Background and Objectives: As Severe Acute Respiratory Syndrome Corona virus 2 (SARS-CoV-2) continues to spread, it is important that we continue to expand our knowledge about its transmission. Asymptomatic cases missed by surveillance and their ability to transmit SARS-CoV-2, will  tamper any hopes of stopping the COVID-19 Pandemic .This study provides various insights in interpreting disease transmission,  evaluating interventions, and setting public health policies.

This study is to determine the rate of secondary infection among Symptomatic and Asymptomatic contacts of individuals with confirmed Corona Virus disease 2019 (COVID-19) in Tertiary care centre, Thanjavur.

Contact tracing is a central public health response to infectious disease outbreaks, especially in the early stages of an outbreak when specific treatments are limited.

Methods: From May, 2020 to September, 2020 (5 months), overall 200718 cases were tested by RT-PCR which includes In-patients, Out-patients, Contacts of Covid-19 infected cases, and general surveillance population.

In this, the Data of Contacts of Individuals with RT-PCR confirmed COVID-19 infection were taken .The contacts were then categorized as Symptomatic and Asymptomatic contacts of RT-PCR positive COVID-19 cases. The Secondary attack rate among the Symptomatic and Asymptomatic contacts were compared and analyzed.

Results: From May, 2020 to September, 2020 (5 months), 23895 contacts Of Covid-19 RTPCR positive patients, among which 6057 were symptomatic (25%) and 17838 were asymptomatic (75%) were traced. They were tested for SARS-CoV-2 infection by RTPCR. Among Symptomatic contacts, 943 were tested positive (16%) and 5114 were tested negative (84%). Among Asymptomatic contacts, 2706 were tested positive (15%) and 15132 were tested negative (85%).

Among the 23895 contacts, 3649 secondary cases of COVID-19 infection (including 2706 asymptomatic infections) were detected, with an overall infection risk of 15.27% (95% CI, 14.82%-15.73%, Taylor series). The secondary clinical attack rate was 15.57% (95% CI, 14.68%-16.5%) among symptomatic contacts and 15.17% (95% CI, 14.65%-15.7%) among asymptomatic contacts.

Discussion: This study explored transmission risk factors for COVID-19 and the proportion of asymptomatic cases that would have been missed by testing symptomatic individuals only. Our analysis revealed a similar clinical attack rate between the contacts that were symptomatic and asymptomatic. This observation was consistent with a reasonably high rate of asymptomatic carriage. For prevention of transmission, early identification of COVID-19 index cases and contacts is important. In view of the significant number of missed diagnoses in a symptom-based testing strategy, testing of all contacts, including asymptomatic individuals, is recommended.

Keywords:  COVID-19, SARS-CoV-2, Symptomatic and Asymptomatic contacts, Secondary attack rate.

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Dr V. Saiprabha