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

Abstract

India had the worst conditions during the second wave, and yet, the cause of the enormous spike in COVID-19 cases in a short time span remains unexplained. We aimed to decipher the cause of the enormous spike in COVID-19 cases in the second wave in India. A comparative retrospective cohort study was conducted between the first wave and the second wave. 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. Between January-2021 and May-2021, 1587 unrestricted social interactions were traced with 437 primary cases for the second wave. While for the first wave, 436 unrestricted social interactions were traced with 112 primary cases between January-2020 and December-2020. In the second wave, 36.8% of primary cases developed 334 secondary cases, while 25.0% of primary cases developed 82 secondary cases in the first wave. The secondary attack rate rose 2.5-fold in the age group >50years, and rose ~86% in the age group <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. Changes in the pattern of unrestricted social interaction among household members and a high affinity of the delta variant virus for infecting people in the age groups of <10 years and >50 years escalated the secondary attack rate and incidence proportion rate that led to the unprecedented surge of COVID-19 cases in the second wave.