Percentage of people with antibodies high, shows Delhi serological survey

Experts interpret results differently

Updated - July 22, 2020 09:06 am IST

Published - July 22, 2020 07:50 am IST - New Delhi

Health care worker taking a swab sample during a rapid antigen test at a COVID-19 antigen testing center in Delhi's Karol Bagh area.

Health care worker taking a swab sample during a rapid antigen test at a COVID-19 antigen testing center in Delhi's Karol Bagh area.

With the results of the Delhi serological survey showing that 22.86% of residents have developed antibodies against COVID-19, experts agreed that the percentage was “high”, but differed on its interpretation.

Blood samples of 21,387 people were taken at random and tested for antibodies between June 27 and July 10. If a person has antibodies, it means he/she was infected by COVID-19 at some point.

‘Herd immunity soon’

Jayaprakash Muliyil, an epidemiologist and former principal of Christian Medical College, Vellore, said the seroprevalence in Delhi is “decently high” and it was expected in the city. “There are some questions on how long the immunity last. I think it will be a long-lasting immunity, but I cannot give you an exact number," Dr. Muliyil told The Hindu .

He said that people who were infected by SARS (Severe acute respiratory syndrome) virus had immunity against the virus for a long time. “This is a cousin (of SARS) and why should it be different?” he said, adding: “The high percentage shows that the real magnitude of the virus is higher than the total number of positive cases in the city.”

Dr. Muliyil said the survey means that 22.86% of Delhi’s population has been infected and they have developed antibodies and recovered.

He added that in an urban area, about 50% of the people have to be infected by the virus and develop antibodies to attain herd immunity. “In a few weeks, Delhi will attain herd immunity,” he said.

When asked whether the government should change strategy after the results, he said: “At this stage, you should stop looking for carriers of the virus and focus only on the sick. The mortality is low and we do not have a dangerous disease.”

‘False positives’

K. Srinath Reddy, epidemiologist and president of Public Health Foundation of India-Delhi, said that the survey reveals a “high level of exposure” to the virus, but differed in its interpretation.

“Surveys in other large cities abroad revealed prevalence rates ranging from 0.1% in Tokyo to 17% in London and 20% in New York. Such surveys have some false positives due to cross-reactivity with other corona viruses and some false negatives due to declining antibody levels. However, serological surveys are good for comparing different populations and for studying time trends in the same population,” he said.

Dr. Reddy added that herd immunity threshold for novel coronavirus is estimated to be over 70% of the population. “Delhi is far short of that. Antibodies seem to decline in a few months. We do not know if cell-mediated immunity confers longer lasting immunity against this virus.”

He said that a large proportion of the population still remains susceptible and containment measures must be continued with full vigour.

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