Fourth sero survey in Mumbai’s wards with fewer Covid-19 cases

The Brihanmumbai Municipal Corporation (BMC) is gearing up to conduct the fourth serological survey at four civic wards in the city that have seen fewer Covid-19 cases since the outbreak of the pandemic last year

The Brihanmumbai Municipal Corporation (BMC) is gearing up to conduct the fourth serological survey at four civic wards in the city that have seen fewer Covid-19 cases since the outbreak of the pandemic last year.

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For the survey, blood samples of only those who have not been vaccinated will be collected from both civic body and private laboratories.

Serological survey examines how many people in a population have been infected with Covid-19 and recovered from it. It is undertaken to gauge the prevalence of the virus in a particular area. The blood serum of a group of individuals is tested using an antibody test, also known as a serology test.

Anticipating a third wave post-August, the civic body’s decision to conduct a fourth sero survey stems from their need to understand the prevalence of Sars-Cov-2, the virus that causes Covid-19, among residents in these wards that have clocked comparatively fewer cases.

In south Mumbai, the BMC has selected two wards— B (Masjid Bunder, Mohd. Ali Road, Dongri and Bhendi Bazar) and C (Pydhonie, Girgaon, Kalbadevi) with around 130,000 and 170,000 population respectively. In the suburbs, densely populated wards— L (Kurla) and M/East (Deonar, Govandi) with a population of more than 800,000 and 900,000, respectively (as per the Census 2011) have been selected.

“These wards have been selected as the prevalence of the infection is much less compared to the density of population. Despite having several densely populated pockets and slums in these wards, the infection rate has remained under control,” said Suresh Kakani, additional municipal commissioner, BMC.

“There is a possibility that herd immunity (when around 60-70% of the population in a community develop immunity to slow down the infection rate) has helped stop the spread of the infection in the wards. To establish the theory, we have decided to do a sero survey in these three wards,” he added.

Till June 5, B ward has recorded only 3,821 cases since last year - the lowest number of Covid-19 cases. This is followed by C ward with 6,624 cases in total. The M-East and L wards have recorded 20,541 and 25,247 cases respectively.

On the other hand, during the same period, K-West (Andheri West) has recorded 51,931 cases, the ward with the highest number of Covid-19 patients. Similarly, R-Central (Borivli) and K-East (Andheri East) have registered 48,802 and 44,194 cases since the outbreak of the pandemic.

When asked the reason for not conducting a city-wide sero survey, Kakani said, “At present, we have decided to conduct the fourth sero survey as a part of the study to understand the epidemiological characteristics of the wards with less Covid-19 cases.”

The first sero survey in Mumbai was conducted in July 2020 followed by a second one in August 2020.

The third survey was conducted in March 2021 in all 24 wards of the BMC which showed the presence of antibodies in 36.30% samples out of the total 10,197.

The third survey’s results showed that seropositivity rate has increased in non-slum areas compared with slum areas. The sero survey in slum areas showed the sero positivity at 41.6%, which was 57% in July 2020 and 45% in August 2020. In non-slum areas, the sero positivity was registered in 28.5% samples, which increased from 16% in July 2020 and 18% in August 2020.

Doctors have welcomed the civic body’s decision to conduct the fourth sero survey that may help understand the factors behind the ward-wise variation in cases.

“Just like an iceberg, a large number of invisible and undiagnosed infectious cases often remain hidden below the waterline. A sero survey at the wards with fewer cases may help to understand the actual infection rate. However, the concept of development of herd immunity is quite debatable. This survey may therefore help in throwing some data,” said Dr Rahul Pandit, part of the state’s Covid-19 taskforce.