NEW DELHI (BLOOMBERG) - High levels of chronic disease in India, such as diabetes and hypertension, helped stoke the brutal coronavirus waves that hit the country during the pandemic, researchers said.

The findings from one of the few large-scale studies of Covid-19 in the world's second-most populous nation showed patients from the southern district of Madurai had a higher risk of dying than those in China, Europe, South Korea and the United States, even though 63 per cent of those tested were asymptomatic.

Chronic health conditions in the community may have played a role, according to the report published in The Lancet.

For years India has faced an escalating non-communicable disease crisis as its middle class expands and leads a more sedentary and affluent lifestyle. That makes them susceptible to ailments such as diabetes and heart disease that account for almost two-thirds of all deaths in the country.

Those existing conditions may have allowed the coronavirus to do more damage, boosting cases and fatalities and potentially fuelling the near collapse of India's health system.

The death rate was 5.7 per cent among Covid-19 patients with at least one existing health condition, compared to 0.7 per cent in those who were otherwise healthy, the researchers found.

The data came from more than 400,000 people who underwent coronavirus testing known as reverse transcription-quantitative polymerase chain reaction (RT-PCR) in Madurai from May 20 to Oct 31 last year, during India's first wave.

"The findings that hypertension and diabetes actually predict acquisition of Covid-19 itself, or at least being tested positive on an RT-PCR test, in itself is a significant finding," Dr Ramanan Laxminarayan, the study's lead author and founder of the Centre for Disease Dynamics, Economics & Policy in Washington, said in an interview.

"Let's just say India had half the diabetes and hypertension that we have, we probably would have seen a far smaller impact of the second wave."

Managing health conditions that are common in the population should be at the top of the list for any government response to curbing the pathogen's toll, he said.

The researchers also highlighted what appears to be a mass underreporting of Covid-19 cases and deaths in India after assessing the ratio between infections and fatalities, and the number of people in Madurai who were already producing infection-fighting antibodies between Oct 19 and Nov 5 last year.

The results showed that testing found only 1.4 per cent of infections, and just 11 per cent of the expected number of deaths were detected.

Some scientists have estimated that as many as five million people may have died after India's hospitals were overwhelmed during the second wave that peaked mid-May, a fraction of the official total tally of about 430,000 during the entire pandemic.

The death toll is lower than what would have been expected given the number of known infections, the researchers said.

Researchers from the state government of Tamil Nadu, the University of California, Berkeley, Princeton University and the Johns Hopkins Bloomberg School of Public Health also contributed to the study.

It comes as India prepares for an expected third Covid-19 wave that some experts forecast will be smaller than the second and could peak in October - blunted in part by a growing wall of vaccinations and naturally acquired antibodies from past outbreaks.

A national survey last month found that two-thirds of Indians above the age of six had been exposed to the coronavirus.