High exposure to virus in urban areas
NEW DELHI: Sero surveys conducted by the states to determine the extent of exposure to the SarsCoV-2 virus that causes the coronavirus disease (Covid-19) have 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. The findings are good news for the cities because the high positivity rates signal a lower risk of reinfection, and conversely, they should worry the states in the hinterland where cases are beginning to spike. In Mumbai, the positivity rate was 57% in three slums; it was 0.7% in six districts of Bihar and 0.5% in three districts of Chhattisgarh, according to state government officials.
Sero surveys are being conducted in select districts by the Indian Council for Medical Research (ICMR) and state governments to detect how many people have developed immunoglobulin (IgG) antibodies to the coronavirus disease and to gauge the extent of infection through random sampling.
Experts said a low sample size could be a reason for under and overestimation of the prevalence of antibodies. Higher antibody positivity rates indicate a lower possibility of reinfection. Lower rates shows that the possibility of Covid-19 spread is still high.
Delhi and Mumbai, the two major cities to have completed the sero surveys, have shown different results. While in Mumbai, 57% of the persons surveyed in three slums showed IgG antibodies, in Delhi it was 23.48%. To be sure, the Delhi survey was more comprehensive.
“This is suggestive of infection during mid-June, and since then, we have seen a decent increase in the number of cases. That means more people are exposed to the virus, and the disease prevalence could be higher,” said Dr Jugal Kishore, head of the community medicine department of Safdarjung Hospital in New Delhi.
Delhi has conducted a second survey the results of which are expected in the coming week.
On July 28, the Brihanmumbai Municipal Corporation (BMC), along with the government think tank NITI Aayog and Tata Institute of Fundamental Research (TIFR), published the results of the survey conducted in three wards—M-West (Tilaknagar and Chembur), F-North (Matunga, Sion and Wadala) and R-North (Dahisar and Mandapeshwar) -of Mumbai. Of the total sample size of 6,936 persons, 4,232 were tested in the slums and 2,702 in non-slum residential areas. While in the slums, 57% of the individuals surveyed had once been infected with Covid-19, the proportion was only 16% in nonslum residential areas, the findings showed.
BIHAR, CHHATTISGARH, ODISHA
“The prevalence of the virus in slums is high due to the densely populated areas. In non-slums, there is better social distancing and maintenance of hygiene which helped to stop the spread of the virus to a larger number of people,” said Suresh Kakani, additional commissioner, BMC.
Bihar performed its first sero survey in six districts, each having a sample size of 400, between May 17 and May 20, when the number of Covid cases in the state was not high because of the low testing ratio of 1,500 per million population. According to baseline data from the first survey, the overall serological positivity rate was 0.7% in the six districts of Bihar.
“This meant that only 17 out of a total 2,400 people surveyed developed antibodies (immunity) against Sars-CoV-2,” said Dr Pradeep Das, director of the ICMR-Rajendra Memorial Research Institute of Medical Sciences, Agamkuan, in Patna. The second of sero survey in the state will start from August 17.
In Chhattisgarh, the survey was conducted in Bijapur, Kabirdham and Surguja districts in July with a sample size of about 400 people in each district.