The second sero-survey conducted in Mumbai has found that 45% of the people residing in the city’s slums were exposed to the coronavirus, compared to the 57% recorded in the first survey in July, PTI reported on Friday. The drop in numbers – by 12% – indicates the infection spread in slums of the city may have come down, the Brihanmumbai Municipal Corporation said in a statement.

The Brihanmumbai Municipal Corporation’s second round of survey covered 5,384 people in three wards – Matunga (F North), Dahisar (R North) and Chembur (M West). These are the same wards in which the first survey was conducted on 6,936 people people in July.

The sero-prevalence, or the number of persons in a population who test positive for a specific disease, for non-slum sectors and buildings was found to be 18%, a rise of two percentage from the 16% recorded in the first survey. This means, the city’s sero-survey has found a rise in the level of antibodies in non-slum areas, but a fall in number of people exposed to the virus in shanty colonies.

Presence of antibodies in the blood indicates that the person has been exposed to an infection. Although having antibodies is not the same as having immunity to the virus, research shows that antibody levels are closely linked with the ability to disarm the virus.

Healthcare workers wearing personal protective equipment (PPE) and volunteers walk through a slum in Dharavi. (Credit: Francis Mascarenhas/ Reuters)

Of the 728 healthcare workers surveyed in slums in this second round, 27% had developed antibodies against Covid-19. BMC officials told The Indian Express the lower rate of exposure in healthcare workers as compared to the rest of the slums is due to better prevention methods adopted by them.

The latest findings of the sero survey in Mumbai are similar to Delhi’s third sero survey, which had also shown a fall in sero prevalence by almost 4%.

“The two sero surveys [in Mumbai] show that the level of sero prevalence is different from one locality to another even within the city,” Dr Jayanthi Shastri, head of microbiology at BYL Nair hospital, and a part of the survey team, told The Indian Express.

Dr Lalit Kant, former head of epidemiology and communicable diseases division of the Indian Council of Medical Research said the decline in sero-prevalence in slum areas could be because an entirely different set of people was surveyed the second time. He added that the findings for non-slum pockets, where sero prevalence grew from 16% in the July to 18% in August, is “as per estimation”.

“It is possible the second survey was conducted in areas where there were fewer Covid-19 cases as compared to the locality selected in the first survey, hence the varied results. A second possibility is that the infection came to these slum pockets in March-April and five months later, the antibodies have started decaying in the people who were exposed.”  

— The Indian Express

Other findings

The sero-prevalence of Covid-19 infection in women was once again marginally higher than men, the sero survey said.

In the second survey, the sero-prevalence in the age group above 40 was slightly higher.

“Emerging scientific evidence hints at fall of antibody levels in recovered patients/asymptomatic patients over a period of time and this might have contributed to the trend between two rounds of sero-survey,” the BMC said. “The impact of this on immunity, if any, is still unknown.

What is a sero survey?

According to the US’ Centers for Disease Control and Prevention, serology tests are “those that look for antibodies in blood”. “If antibodies are found, that means there has been a previous infection,” it says. “However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”

Serological surveys reveal how many people may have been infected with the novel coronavirus in an area but they do not show how many people are immune to the virus. It still isn’t clear how long antibodies last in infected persons and how many antibodies are needed to protect a person from reinfection.