Secondary attack rate of COVID-19: analysis of contacts of COVID-19 cases admitted in a tertiary care centre, Northern district of Kerala, India-a cross-sectional study

Anitha S. Saraswathy, Rameela Sanya, Jayasree A. Kumaran


Background: Secondary attack rate is the proportion of primary contacts that develop infection or symptom within one incubation period after contact with an index case. The secondary attack rate will help in studying and understanding the transmission dynamics of infectious diseases. As coronavirus disease (COVID-19) is a novel disease which has resulted in a pandemic, understanding the transmission dynamics is very important for strengthening preventive measures.

Methods: The present cross-sectional study was conducted to estimate the secondary attack rate among primary contacts of COVID-19 cases admitted in a tertiary care centre in Northern Kerala. Secondary attack rate was calculated as the percentage of primary contacts that tested positive for COVID-19 with RT-PCR test within 14 days of exposure.

Results: Secondary attack rate was found to be 5.88%, secondary clinical attack rate 2.35% and secondary infection rate 3.53%. Among those contacts that developed disease, 80% were household contacts.

Conclusions: Secondary attack rate was 5.88% and household contacts were the majority to get the disease.


COVID-19, Kerala, Secondary attack rate, Secondary clinical attack rate, Secondary infection rate

Full Text:



Rolling updates on corona virus disease (COVID-19). Available at: diseases/novel-coronavirus-2019/events-as-they-happen. Accessed on 24 April 2020.

WHO coronavirus disease (COVID-19) sash board. Available at: Accessed on 20 August 2020.

Ministry of health and family welfare, Government of India, COVID-19 state wise status. Available at: Accessed on 20 August 2020.

Directorate of health services, Kerala. COVID-19 outbreak control and prevention state cell. Available at: 08/Bulletin-HFWD-English-September-09.pdf. Accessed on 20 August 2020.

Yang L, Eggo RM, Kucharski AJ. Secondary attack rate and super spreading events for SARS-CoV-2. Lancet. 2020;395(10227):e47.

Questions and answers on corona viruses: COVID-19. Available at: Accessed on 24 April 2020.

Homecare for patients with COVID-19 presenting with mild symptoms and management of their contacts. Available at: file:///C:/Users/user/ Downloads/WHO-nCov-IPC-HomeCare-2020.3-eng.pdf. Accessed on 24 April 2020.

Household transmission investigation protocol for corona virus disease 2019 COVID-19. Available at: considerations-in-the-investigation-of-cases-and-clusters-of-covid-19. Accessed on 24 April 2020.

Shah K, Saxena D, Mavalankar D. Secondary attack rate of COVID-19 in household contacts: a systematic review. QJM Int J Med. 2020;1-10.

Qin-Long J, Ming-Jin L, Zhou-Bin Z, Li-Qun F, Jun Y, An-Ran Z, et al. Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study. Lancet Infect Dis. 2020;20(10):1141-50.

Qin J, You C, Lin Q, Hu T, Yu S, Zhou XH. Estimation of incubation period distribution of COVID-19 using disease onset forward time: a novel cross-sectional and forward follow-up study. Preprint. medRxiv. 2020.

Jing QL, Liu MJ, Yuan J, et al. Household secondary attack rate of COVID-19 and associated Determinants. Preprint. medRxiv. 2020.

Epidemiology and case management team, Korea centers for disease control and prevention. Coronavirus disease-19: summary of 2,370 contact investigations of the first 30 cases in the Republic of Korea. Osong Public Health Res Perspect. 2020;11(2):81-4.

Burke RM, Midgley CM, Dratch A, Fenstersheib M, Haupt T, Holshue M et al. Active monitoring of persons exposed to patients with confirmed COVID-19 - United States, January-February 2020. Morb Mortal Wkly Rep. 2020;69(9):245-6.