The figures of the latest serosurvey announced by the Delhi government that indicates a 56.13 percent seroprevalence in the national capital, should be interpreted with caution, experts say. On Tuesday, the Delhi government announced the results of the fifth serosurvey conducted in the national capital. This survey had a sample size of 28,000 that was distributed across 11 districts in Delhi.

The survey has found a seroprevalence of 56.13 percent in Delhi. As per the survey, Southwest Delhi had the highest seroprevalence at 62.18 percent while North Delhi had the lowest at 49.09 percent. The serosurvey results indicate that perhaps the national capital could be the first in the country to achieve herd immunity. This is the largest serosurvey conducted in the country so far, and also the largest seropositivity number reported for any Indian city.

Speaking about the survey results, Delhi Health Minister Satyender Jain said Delhi was moving towards herd immunity. Better not to get into this only experts will be able to give a clear picture, he however added. Experts say while the findings of the serosurvey are important, the conclusion that the capital is moving towards herd immunity should be taken with a pinch of salt.

The findings are helpful to assess the rate of spread of the disease but need to be taken with caveats, immunologist Satish Devadas told PTI. He added, Even if the sample size is relatively small, it must be taken to account. However, the conclusions on herd immunity have to be taken with a pinch of salt. Not just Devdas, other experts to believe conclusions about herd immunity should be taken with caution.

Speaking to PTI, experts said while the results of serosurvey suggest the possibility that herd immunity may break the chain of Covid-19 transmission and the findings are insightful to get an idea of the community spread of the coronavirus in the city, the dangers may be far from over. Herd immunity occurs when a significantly large number of people in a community become immune to a contagious disease after recovering from the infection, thereby curbing its spread across the population.

Experts noted that it is not known what the number of recovered individuals need to be in a community to break the chain of viral transmission with herd immunity. Our best bet is still in the 60-70 percent range but this is based on theory, and not on the specific information we have about the duration of immune protection following infection, epidemiologist Ramanan Laxminarayan told PTI.

Laxminarayan, founder and director of the Center for Disease Dynamics, Economics & Policy (CDDEP) in the US, said many pockets within urban India may have already reached this threshold. Scientists also cited the example of the Brazilian city of Manaus, where immunity against the infection may have begun to wane already.

A study of blood donors in the city, published in the journal Science, indicated that about 76 percent of the population may have been infected with SARS-CoV-2 by October 2020 an infection rate that would have been above the theoretical herd immunity threshold. Post lockdown the restrictions on movement have decreased and hence communities cannot remain in isolation. Thus a general comment about herd immunity or community immunity is unrealistic at this juncture, added immunologist Vineeta Bal, affiliated with the Indian Institute of Science Education and Research, Pune.