The Sars-Cov-2 virus is much less deadly and far more transmissible than previously thought, results from a serological survey conducted in the national capital indicated, and its findings suggested the city could be closer to achieving herd immunity than previously believed, according to experts.

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The National Centre for Disease Control (NCDC) tested 21,387 people at random from across Delhi between June 27 and July 10, a survey that showed that 23.48% of these had antibodies for the virus, which causes Covid-19, in their blood. Adjusting for false positives and negatives based on the performance of the kit, 22.86% of the population may have been exposed to the virus, officials announced.

Extrapolating the serological prevalence data against recorded deaths in Delhi puts the state’s Covid fatality rate at 0.08%, against 9.7% for severe acute respiratory syndrome (Sars), 34% for middle eastern respiratory syndrome (Mers), 2.5% for Spanish Flu 1918, and 1.3% for H1N1 2009/10 (swine flu). The new coronavirus, according to these results, may therefore be less deadly than these, but far more transmissible since it has caused manifold more infections.

Delhi’s 22.86% number is higher than what has been established till now for New York, London and Tokyo. According to studies from these cities, New York had a Covid-19 serological prevalence of 19.3%, London 17.5%, Madrid 11.3%, Geneva 10.8%, and Tokyo of 0.1%.

“The antibody positivity rate reported in the Delhi Covid-19 antibody survey is among the highest in the world, matched only by New York. Even while accounting for false positive results from other coronviruses and the higher likelihood of such results in asymptomatic persons, the result indicates a high level of transmission,” said Dr K Srinath Reddy, president, Public Health Foundation of India.

High prevalence because of undetected infection means India’s case fatality rate would be significantly lower than 2.43%, which is calculated based on recorded deaths among diagnosed cases.

Prevalence is likely to be higher still since people who may have had mild disease in the early course of the pandemic may have lost protective antibodies. “Antibodies have been known to decline in three months, some of those infected early on may not have tested positive. These false negatives may partly balance the false positives,” said Dr Reddy. “While this is good, we must continue to vigorously implement public health measures to contain the transmission so that lives can be saved,” he added.

This means herd immunity may have already started setting in, said another public health expert.

“Around half the population is not susceptible to infection because of various reasons, such as cross immunity or geographical isolation. This unexplained phenomenon is what Dr Karl Friston calls ‘immunological dark matter’, because we know it exists to account for what is happening but we can’t see it. So if you account for 20-25% prevalence and 50% natural immunity, you have at least 70% of the population with some amount of immunity to Covid-19,” said Dr Dileep Mavlankar, director, Indian Institute of Public Health-Gandhinagar.

Dr Friston is a neuroscientist at University College London in the UK, and has done several modelling studies to predict the progression of the Covid-19 pandemic.

“This means that the benefits of herd immunity against Covid-19 may set in even with a seroprevalence of 20-30%,” Mavlankar said.

“Of the nearly 3,000 antibody samples that we analysed, we did not find any difference in the quality of antibodies among the various age groups. Although samples from people 60 years and older were few, the proportion with antibodies against the infection was the same as young age groups. The belief that older adults do not develop antibodies that leads to higher mortality rates among them is simply not true,” said Dr SK Sarin, director, Institute of Liver and Biliary Sciences.

The focus should be on preventing a potential second wave in a few months. “We don’t know how long the immunity last, as it could be short duration immunity, as with common cold caused by coronaviruses, or last for decades. Testing-and-tracing people must continue to prevent a potential second wave till a vaccine is available,” said Dr Reddy.

“The survey results indicate that about a quarter of the population has been exposed to the infection. Our response should continue to focus on core public health measures – detect, test, trace contacts, isolate and treat – while promoting physical distancing, hand hygiene and wearing of face masks. All possible efforts should continue to be made to break the virus transmission chains,” said Dr Poonam Khetrapal Singh, regional director, World Health Organisation South East Asia Region.