Role of Secondary Attack Rate in The Surge of COVID 19 Cases: A Comparative Retrospective Cohort Study between First Wave and Second Wave of COVID 19

Objective: A comparative retrospective cohort study was conducted between the first wave and the second wave to decipher the cause of the enormous spike in COVID-19 cases in the second wave.<br><br>Methods: In this retrospective cohort study, an individual primary case was interviewed using the "COVID-19 transmission questionaries sheet" to trace the total number of primary cases and age-wise unrestricted social interaction and secondary cases. We assessed the comparative secondary attack rate, incidence proportion rate and to what extent unrestricted social interaction contributed to the surge of COVID-19 cases during the incubation period.<br><br>Results: Between January-2021 and May-2021, a total of 1587 unrestricted social interactions were traced with 437 primary cases for the second wave. Of those, 36.8% primary cases developed 334 secondary cases. While for the first wave, a total of 436 unrestricted social interactions were traced with 112 primary cases between January-2020 and December-2020. Of those, 25.0% primary cases developed 82 secondary cases. The secondary attack rate rose 2.5-fold in the age group &gt;50years, and rose ~86% in the age group &lt;10years, while it remained stable in the age group 10-50years. The overall 58% increment in secondary attack rate in the second wave indicates changes in the pattern of unrestricted social interaction amongst household members. This was further supported by the incidence proportion rate that surprisingly rose to ~39% in households of two- and three members in the second wave, while it was negligible in the first wave.<br><br>Conclusion: Changes in the pattern of unrestricted social interaction, which escalated the secondary attack rate, particularly in the age groups of &lt;10 years and &gt;50 years, and a higher incidence proportion rate in the household size of two- and three members in the second wave, could have led to the unprecedented surge of COVID-19 cases in the second wave.