Background: Coronavirus disease 2019 (COVID-19) has become a major public health problem since its inception disrupting public life and crippling health systems. The mutated variant of the causative virus, Delta, has been notorious in causing rapid upsurge in cases compared to the Alpha variant. The current study tries to find out the household secondary attack rate (HSAR) of COVID-19 and factors associated with it during the second wave of cases in Kerala. Methodology: A retrospective cohort study was performed among 313 household contacts of 76 COVID-19 patients who had been admitted in Government Medical College, Thrissur, in the southern state of India, Kerala. Data from the participants were collected via phone using a semi-structured interview schedule, and analysis was performed with SPSS software. Results: The HSAR among household contacts was 59.1% (53.4–64.6%). The risk of acquiring COVID infection among household contacts was higher among contacts of symptomatic index cases with a P value of 0.001 and an odds ratio of 11 (3.7–32.4). index cases were having a home isolation P value of 0.001 and an odds ratio of 3.2 (2–5.1), with delay in COVID-19 testing for index cases with a P value of 0.006. Regarding characteristics of household contacts, higher age groups (p = 0.048), groups living in the same room with an index case P value of 0.021 and an odds ratio of [1.71 (1-2.8)], groups having physical contact with an index case P value of 0.001 and an odds ratio of [3.7 (2.1-7)], groups with touched or cleaned linen/articles with an index case P value of 0.02 and an odds ratio of [1.8 (1-3.1)], and groups having co-morbidities, especially diabetes mellitus (p = 0.0020), were significantly associated with chances of acquiring infection. However, the history of previous COVID positivity in household contacts was a protective factor against the infection P value of 0.009 with an odds ratio of [0.09 (0.01-0.78)]. Conclusion: The study concludes that the second wave of COVID-19 in Kerala was primarily caused by a high SAR, especially among household contacts, and this could have been the reason for the difficulty in control measures during the wave.