The Greater Chennai Corporation will give additional focus to vaccination in zones where seroprevalence is relatively lower.
The recently released results of the fourth round of a survey conducted in early July showed an overall seroprevalence of 78.2%. The study was done by the Corporation along with the Indian Council of Medical Research’s National Institute of Epidemiology (NIE) and the National Institute for Research in Tuberculosis (NIRT) based on samples collected from 204 streets in 51 wards spread across all zones. Samples were collected from around 7,000 individuals in the age group of six and above. Among the 15 zones, Tondiarpet had the highest seroprevalence of 83.3% and Madhavaram the lowest of 72.5%.
While seroprevalence has jumped from 49.2% observed during the previous survey conducted prior to the second wave, officials and experts said this should not lead to complacency.
Corporation Commissioner Gagandeep Singh Bedi said from an administrative perspective, the results should be used to fine-tune preventive strategies.
He said a key outcome would be to increase the vaccination coverage in zones like Madhavaram, Alandur and Perungudi, where seroprevalence was low. “The variation is not significant. However, in these zones the seroprevalence is relatively lower, and consequently, they could be more prone to newer clusters of cases,” he said.
P. Ganeshkumar, assistant director and scientist D, ICMR NIE, currently supporting the Greater Chennai Corporation on COVID-19, said the survey measured only immunoglobulin G (IgG) antibodies related to the SARS-COV-2 virus and not the neutralising antibodies, which indicate immunity against COVID-19.
“However, detecting neutralising antibodies is a relatively costly affair and not feasible during such surveys. Even if we detect neutralising antibodies, their presence cannot be used to easily estimate the immunity levels in the population as there are more nuances involved,” he said.
Highlighting that the presence of IgG antibodies only indicated that a person either had exposure to the virus or was vaccinated, Dr. Ganeshkumar said the purpose of such serosurveys were to observe the trend and equip ourselves better against COVID-19.
Apart from targeting vaccination in zones where seroprevalence was relatively low, he said results could also be used in the earlier identification of new variants of SARS-CoV-2. “For instance, if we see new clusters in zones like Tondiarpet where seroprevalence is high, it is likely that a new variant is behind it. We should send samples from such cases to genomic study,” he said.
He said serosurveys should not be used to conclude whether herd immunity levels had been reached or not, particularly with the emergence of new variants and the possibility of re-infection and infection after vaccination, although the chances were meagre.
The focus on COVID-19 control measures like continuous surveillance, testing, effective contact tracing and strict isolation must continue.