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Published: 2025-05-01 14:32:00
The Kumbh Mela festival
New research into the devastating Covid-19 Delta wave in India provides invaluable information for future pandemic management.
A team from the University of Southampton’s WorldPop research group has, for the first time, delved into detailed data from 2020 and 2021 on population movements across India, alongside timings of public events and government-level interventions.
Their findings, published as we pass five years since the global pandemic, show that a variety of circumstances coincided in the weeks preceding the Delta wave, resulting in rapid transmission throughout India and the rest of the world, high numbers of daily recorded cases, and substantial mortality compared with the initial pandemic wave.
The research team used an advanced data analysis framework incorporating drivers of transmission and their lagged impacts, which can provide insights for future pandemic responses.
Dr Eimear Cleary , Senior Research Fellow at WorldPop, said: “The first Covid-19 wave in India was relatively mild, but the second wave which began in March 2021 – during which the Delta variant emerged – was severe, with very high associated mortality.”
In April 2021, India had the highest number of new and active cases of Covid in the world, and between April and July 2021 there were an estimated 2.7 million Covid-19-related deaths.
Dr Cleary said: “We looked at the movement of people on a population level, climate drivers associated with transmission – such as temperature, UV index and precipitation, timings and stringency of government restrictions, public events, and the lagged impact of drivers of transmission on Covid-19 incidence.
“ Publications at the time speculated that pandemic fatigue may have been emerging during the initial stages of the Delta wave in India and that there perhaps was a feeling of relative safety as it seemed that Covid transmission was under control. This was coupled with the emergence of the Delta variant, which was more virulent and more transmissible.”
The Delta variant also coincided with major public events including the Kumbh Mela in April 2021, a religious festival held on the banks of the Ganges.
“The increase in mobility, together with a relaxation of restrictions and the emergence of the Delta variant seem to have been the biggest contributory factors to the high levels of Covid-19 transmission observed during the second pandemic wave,” explained Dr Cleary.
A framework for the future
The WorldPop team used a framework incorporating data on climate, demographic, and behavioural drivers of transmission. This allowed them to explore what happened in fine detail and to examine any differences in factors that led to transmission peaks.
They found that people’s movements returning to almost pre-pandemic levels was one of the main contributors to Covid-19 transmission during the Delta wave.
During the first wave, stringent interventions such as lockdowns had a big impact on preventing the spread of the virus, especially in rural areas. In urban areas, public holidays – and, therefore, people getting together – played a significant role in its spread.
Dr Cleary said: “This research highlights the value of advanced data analysis frameworks incorporating lagged effects for estimating the impact of interventions on transmission, which will be extremely beneficial for managing future epidemics and outbreaks.”
She added: “Taking the lagged impact of drivers on transmission into account is important for pre-emptively implementing interventions in future pandemics. Once peaks in transmission are seen, implementing control measures will likely have a delayed impact on reducing case numbers.
“Nevertheless, from an infection prevention point of view, interventions can still be effective at controlling the spread of new variants with severe clinical outcomes, particularly during the early stages of transmission waves.”
The research is published in the journal PLOS Global Public Health .
Throughout the pandemic, WorldPop conducted extensive research to track and map the spread of Covid-19 around the world . This work enabled prediction and early warning for virus resurgence and analysed the effectiveness of measures such as lockdowns, social distancing and self-isolation.