Results of a serological survey conducted in Delhi have brought both good and bad news. The good news is that the coronavirus causing coronavirus disease (Covid-19) is much less deadly now and that the national capital could be closer to achieving herd immunity; the bad news is, it is far more transmissible than previously thought.
During the survey, from June 27 to July 10, the National Centre for Disease Control (NCDC) tested 21,387 people at random from across Delhi. The results showed that 23.48% of these people had antibodies for the virus in their blood. Government officials said after adjusting false positives and negatives based on the performance of the kit, it could be said that 22.86% of the population may have been exposed to the virus.
This number for Delhi 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%.
Regional director of the World Health Organisation for the South East Asia Region, Dr Poonam Khetrapal Singh, said 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,” she added.
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.
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.
“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.
Dr Dileep Mavlankar, director of Indian Institute of Public Health in Gandhinagar, said: “Around half the population is not susceptible to infection because of various reasons, such as cross immunity or geographical isolation. This unexplained phenomenon is called ‘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.”
This, he added, meant that the benefits of herd immunity against Covid-19 may set in even with a seroprevalence of 20-30%.
The focus should be on preventing a potential second wave in a few months. “We don’t know how long the immunity lasts, 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.