A serological survey conducted on 6,936 people in three wards Mumbai found 40.5 per cent of them were exposed to SARS-CoV-2, a virus that causes Covid-19, and had developed IgG antibodies against it.
Mumbai’s latest findings are among one of the highest exposure rates found globally. A sero-survey in Spain covering 61,075 participants between April to May found 5 per cent seroprevalence. A recent Delhi sero survey covering 11 districts and 21,387 people found 23.48 per cent population had developed IgG antibodies. In New York, 3,14,000 samples found antibodies in 26 per cent samples, except for a few smaller pockets that reported seroprevalence of 56 to 68 per cent.
The survey in Mumbai has thrown up some interesting findings. The transmission pattern varies from region to region, especially from slum to a high-rise, and more such surveys will be needed in future to assess a pattern. While the survey has popped questions about herd immunity, critiques have also raised concern over a small sample size that may not be truly representative of a city with a population as massive as 1.8 crore. The survey covers 0.00038 per cent of the population.
In May, BMC selected three wards for survey of 8,800 people: Matunga (F-North ward) that had the highest growth rate and caseload, and proximity to Dharavi, city’s biggest slum that had become a hotspot in April. It also selected Chembur (M-West ward) for medium growth rate in Covid-19 cases and Dahisar (R-north) that had the lowest growth rate in the city. Eventually, 6,936 people agreed to give their samples — 4,234 from slums and 2,702 from non-slums.
The survey found Matunga slums had the highest sero prevalence at 57.8 per cent. “The other two regions of Dahisar and Chembur had lower prevalence but not statistically significant,” said Sandeep Juneja, professor and dean of School of Technology and Computer Science, in Tata Institute of Fundamental Research (TIFR). TIFR partnered with BMC and Niti Aayog to conduct the serosurvey.
In slums, 57 per cent had seroprevalence as opposed to 16 per cent in non-slum areas. Dr Ullas Kolthur Seetharam, from TIFR, says high population density, poor physical distancing, shared facilities like toilets, washing areas, and people living in close quarters have contributed to a faster viral transmission in slums. In high-rises and gated societies, individual toilets and low-population density have led to lower exposure to the virus. This means slum dwellers can maybe rejoice a little to have inched closer to herd immunity but quarantining exercise (to shift high-risk contacts in covid care centres) will have to continue. And the residential population cannot afford to let their guard down, they need to continue all safety precautions like masks, hand hygiene, and physical distancing.
If these findings are extrapolated, it may mean around 70 lakh people in Mumbai have already been exposed to Covid-19. It also shows that the proportion of asymptomatic Covid-19 cases is much higher than expected, since none of those who tested positive for antibodies had undergone an RT-PCR test– their symptoms were not strong enough to warrant a test. Until now around 70-80 per cent Covid-19 cases diagnosed were asymptomatic. This survey indicates perhaps more than 90 per cent people are in fact asymptomatic
The survey finds that the actual fatality rate, in that case, will be much lower at 0.05 to 0.10 per cent. Confirmed covid cases currently in Mumbai are 1.10 lakh with 6,187 deaths (5.5 per cent fatality rate).
Men vs women
Until now RT-PCR tests of all confirmed cases in Maharashtra have shown 61 per cent men have been infected and 39 per cent were women. Of 6,187 deaths in the state, 65 per cent were men and 35 per cent women.
In Mumbai, men account for 55 per cent of total cases and women 45 per cent.
The serosurvey has thrown up a surprising finding. More women were in fact exposed to Covid-19 and more women developed immunity than men. Data showed that in slums 59.3 per cent of 2,297 women tested, and 53.2 per cent of 1,937 men had antibodies against Covid-19. In non-slum areas, 16.8 per cent of women and 14.9 per cent of total men tested had developed antibodies.
What could this mean? “We need to have more serosurveys in other geographical settings to check if the same trend continues,” says clinical scientist Dr Gagandeep Kang.
Kang says global data shows the infection occurs equally in men and women, but women seem to have lesser mortality risk than men. “Oestrogen may play a role here. While it does not protect from infection, it may protect from mortality,” Kang says. Experimental oestrogen therapy is being used to save the lives of critical male patients in several western countries, including the USA, but findings are awaited.
Experts believe women perhaps show lesser symptoms and develop quicker antibodies against Covid-19 than men. “But more research is required to get answers,” Dr Seetharaman from TIFR says.
Dr Jayanthi Shastri, head of microbiology in Nair Hospital, Mumbai, said women in India tend to seek medical care later than men, “only when their symptoms become worse”, and this may also be why lesser women turn up for RT-PCR testing than men. “This may explain why we have more men who are positive than women,” she said.
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The seroprevalence of 40.5 per cent , Shastri says, is an indicator that “we have come closer to herd immunity”. Shastri is one of the members heading the survey. She says a subset of the antibodies will be sent for further analysis to study neutralising antibodies and get more information on immunity, like how long can it last against Covid-19.
Clinical scientist Kang also says that at 57 per cent, slums are close to herd immunity. “Maybe the drop in cases from slums in Mumbai could be because most people in crowded localities were already infected. This also has implications for vaccines,” she said.
Globally, experts believe if 60 per cent population is infected, herd immunity can be achieved for Covid-19. But Maharashtra state epidemiologist Dr Pradeep Awate points towards a few challenges. “Yes, slums will achieve herd immunity faster than the general population in Mumbai. But we do not know how long IgG antibodies last. Cellular immunity has to be developed and we still don’t have answers to that. We also need to survey all 24 wards of Mumbai to get a true representation of the entire Mumbai. This survey covers only three wards.”
Dr Sujata Baveja, microbiologist in Sion hospital, also says that IgG antibodies (that the seroprevalence survey studied) is produced within two to three weeks of infection and indicates the person had recent infection. “But how much antibodies are required to protect from future infection is a question,” she said.
The BMC plans to conduct a second sero survey in other pockets of the city in a fortnight to measure whether viral transmission pattern is similar, whether infection growth has stabilised and whether same rate of seroprevalence exist.
What private labs in Mumbai found
Data of 9,590 samples tested for antibodies in Mumbai’s two private laboratories showed overall 24.3 per cent had antibodies against Covid-19. These samples were collected from people who could afford to pay and included office-goers, businessmen, salon staff, health workers, and residential societies. Thyrocare tested 5,485 people, and found antibodies in 1,501 (27.3 per cent). Likewise. Suburban Diagnostics lab tested 4,105 people and found 830 (20.2 per cent) had antibodies.
The private labs may not have got the slum population to test, hence the overall prevalence is lower than BMC survey. But since these labs tested middle-class and upper-class sections, it may indicate that the seroprevalence is higher than 16 per cent (found in BMC survey).
BMC officials told The Indian Express that the viral transmission pattern changes from ward to ward in Mumbai. Areas like Colaba, Cuffe Parade and Malabar Hills (A, B and D Ward) have lower viral transmission and lesser cases, as compared to, for instance, Dharavi (G-North) that has high case load.
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