transmission Dynamics of Symptomatic vs. Asymptomatic Infections was reviewed and approved by the NIMHANS Institutional Ethics Committee (Basic and Neurosciences) in its 24th meeting held on 3rd June 2020 (Ref No. NIMHANS/IEC/2020-21)