A second sero-survey is slated to begin in Mumbai from August 13 to assess whether the prevalence of Covid-19 exposure has changed since the first survey was conducted.
In the first fortnight of July, blood samples of 6,936 people from three wards found that 40.5 per cent people had developed IgG antibodies against SARS-CoV-2, a virus that causes Covid-19. In slums, the exposure was 57 per cent and 16 per cent in non-slums.
People surveyed were from three wards — Chembur (M-West ward) where medium growth rate of Covid-19 cases was found, Dahisar (R-north) that had lowest growth rate and Matunga (F-North ward) that had highest growth rate in May.
The second survey will also be conducted by Brihanmumbai Municipal Corporation in association with Tata Institute of Fundamental Sciences (TIFR), in the same three wards to measure whether the exposure rate of virus remains constant or has increased to get indications of how close the city is towards herd immunity in slums.
Civic officials said the survey findings will also help frame policy for lifting lockdown and reopening of the city. The researchers are keen to assess whether the exposure rates have risen in non-slums areas with the phased lifting of the lockdown. The exposure rate was between 11-18 per cent in non-slum areas in the three wards in first survey. The survey will also look at infection fatality rate in both slums and non-slums.
According to Dr Ullas Kolthur, professor in TIFR, the team will begin visits after Janmashtami festival. “We will try to cover around 6,500 population, but different from the previous survey. This time we are also going to ask people if they remember having any symptoms,” Kolthur said.
The team is also going to study how long antibodies last in those who test positive to understand till when immunity lasts. The sero survey will take two weeks followed by one week for analysis, civic officials said.
In the first survey, females were found to be more exposed to Covid-19 than men in both slums and non-slums. The survey will relook at the numbers to check if the trend continues.
In the first survey, 3,327 samples were collected in Matunga; 57.8 per cent people in slum were exposed and had developed antibodies, as opposed to 17.4 in non-slum pockets. In Chembur (M-West) 2,460 samples were taken, slums had seroprevalence of 56.7 per cent and non-slums 15.6 per cent. Dahisar (R-North), where 1,149 samples were collected, has seroprevalence of 51 per cent in slums and 11.4 per cent in non-slums.