Since the first serosurvey of slumdwellers conducted in July, there has been a 12 per cent decrease in the Covid-19 antibodies in their blood serum. Correspondingly, as more cases emerge from housing societies, there has been a two per cent increase in the presence of antibodies in the blood serum of residents of these areas. But civic officials have ruled out the chances of herd immunity. According to the state Covid task force, such studies can only indicate exposure to the virus and have advised the public to continue following basic Covid-19 protocols, to protect themselves from contracting the infection.
The second serosurvey revealed that the prevalence of the novel coronavirus in slum areas had reduced to 45 per cent from the 57 per cent reported in the first round. On the other hand, seroprevalence in non-slum areas had increased, from the earlier 16, to 18 per cent. Moreover, seroprevalence among health care workers in both the rounds was nearly 27 per cent (on average) in four categories of workplaces viz. health posts, dispensaries, health office and programme field staff. Seroprevalence is the percentage of individuals in a population who have antibodies to a pathogen, as measured in their blood serum.
Additional Municipal Commissioner Suresh Kakani said the second sero-survey report indicates that there could have been a reduction in the spread of infection in slums. Moreover, seroprevalence in the 40-plus age group was slightly higher. “There has been a decrease in the rate of infections in slum areas, as most cases in the city are now coming from the non-slum areas, so we can see a 12 per cent drop in seroprevalence in slum areas. Significantly, seroprevalence in women was marginally higher than in men in both the rounds while the age-wise prevalence was comparable in the first round in all ages,” he said.
The serosurvey for SARS-CoV2 was commissioned by the NITI Aayog, the Municipal Corporation of Greater Mumbai (MCGM) and the Tata Institute of Fundamental Research (TIFR), and partner organisations viz. Kasturba Molecular Diagnostic Laboratory, Translational Health Science and Technology Institute (THSTI), A T E Chandra Foundation and IDFC Institute.In the first round, 6,936 samples (of an estimated 8,870) from the public were collected from three wards (R/N, M/W & F/N) in slum and non-slum areas. Participants were recruited following informed voluntary consent. The same study was conducted in the same three wards in the second half of August. Of the total 5,840 target samples, 5,384 participants (92 per cent) were recruited from the public.
In all, 728 healthcare workers were recruited for the study, for both rounds combined. Moreover, around 1-2 per cent of the samples were from those who had participated in both rounds.Dr Shashank Joshi, a member of the state's Covid-19 taskforce, said there was no comparison between the first and second serosurveys, as they only showed the exposure of the participants to the virus.
Further, he felt there was a rapid decrease in antibodies, which would further decrease were citizens to adhere to Covid-19 protocol. “We are nowhere near herd immunity and the sample size for the study is very small, so we cannot come to conclusions. The study only provides information about the spread of infection in the population and in order to learn about the actual prevalence of the SARS-Cov-2 virus, there should be a 70 per cent sample size with T-cell response,” he said.