Close to 31.5 million people exposed to Covid in Karnataka: Sero study

The researchers collected samples from close to 3,000 households in 20 districts in the state from mid-June through August 2020

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Close to 31.5 million people in Karnataka are projected to have been infected by the Sars-COV-2 virus, according to a seroprevalence study carried out by an independent group of researchers, which suggests the actual number of infections in the state could be close to 100 times higher than the 327,000-odd cases confirmed till end of August.

The research uses a survey design that has previously been utilised to gather detailed household econometrics, making it one of the most comprehensive such exercises yet. Sero prevalence studies, which involve drawing blood to determine presence of antibodies, typically require careful consideration in sampling in order to get a true picture of prevalence in a given area.

“Adjusted seroprevalence across Karnataka was 46.7%, (95% CI: 43.3-50.0), including 44.1% (95% CI: 40.0-48.2) in rural and 53.8% (95% CI: 48.4-59.2) in urban areas… The relatively high prevalence in rural areas is consistent with the higher level of mobility measured in rural areas, perhaps because of agricultural activity,” said the researchers from IDFC Institute in their paper published as a preprint on medRxiv.

The researchers collected samples from close to 3,000 households in 20 districts in the state from mid-June through August 2020 and also swabbed respondents for RT-PCR tests to determine active infections. “The proportion of those testing positive on RT-PCR, ranged from 1.5 to 7.7% in rural areas and 4.0 to 10.5% in urban areas, suggesting a rapidly growing epidemic,” they added.

“Our findings provide new evidence that the Covid-19 epidemic in India has affected rural areas almost as severely as urban areas, despite early attention to the epidemic in urban areas,” the researchers adding, identifying two key factors: “First, the release of the lockdown imposed in March was immediately followed by a large migration of daily labourers who lost their sources of income in urban centers and returned home to rural parts of the country…

“A second contributing factor was that, while urban areas experienced severe lockdowns, rural areas experienced fewer restrictions on mobility because agricultural activity was deemed an essential sector,” they said.

Survey design is a “critical challenge,” said lead author Manoj Mohanan, a professor at Duke university and a fellow at IDFC Institute. He added that for the study, they partnered with CMIE, which runs the world’s largest household survey panel known as the Consumer Pyramids Household Survey covering a little under 175,000 households in India. Among these, 9717 are in Karnataka.

“We worked with a randomly drawn subset of a quarter of these households and then followed up with each of them (and their household members) to collect swabs and blood samples. This strategy has two major advantages: First, because we have data from CPHS survey in previous rounds on exactly who is in these households, we are able to adjust the proportion of tests positive to reflect the composition of households… Second, it gives us a sampling frame that is very well defined and we know exactly how to extrapolate findings from this CPHS population to arrive at a statewide number,” he added.

The researchers add that their findings have several policy implications. “With nearly half the population in the state being infected with Covid as of August 2020, stringent suppression policies across the general population will impose significant costs on those who are already infected,” they said in their paper.

To address this, Mohanan, in a separate conversation over email with HT, said states could “consider strategies that are more targeted, to focus on areas where there is a growing infection and limit opportunities for transmission there rather than general statewide suppression measures”.

Mohanan added that the team is working on future studies that focus on understanding how antibodies and immunity might progress over time. “This will be important to understand any risks of re-infection and also to help inform policies on vaccination, which will be a key challenge in coming months,” he said.

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