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New Delhi, Oct. 1 (PTI): The transmission of Covid-19 in India has been driven mainly by superspreaders or a small percentage of infected people, according to the largest contact tracing study in the country which also shows that children are key to the spread of the novel coronavirus.
The researchers, including those from the Government of Tamil Nadu, and Andhra Pradesh, found that over 70 per cent of Covid-19 infected patients in the country did not infect any of their contacts, while 8 per cent of infected individuals accounted for 60 per cent of observed new infections.
The study of over half-million people in India found that both cases and deaths due to the disease have been more heavily concentrated in the 40-69 year age group than is seen in high-income countries, among other trends.
The researchers also found a high prevalence of infection among children who were contacts of other cases around their own age.
The study, published in the journal Science on Wednesday, 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 per cent at ages 5-17 years to 16.6 per cent 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 per cent of infected individuals did not infect any of their contacts, while 8 per cent of coronavirus positive patients accounted for 60 per cent of observed new infections.
The researchers said this finding presents the largest empirical demonstration of superspreading, which is when an unusually large number of individuals are infected by few other people.
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 per cent of those who died had at least one co-morbidity, and 36 per cent had two or more underlying health conditions that made them more susceptible to the disease.
About 45 per cent 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 per cent of Covid-19 deaths occurring on or before 1 August, 2020 were among people older than 75, compared with 58.1 per cent 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.