Sero survey

nerable and people cannot afford to lower their guard and abandon precautionary measures such as social distancing or wearing masks.

If the findings are extrapolated to the estimated 19.98 million population of the national capital, the 22.86% translates to 4.53 million people who may have been infected by the Sarscov-2 virus and recovered, potentially building an immune response that – at least in the short term – protects them from reinfection.

The statistics also have an implication for how deadly Covid-19 is believed to be. To date, the Capital has recorded 125,096 confirmed cases and 3,690 deaths, translating to a fatality ratio (in this case, known as case fatality ratio) of 2.95%. But if the number of actual cases is taken to be 4.53 million, the fatality rate comes to 0.08%.

The 22.86% would have likely grown since July 10, the last day when samples were collected.

A growing number of people who may be protected from infection is crucial for the epidemiological phenomenon known as herd immunity, in which an infectious disease stops spreading in a population if it does not find adequate number of vulnerable people. Experts have put this number between 40% and 65% for a disease like Covid-19.

“The findings are suggestive of infection during mid-june, and since then we have seen a decent increase in the number of cases that means more people are exposed to the virus, and the disease prevalence could be higher. Also, it wasn’t a door-todoor sample collection that also means about 5-10% of the population probably was missed. If you factor in all these variables then the prevalence should easily be around 40%, which is good,” said Jugal Kishore, head, community medicine department, Safdarjung Hospital.

“It also means the susceptible population is going down, which suggests that epidemic is moving downwards towards its end,” he added.

The findings come a day after peer-reviewed studies showed two vaccine candidates were safe and effective, including the one developed by UK’S Oxford University that is largely seen as the front runner. “The biggest silver lining for me is that if we have such high population that was exposed in past but did not largely display serious symptoms or succumbed then it as good as a vaccine,” said NCDC’S Singh. There are, however, still several unanswered questions.

“Since these many people have developed antibodies, there will be some immunity against the disease; however, what is pertinent to determine is the level of immunity; whether there will be reinfection among recovered patients, or will there be longer immunity against it,” he added.

The results are being studied by the Delhi government to determine if there needs to be a change in strategy. “This captures the state of recovery from Covid-19 infection in mid-june, which is a month ago. We will consult with public health experts and epidemiologists to determine if the future course of Delhi’s strategy against Covid-19 should change in light of these survey results or not,” Delhi chief minister Arvind Kejriwal’s office said in a statement on Tuesday.

In an interview to HT on Saturday, Kejriwal had said he was eagerly awaiting the results because they would indicate that the public was approaching the herd immunity level. “I am anxiously awaiting the results of the survey -- the findings will indicate to what extent the virus has spread. And, most importantly, whether Delhi is inching towards herd immunity,” he said. The Union health ministry on Tuesday said that the findings showed “only 23.48% of the people are affected in Delhi, which has several pockets of dense population”. “However, a significant proportion of the population is still vulnerable. Therefore, the containment measures need to continue with the same rigour,” it added.

A serological test involves collection of blood of individuals to check for the prevalence of IGG antibodies that develop roughly around a week after infection and can be detected for months.

Delhi’s results mirror findings in other cities with large outbreaks, such as New York city. In a study published by researchers from Mount Sinai Hospital on June 29, around 19.3% of people in the city had been exposed to the virus through April 19. New York city, like Delhi was in India, was one of the first hot spots in the United States. It has now brought its infection under control while new clusters pop up across the country, similar to the case with Delhi and rest of India.

The findings of any serological study into Covid-19, however, is subject to reliability of the test. According to an advisory by the United States’ Centers for Disease Control updated on June 30, a positive antibody test may also mean that a person has “antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold”.