As much as 23.48% of Delhi’s population or a whopping 4.7 million people have been exposed to the virus that causes Covid-19, according to the Union health ministry’s data from a sero-prevalence survey released on Tuesday.
“The results of the sero-prevalence study show that on an average, across Delhi, the prevalence of IgG [Immunoglobulin G] antibodies is 23.48 %. The study also indicates that a large number of infected persons remain asymptomatic,” the health ministry said in a statement.
The ministry had commissioned a sero-surveillance study in Delhi, which has a population of about 20 million, and it was jointly conducted by the National Centre for Disease Control (NCDC) and the Delhi government. It was done following a rigorous multi-stage sampling study design between June 27 and July 10.
Experts in community medicine say that since the samples were collected in end-June and the beginning of July, there is a possibility more people may have developed antibodies against the virus by now.
“This is 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,” said Jugal Kishore, head of the community medicine department of Safdarjung Hospital.
“Also, it wasn’t a door-to-door sample collection, that also means about 5% to 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.”
He added, “It also means the susceptible population is going down, which suggests that the epidemic is moving downwards towards its end.”
Survey teams were formed for all the 11 districts of Delhi, and blood samples were collected from 21,387 individuals. The samples were tested in labs for Immunoglobulin G (IgG) antibodies.
IgG is the most common antibody in blood and other body fluids that the immune systems produces to protect against bacterial and viral infections and allergens. IgG can take time to form after an infection, and is useful in detecting a past infection, according to the Indian Council of Medical Research (ICMR).
Epidemiologists say merely knowing the presence of antibodies is not enough, as one needs to know the quality of antibodies to make some meaningful difference.
Lalit Kant, former head of the epidemiology and communicable diseases division of ICMR, said: “We need more information to make sense of the survey, such as whether these antibodies are neutralising antibodies that can kill the virus, or for how long do these antibodies stay in a person’s system. Is it a long-term immunity or short-term? More relevant information needs to be put out.”
The antibody test is not a diagnostic test but provides information about past infection due to SARSCoV-2 in individuals who test positive. Antibody testing repeatedly done over time, in the form of sero-surveillance, generates important evidence for assessing the spread of the pandemic from time to time, the health ministry’s statement said.
“This implies the following: nearly six months into the epidemic, only 23.48% of the people are affected in Delhi, which has several pockets of dense population. This can be attributed to the proactive efforts taken by the government to prevent the spread of infection, including prompt lockdown, effective containment and surveillance measures, including contact tracing and tracking, as well as citizen’s compliance to COVID Appropriate Behaviours,” it said.
“However, a significant proportion of the population is still vulnerable. Therefore, the containment measures need to continue with the same rigour. Non-pharmacological interventions such as physical distancing, use of face mask or cover, hand hygiene, cough etiquette and avoidance of crowded places etc, must be followed strictly,” it added.
This was one of the largest sero-prevalence studies conducted in India using ELISA testing. ICMR-National Institute of Virology (NIV) at Pune developed and validated the indigenous blood-based IgG ELISA (Enzyme-Linked Immunosorbent Assay) test “Covid Kavach Elisa” for antibody detection for Covid-19, which was used for the sero-prevalence survey in Delhi.
While real time reverse transcription-polymerase chain reaction (rRT-PCR) test is the frontline test for clinical diagnosis of SARS-CoV-2, robust antibody tests are critical for surveillance to understand the proportion of population exposed to infection. It takes about 2.5 hours to get results and can be deployed even in districts, says the research body.
