Serosurvey sheds light on Covid re-infection chances; more antibodies in urban India

The serosurvey to determine the extent of Covid exposure conducted by the Indian Council of Medical Research (ICMR) in coordination with the state governments has found interesting results. The serosurvey has detected high antibody positivity rates among the population in cities such as Mumbai and Delhi and low rates in primarily rural states such as Bihar, Jharkhand and Chhattisgarh. 

This means a considerably lower risk of reinfection in cities due to high antibody positivity rates. On the contrary, the survey causes a reason to worry for the rural areas where the antibody positivity rates were found to be low. 

A serosurvey involves testing of blood serum of a group of individuals and this will be used to monitor trends in the prevalence of the novel coronavirus, or SARS-COV-2, infection at the district level. The serosurveys are being conducted in select districts by the ICMR to detect the number of people who developed immunoglobulin (IgG) antibodies to the coronavirus disease.

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Mumbai and Delhi have higher rate of antibody positivity

When checked in Mumbai, the positivity rate was 57% in three slums the Brihanmumbai Municipal Corporation (BMC), along with the government think tank NITI Aayog and Tata Institute of Fundamental Research (TIFR) conducted the survey. Out of the total sample size of 6,936 persons, 4,232 were tested in the slums and 2,702 in non-slum residential areas. The results of the survey showed, 57% of the individuals from slums had once been infected with Covid-19, while only 16% in non-slum residential areas.

Delhi showed a rate of 23.48% for IgG.antibodies to the coronavirus disease. However, the survey result showed antibodies positivity rate of 0.7% in six districts of Bihar and 0.5% in three districts of Chhattisgarh, substantially lower as compared to cities.

Higher antibody positivity rates indicate a lower possibility of reinfection and vice versa. However, Experts are of the opinion that the varying rates of antibody positivity could be due to low sample size.

Bihar conducted the serosurvey in six of its districts with a sample size of 400 in each district and the survey was conducted when the testing ratio was low i.e. 1,500 per million population, reports stated.

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Whereas, three of Chhattisgarh's districts had conducted a serosurvey resulting in positivity rate of 0.5%. The health officials have opined the reason for low antibody rate could be due to a large number of false negatives, as the reliability of the test is said to be about 60% which means there are high chances of false negatives to be up to 40%, according to reports quoting health department official in Chhattisgarh.

In Odisha, the serosurvey shows antibody positivity rate of 31% in Ganjam district whereas Bhubaneswar showed 1.4% while Puro showed zero antibody positivity. However, an ICMR official opined that the rates would have been higher if the serosurvey would have been conducted by August end.