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Epidemiology and transmission dynamics of COVID-19 in two Indian States

Our take —

Several CDC case reports and published case series suggest that multisystem inflammatory Through August 1, 2020, more than 400,000 cases of COVID-19 were diagnosed in Tamil Nadu and Andhra Pradesh, India; more than 3 million contacts were investigated. Incidence declined following lockdowns. Similar to other settings, age and co-morbid conditions were associated with higher risk for mortality and the case fatality rate was highest among those aged >85 years of age (16.6%). The secondary attack rates among low risk contacts was 4.7% and 10.7% among high risk contacts and the highest risk of transmission was between case-contact pairs from the same age group, though there was also inter-generational transmission in households. The number of infections among children were likely underestimated since people with symptoms were more likely to be tested in public health campaigns.

Study design


Study population and setting

This study uses data from COVID-19 public health surveillance and contact tracing efforts through August 1, 2020 from Tamil Nadu and Andhra Pradesh, two states in India.

Summary of Main Findings

In this comprehensive and wide-ranging analysis, the authors investigated the trajectory of the outbreak in the two states, secondary attack rates by age of cases and contacts, and mortality risk. Through August 1, 2020 there were 263,330 cases reported from Tamil Nadu and 172,209 from Andhra Pradesh; many cases were likely missed early during the pandemic when testing was inadequate and cases declined following lockdowns. Contact tracing efforts identified more than 3 million contacts, and complete data were available from 575,071 contacts of 84,965 cases for analysis. The authors estimated that the overall secondary attack rate for contacts with high risk exposures was 10.7%, and 4.7% for those with low risk exposures. The highest probability of transmission occurred among case-contact pairs of similar age, but contacts of cases aged 20-44 years were at highest risk overall. Among 124,884 cases followed-up to ascertain outcomes, 2% died with risk increasing by age; 0.05% of cases aged 5-17 years died, compared to 16.6% of those aged >85 years.

Study Strengths

The analysis includes hundreds of thousands of cases and contacts with detailed information about risks and outcomes. The findings are very detailed, including a lengthy supplementary file accompanying the paper, which provide insights on multiple areas of epidemiology and transmission in these areas of India.


Similar to all studies reliant upon public health surveillance data, there are a lot of missing data and many cases were likely missed. In particular, people with symptoms were much more likely to be tested, so this study likely underestimated cases and secondary attack rates among younger age groups, who are more likely to be asymptomatically infected with SARS-CoV-2.

Value added

This is the most comprehensive analysis to date about SARS-CoV-2 in South Asia available, and contributes importantly to our understanding of incidence, mortality, and transmission risks, by age and setting.

This review was posted on: 30 October 2020