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Explained: What do Delhi’s serosurvey results suggest about herd immunity?

Many researchers believe that if 60 per cent or more of the population has developed antibodies against Covid-19, there is a possibility of acquiring herd immunity.

Written by Astha Saxena , Edited by Explained Desk | New Delhi |
Updated: January 27, 2021 8:19:32 am
Health workers collect blood samples during a serological survey at a dispensary in Majnu Ka Tila in New Delhi. File/Express Photo by Abhinav Saha

The initial findings of the fifth round of serological survey conducted in Delhi suggests that over 50 per cent of the people who were surveyed during the exercise have developed antibodies against Covid-19.

The survey was conducted between January 11 and January 22, and a total of 28,840 samples were picked up from the 11 districts of Delhi.

What do these results mean for Delhi?

The results of the latest serosurvey in Delhi has led researchers and experts to surmise that a large section of the city’s population has already developed antibodies against Covid-19.

The presence of antibodies among a large percentage of the population could be a reason for the decline in the daily number of Covid-19 cases. As more people are able to resist infection, it will help to break the chain of transmission of the virus.

How many such surveys have been conducted so far?

Five serological surveys have been carried out in Delhi so far, including the present one, which was conducted in January.

The first serological survey was jointly carried out by the Ministry of Health and Family Welfare and the National Centre for Disease Control (NCDC) in the last week of June.

The next four studies were initiated and planned by the Delhi government in the months of August, September, and October 2020, and January 2021.

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What did those previous surveys show?

The serosurvey conducted by NCDC in July last year suggested the presence of antibodies in 23 per cent of those surveyed. In August, the survey conducted by the Delhi government showed 29.1 per cent had antibodies.

The figure dropped to 25.1 per cent in September, and stood at 25.5 per cent in October.

What is the relevance of conducting this exercise on a regular basis?

Carrying out repeated serological surveillance on the same population gives an idea of how the disease is behaving. Dr Jugal Kishore, head of community medicine, Safdarjung Hospital and Vardhman Mahavir Medical College, said, “It is always good to have surveillance regularly to understand the trends. Having a robust surveillance is always beneficial, it may not be too close, but it may help us in giving an idea, even of the natural history of the disease.”

What do the data suggest about herd immunity?

Many researchers believe that if 60 per cent or more of the population has developed antibodies against Covid-19, there is a possibility of acquiring herd immunity. “In Delhi, it is quite indicative, as the number of cases is also going down. This shows that we are moving closer towards acquiring herd immunity,” said Dr Kishore.

What should be the next course of action for the city?

Dr Samiran Panda, Scientist and Head of Epidemiology and Communicable Diseases Division at Indian Council of Medical Research (ICMR) said, “It is time for us to look at the data carefully and try to understand if it is a homogeneous picture that we have seen and is applicable for the entire Delhi; or is it about the average which needs to be segregated in terms of urban/slum and rural settings.

“Looking at the data from the top won’t help us. We have to get to the details, and that would help us to understand the exact situation. We need to have our intervention measures ramped up, rather than relying completely on herd immunity.

Will the city be carrying out more such surveys in the coming months?

According to the senior officials in the Delhi government, the state health department plans to continue with carrying out the serological surveys at regular intervals until such time when a large proportion of the city’s population has developed antibodies against the virus.

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