Researchers, including those from the Government of Tamil Nadu, and Andhra Pradesh, have conducted one of the largest analysis of COVID-19 epidemiology to date, and have found that both cases and deaths due to the disease have been more heavily concentrated in the 40-69 year age group in India than is seen in high-income countries, among other trends.

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The study, published in the journal Science on September 30, assessed the disease transmission patterns in 5,75,071 individuals exposed to 84,965 confirmed cases of COVID-19 in the two states based on data collected by tens of thousands of contact tracers. According to the scientists, including Ramanan Laxminarayan from the Center for Disease Dynamics, Economics, and Policy in New Delhi, the findings offer a peek into the pandemic's trajectory in a low- and middle-income country, where most COVID-19 cases have occurred.

Based on the data, the scientists said both cases and deaths in the two Indian states were concentrated in younger cohorts than expected from observations in higher-income countries. The study noted that contacts with the same age were associated with the greatest infection risk -- a pattern, which the researchers said, was strongest among children between 0-14 years of age, and among adults older than 65 years. They said the proportion of deaths to the number of infected people, known as the case-fatality ratio (CFR), spanned 0.05 percent at ages 5-17 years to 16.6 percent at ages exceeding 85.

The researchers also noted that COVID-19 patients in the two states have a median stay of five days in the hospital before death compared to 13 days to death from the date of hospital admission in the US. Follow-up testing of exposed contacts revealed that 70 percent of infected individuals did not infect any of their contacts, while 8 percent of coronavirus positive patients accounted for 60 percent of observed new infections.  The researchers said this finding presents the largest empirical demonstration of superspreading. They said the findings "may indicate the identification of less-severe infections through active case-finding."

Andhra Pradesh and Tamil Nadu are among the Indian states with the largest healthcare workforce and public health expenditures per capita, and are known for their effective primary healthcare delivery models, the study noted. It said 63 percent of those who died had at least one co-morbidity, and 36 percent had two or more underlying health conditions that made them more susceptible to the disease. About 45 percent of those who died were diabetic, the scientists said.

Unlike observations in high-income countries, they said the deaths in India are concentrated at ages 50-64 years. In the two Indian states in particular, they said only 17.9 percent of COVID-19 deaths occurring on or before 1 August, 2020 were among people older than 75, compared with 58.1 percent of deaths due to the disease in the US. "This study was made possible by the significant contact tracing effort in Andhra Pradesh and Tamil Nadu, which involved tens of thousands of healthcare workers," Laxminarayan said. "The results on disease transmission and mortality have the potential to inform policy to fight COVID-19. The study also speaks to the capacity of research emerging from India to help inform the global response to COVID-19," he added.