The Question

A study led by CDDEP, representing coordinated efforts between researchers from Princeton, University of California, Berkeley, Johns Hopkins Bloomberg School of Public Health, and the Government of Tamil Nadu and Andhra Pradesh, investigated disease transmission patterns in 575,071 individuals exposed to 84,965 confirmed cases of COVID-19, in one of the largest contact tracing studies to date.

Andhra Pradesh (population 50 million) and Tamil Nadu (population 68 million) 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. Both states initiated rigorous disease surveillance and contact tracing early in response to the pandemic. Procedures include syndromic surveillance and SARS-CoV-2 testing for all individuals seeking care for severe acute respiratory illness or influenza-like illness at healthcare facilities; delineation of 5km “containment zones” surrounding cases for daily house-to-house surveillance to identify individuals with symptoms; and daily follow-up of all contacts of laboratory-confirmed or suspect COVID-19 cases, with the aim of testing these individuals 5-14 days after their contact with a primary case, irrespective of symptoms, to identify onward transmission.


What We Found


COVID transmission

  1. Risk of transmission from an index case to a close contact ranges from 2.6% in the community to 9.0% in the household and does not differ significantly with respect to the age of the index case.
  2. Infection probabilities ranged from 4.7-10.7% for low-risk and high-risk contact types, respectively. Same-age contacts were associated with the greatest infection risk.
  3. The study found high prevalence of infection among children who were contacts of cases around their own age; this finding of enhanced infection risk among individuals exposed to similar-age cases was also apparent among adults.
  4. Not all infected individual transmit COVID-19. Prospective follow-up testing of exposed contacts revealed that 70% of infected individuals did not infect any of their contacts, while 8% of infected individuals accounted for 60% of observed new infections. This study presents the largest empirical demonstration of superspreading that we are aware of.


  1. Case-fatality ratios spanned 0.05% at ages 5-17 years to 16.6% at ages ≥85 years.
  2. Men were 62% more likely to die than women.
  3. 63% of those who died had at least one co-morbidity. 36% had two or more co-morbidities. 45% of those who died were diabetic.
  4. Unlike observations in high-income settings, deaths in India are concentrated at ages 50-64 years. The figure below shows case fatality ratios (CFR) at various age groups in India compared to other countries. The CFR is higher in the 40-70 age group in India than in any of the four comparison countries (see figure below). For the age category above 80, the CFR is India is in line with other countries indicating a potential survival effect.
  5. Contrary to long hospital stays reported in high-income settings, the median time to death is 6 days following admission (compared to 13 in the United States).

Effect of the Lockdown

  1. There are substantial reductions in the reproductive number Rt associated with implementation of India’s country-wide shutdown, which have not previously been shown empirically.
  2. Case-fatality ratios (proportion of cases that died) have decreased over the course of the epidemic. Individuals who tested positive between in July were 26% less likely to die than those tested in March and April. Those who tested positive in May and June were 13% less likely to die than those tested in March and April.


Why It Matters

“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. 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”, said the director of CDDEP, Dr Ramanan Laxminarayan.

The study titled, “Epidemiology and transmission dynamics of COVID-19 in two Indian states” was published on September 30, 2020 in Science and is available online here.


In the Media

The study’s findings and its implications in the public health context have been discussed in depth in several major international media outlets:

The Economist this week: our coverage of the coronavirus – The Economist

Huge Study of Coronavirus Cases in India Offers Some Surprises to ScientistsThe New York Times

Largest study of COVID-19 transmission highlights essential role of super-spreadersLos Angeles Times

 Kids And Superspreaders Are Driving COVID-19 Cases In India, Huge Study FindsNPR

 Are children the biggest Covid-19 spreaders?BBC News, World Service

 Big contact-tracing study shows role of kids and superspreaders in coronavirus pandemicCNN

Studies show: High prevalence of infection among children who were in contact with their own age group – The Hindustan Times

India Surge Driven by Super-SpreadersBloomberg

 What a Positive Test Won’t Change About Trump and the PandemicThe New Yorker

Check the study’s Altmetrics here


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