Transmission dynamics of COVID-19 in two Indian states

Indian and American researchers collaborated to study the trends of transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in two Indian states – Tamil Nadu and Andhra Pradesh. Their study titled, “Epidemiology and transmission dynamics of COVID-19 in two Indian states,” was published in the latest issue of the journal Science last week.

Study: Epidemiology and transmission dynamics of COVID-19 in two Indian states. Image Credit: Talukdar David / Shutterstock
Study: Epidemiology and transmission dynamics of COVID-19 in two Indian states. Image Credit: Talukdar David / Shutterstock


The COVID-19 pandemic has affected over 36 million people worldwide, and India ranks second among the most affected nations. India being a relatively resource-poor nation with the second largest population globally, handling the pandemic with a highly infectious disease has been a challenge over the past half-year. The epidemiology of the infection is not clearly known in India. This study attempts to show the trends of the spread of the infection in two states in India, namely Tamil Nadu and Andhra Pradesh.

COVID-19 in India

Individuals in low- and middle-income countries (LMICs) such as India are at an “increased risk of severe outcomes” and also have inadequate access to healthcare services. The dynamics of the infection in these countries, thus different from countries in Europe and developed nations such as the United States.  The studies with mathematical modeling that look at the epidemiology of the infection have been performed in the developed nations to date.

In India, the risk of the infection was to over its 1.3 billion population, and thus large-scale containment strategies were implemented at national, state, and local levels. The first documented case of COVID-19 was an Indian national from China.

Andhra Pradesh and Tamil Nadu

Andhra Pradesh and Tamil Nadu are two states in the south of India, with a population of 127.8 million or 10 percent of the total population of India. The healthcare workforces of these two states are better than other states. Their public health expenditures per capita are also more significant with a better and more effective primary healthcare delivery as per pre-COVID data.

As the pandemic began, like other states, here too surveillance and contact tracing measures were adopted early. All individuals seeking care for severe acute respiratory illness or influenza-like illness at healthcare facilities were tested for SARS CoV-2, and their contacts were followed up. 5 km zones were marked as “containment zones” surrounding cases.

Screening process and testing

Tamil Nadu, like other states, initiated airport screening for severe acute respiratory infection for travelers along with thermal and clinical screening since 4 March 2020. All symptomatic persons and their contacts within 14 days were screened.

Tamil Nadu and Andhra Pradesh recorded their first cases on 5 March 2020. Testing capacity was low initially, and the percentage positives on testing were 39.7 percent in Tamil Nadu and 33.5 percent in Andhra Pradesh on 30 and 31 March 2020, respectively.

Positive cases and trends

Throughout April 2020, the number of tests rose in both states, and by 1 August 2020, Tamil Nadu and Andhra Pradesh had 263,330 and 172,209 cases, respectively.

  • Chennai is the capital city of Tamil Nadu, and since March, there was the highest cumulative incidence of COVID-19 there totaling 102,199 cases or 204.6 per 10,000 population by 1 August 2020.
  • The surrounding districts of Ariyalur, Cuddalore, Perambalur, and Villupuram in Tamil Nadu saw a cluster of cases starting on 28 April with a total of 1,142 cases by 15 May.
  • Cases rose in southern districts of Tamil Nadu surrounding Madurai during June
  • Cases rose in all districts of Andhra Pradesh in June

Reproduction number

The Reproduction number Rt that determines the number of secondary infections each infected individual would be expected to lead to also changed over time.

  • The Rt was 1.7 - 3.0 in Tamil Nadu over the period of 10th to 23 March and was 1.0 - 1.3 by the third week of lockdown that started on 23 March 2020.
  • The Rt was 1.4 - 4.3 in Andhra Pradesh over the period of 10th to 23 March and was 1.0 - 1.3 by the third week of lockdown that started on 23 March 2020.
  • Rt was in the range 1.1 - 1.4 from 15 May till 1 August in both states. This was due to social distancing interventions, and well as effective contact tracing and rise in the number of testing said the researchers.
  • By 1 August, 3,084,885 known exposed contacts of confirmed cases had been traced, and 575,071 had been tested. These contacts were generally younger and were more likely to be females. The team found, “test-positive individuals identified through contact tracing were, on average, 1.3 years younger and 4.5% less likely to be male than the overall population of COVID-19 cases in the two states.”
  • The average number of contacts tested per index case was 7.3, and 0.2 percent of index cases were linked to over 80 tested contacts.
  • No positive contacts could be found for 70.7 percent index cases, though.

Death rates

  • Death rates were seen among 102,569 cases in Tamil Nadu and 22,315 cases in Andhra Pradesh
  • The overall case-fatality ratio was 2.06 percent
  • Age-specific case fatality rates were lowest at 0.05 percent (0.012-0.11%) at ages 5 to 17 years and highest at 16.6 percent (13.4-19.9%) at ages more than 85 years.
  • The risk of death was more significant for males than females, older than younger individuals, and those in high-income settings.
  • Nearly half of the deaths in Tamil Nadu and Andhra Pradesh were seen within 6 days of detection. The median time-to-death from the date of hospital admission was 13 days, and this was this was lower than what has been seen around the world. Authors wrote, “Our observations likely indicate a substantial proportion of patients in Tamil Nadu and Andhra Pradesh are diagnosed late in their disease course...”
  • Common comorbid conditions associated with deaths were “diabetes (45.0%), sustained hypertension (36.2%), coronary artery disease (12.3%), and renal disease (8.2%)”

Conclusions and implications

The overall case-fatality rates in the two states of India were similar to other countries. There was a lower incidence of the infection among older adults, and thus the overall case-fatality ratio and age distribution of cases were different from the worldwide picture.

The team wrote, “Prospective testing of a large sample of exposed individuals through integrated active surveillance and public health interventions in Tamil Nadu and Andhra Pradesh provided an opportunity to characterize secondary attack rates...” They call for more studies to see the efficacy of epidemic and pandemic control measures in the public domain, especially in low-resource settings such as India. The team concluded, “Primary data are urgently needed from low-resource countries to guide control measures.”

Journal reference:
  • Epidemiology and transmission dynamics of COVID-19 in two Indian states, Ramanan Laxminarayan, Brian Wahl, Shankar Reddy Dudala, K. Gopal, Chandra Mohan, S. Neelima, K. S. Jawahar Reddy, J. Radhakrishnan, Joseph A. Lewnard, Science  30 Sep 2020: eabd7672, DOI: 10.1126/science.abd7672,
Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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