More than 23% of Delhi’s 1.9 crore population showed evidence of past exposure to Sars-Cov-2, which causes the coronavirus disease, in almost six months and that a large number of infected people remain asymptomatic, the result of the sero-prevalence study conducted in the national capital shows.
The Union health ministry said in a statement that 21,387 samples were collected in accordance with laboratory standards and were tested between June 27 and July 10 to determine the extent of the spread of the coronavirus disease in Delhi.
“Nearly six months into the epidemic, only 23.48% of the people are affected in Delhi, which has several pockets of dense population,” the Union health ministry, which commissioned the study, said in a statement.
The seroprevalence study was conducted jointly by the National Center for Disease Control (NCDC) and the Delhi government “following a rigorous multi-stage sampling study design.”
Blood samples were taken from randomly selected people as part of the survey, which involves rapid tests for antibodies, to study the scale of undetected infections. The samples were tested to ascertain the population-level presence of the IgG antibody, which indicates past infection.
“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,” the health ministry said in the statement.
The ministry, however, warned that 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/cover, hand hygiene, cough etiquette and avoidance of crowded places etc, must be followed strictly,” it added.
A diagnostic method, serological tests are used to identify antibodies, which are created by the immune system when someone is infected with the virus, and antigens in a person’s blood.
Serology (antibody) tests are largely used for surveillance among communities and can be used on people who have already been tested positive for the virus or even those who are asymptomatic.
Epidemiologists often use serological surveys to determine who has antibodies and find out who has been infected with the virus, even if someone never reported a positive test or experienced symptoms.
A serological survey includes IgG Enzyme Linked Immunosorbent Assay (ELISA) test. It estimates the proportion of the population exposed to Sars-Cov-2 infection.
The IgG test is not useful for detecting acute infections but it indicates episodes of infections that has happened in the past. The test is approved by ICMR for its high sensitivity and specificity.
The Indian Council of Medical Research (ICMR) had conducted a pilot sero survey across 83 districts in 21 states in May.
The results showed that the percentage of the general population to have been infected in the past was 0.73%, with urban areas having shown higher Covid-19 prevalence of about 1.09%.
Delhi, which has overtaken Mumbai as the worst-affected city in the country, has 123,747 infections so far and 3,663 people have succumbed to the viral disease.