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U. researcher finds COVID-19 infections in South India driven by superspreaders in largest contact tracing study to date

Coronavirus model
Photo courtesy of U.S. Department of State

Research led by Ramanan Laxminarayan, a senior research scholar at the Princeton Environmental Institute (PEI), found that most COVID-19 infections in the Indian states of Andhra Pradesh and Tamil Nadu are spread by a small number of infected individuals known as superspreaders.         

“No secondary infections were linked to 71% of cases whose contacts were traced and tested,” the researchers found. Rather, 8 percent of those infected accounted for 60 percent of new cases.


“Superspreading has been suspected, but not really documented,” Laxminarayan told CNN. 

Laxminarayan worked with researchers from Johns Hopkins University and the University of California, Berkeley and Indian public health officials to examine infection rates among 575,000 contacts of 84,965 confirmed COVID-19 cases. This is the largest contact tracing study done to date. 

“Previous studies of Covid transmission have been fairly small and these pose a challenge for arriving at any definitive results on who spreads the disease, in what contexts and to what extent,” Laxminarayan told The Daily Princetonian.

Researchers traced instances where infection spread from an initial — or index — case and organized the data according to the sex and age of those infected by the index case. They found that contacts among individuals in the same age group carried the highest risk of infection. 

“Patterns of enhanced transmission risk in similar age-pairs were strongest among children ages 0–14 years and among adults 65 years and older,” the study found. 

The researchers were able to “identify high prevalence of infection among children who were contacts of cases around their own age,” though they say “the role of children in transmission has been debated.”


Laxminarayan and his co-authors concluded that while “school closures and other non-pharmaceutical interventions” may have helped reduce transmission during the study period, social interactions among children may still be conducive to transmission.

However, the greatest proportion of test-positive contacts in most age groups stemmed from “index cases aged 20–44 years.”

Mortality plateaued above age 65, whereas in the United States, most deaths occur in those over 75. The researchers suggest that differing stay-at-home orders, social welfare programs for seniors, and India’s lower life expectancy explain this disparity.

Contact tracing in the study was done manually, without any app or website. 

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“This was old fashioned, shoe leather epidemiology that involved calling or visiting those with a positive test to ask them about people whom they may have come in close contact with,” Laxminarayan said.

India launched a COVID-19 contact tracing app in late March, which has since been downloaded by over 100 million users. Some have posed privacy concerns, echoing those raised in the United States as Congress tries to pass the TRACE Act — legislation which would grant $100 billion to health care service providers for contract tracing. 

In the future, the research team hopes to gather data on when a person was exposed to an index case and track the onset of symptoms after a positive test. Laxminarayan also hopes for universal acknowledgment of the importance of social distancing. 

“The superspreading information would indicate that there should be a greater focus on stopping opportunities for large gatherings particularly in indoor spaces unless there is aggressive testing to ensure that individuals participating in these events are not infectious,” he said.

The study was published in Science on Sept. 30.