Of the 37,646 blood samples tested since June, 14,080 were found to have antibodies. (HT FILE)
Ten months after the outbreak of the Covid-19 pandemic in the city, the number of people with presence of antibodies against Sars-CoV-2, the virus that causes the disease, has risen to 46.8% in November from 27.9% in June, according to data from a Mumbai-based private laboratory, Suburban Diagnostics.
The laboratory conducted blood tests on 37,646 people over six months to detect the presence of Immunoglobulins-G (IgM) antibodies that are produced by the body’s immune system on being exposed to a foreign particle such as a virus. It helps identify individuals who were previously infected with the Sars-Cov-2 virus and have recovered. Of the 37,646 blood samples tested since June, 14,080 were found to have antibodies. Most individuals who undertook the tests at the laboratory comprised office-goers, business persons, hotel staffers, and health workers, including doctors. The cumulative antibody positivity rate was 37.40%.
From 27.9% in June (190 total blood tests), the antibody positivity rate dropped to 21.8% (6,204) in July, but gradually increased to 30.6% (6,234), 36.7% (10,535) and 48% (9,238) in August, September and October, respectively. In November, the Covid-19 antibody positivity rate stood at 46.8% (5,245 blood tests). “A total of 63% of those who underwent the antibody tests were from working groups, including doctors,” said Dr Anupa Dixit, laboratory director, Suburban Diagnostics.
A large number of recovered symptomatic patients also opted for the test to donate their blood plasma for treatment of other Covid-19 patients, said the laboratory. Medical experts, however, said the city is still far from attaining herd immunity – when a sufficient percentage of a population develops immunity against an infection either through vaccination or previous infections. “It is a good sign if antibodies are increasing among people, but we need more research to say that we are reaching herd immunity,” said Dr Sujata Baveja, head of microbiology at the Lokmanya Tilak General or Sion hospital.
To check the presence of antibodies to fight Covid-19 infection, the Brihanmumbai Municipal Corporation (BMC) in collaboration with NITI-Aayog and Tata Institute of Fundamental Research (TIFR) had conducted two serological surveys in June and August. Blood samples were collected from residents of slum and non-slum areas through random selection. As per the second report, 45% slum and 18% non-slum residents surveyed across three wards in the city were exposed to Sars-CoV-2 and had recovered due to the presence of antibodies. Suburban Diagnostics has only analysed blood samples collected from individuals who approached them to undergo antibody testing.
Suresh Kakani, additional municipal commissioner (health), BMC, said, “I won’t be able to comment on the findings of the private laboratory. But in fact, in our sero survey, we have seen a similar trend where the presence of antibodies among the general population is increasing gradually. The presence of antibodies is more in slums due to the higher population density.”
90% had antibodies even after six months
As per the laboratory’s research on 83 patients with a known history of Covid-19, the maximum antibodies are produced between 60 and 90 days of getting exposed to the virus. Of the 83 patients, 64% (53 patients) were symptomatic. A subsequent follow-up showed that 88% recorded a rise in antibody percentage at 60-90 days, while 4% showed a dip and 8% were still non-reactive. At 90-120 days, 55% showed rising antibody values, 35% a dip in these values, while 2% showed a plateau in antibody values and 8% were still non-reactive. Even after 120-150 days of getting exposed to the virus, 92% had antibodies, while 8% were still non-reactive. “No patient got re-infected with the virus during the follow-up period. However, a longer study will be required for any firm conclusion,” said Dr Amar Dasgupta, director, medical services, Suburban Diagnostics.
Covid positivity rate dips
The positivity rate of the tests conducted by the civic body has dropped from 16% in September to 14.5% in December. Similarly, private labs are also witnessing a plunge in positivity rate. Data shared by Suburban Diagnostics shows that the lab in May witnessed a 29.3% positivity rate which dropped to 11.3% in November. “It is a sign of recovery that the cumulative positivity rate among people is decreasing, while the presence of antibodies against the virus is increasing. However, this percentage differs area-wise,” said Sushant Kinra, chief operating officer (COO) of the lab.