Scientists have discovered unique patterns in coronavirus transmission in one of the largest epidemiological analyses of COVID-19 to date in India. The country now has more than 60 million coronavirus cases and has reported 96,000 deaths from the disease. Results of the study were published on 30 September in the journal Science (1).
The ongoing spread of SARS-Cov-2 seems to be driven by so-called superspreaders – a small percentage of the infected population that pass the virus on to more people. Children and young adults were also found to be much more important in transmitting the virus than previously thought.
The researcher from the United States and India analysed the disease transmission patterns in nearly 600,000 individuals exposed to nearly 85,000 confirmed cases of COVID-19 based on data collected by tens of thousands of contact tracers in two Indian states: Tamil Nadu Andhra Pradesh.
Unlike observations in high-income countries, the scientists found that both cases and deaths due to the disease have been more heavily concentrated in younger cohorts than expected: the 40–69 year age range. Almost two-thirds of those who died had at least one co-morbidity, and one-third had two or more underlying health conditions that made them more susceptible to the disease. Notably, patients died on average within five days of being hospitalized – compared to a few weeks or even up to two months in other countries – possibly due to underlying conditions or poor health overall.
The researchers observed an overall case-fatality rate of 2 per cent, which increased with age. However, unlike in other countries, the death rate began to fall again after 65. Indeed, less than 20 per cent of COVID-19 deaths before 1 August 2020 was among people older than 75.
Most surprisingly, however, only eight per cent of infected individuals accounted for 60 per cent of new infections. Whereas, 71 per cent did not infect any of their contacts, which the researchers say the largest empirical demonstration of ‘superspreading’. In a statement, lead author Ramanan Laxminarayan, a senior researcher at the Princeton Environmental Institute, said: “Our study presents the largest empirical demonstration of superspreading that we are aware of in any infectious disease”.
“Superspreading events are the rule rather than the exception when one is looking at the spread of COVID-19, both in India and likely in all affected places”.
Children and young adults made up one-third of the examined COVID cases and were key to transmitting the virus, which has not been firmly established in previous studies, according to Laxminarayan. In addition, the authors noted that this age group is more likely to contract coronavirus from people their own age.
The findings provide extensive insight into the spread and deadliness of COVID-19 in low-mid income countries, like India, where most of the cases have occurred. The large-scale study also provides evidence that implementation of a countrywide lockdown in India considerably reduced coronavirus transmission.
(1) Laxminarayan, R. et al.Epidemiology and transmission dynamics of COVID-19 in two Indian states. Science (2020). DOI: 10.1126/science.abd7672.