COVID-19 in Mumbai: Antibody testing by two private labs shows positivity rate of 24.3%; Here’s what it means

A COVID-19 patient requires two weeks for antibodies against the infection to develop.

Coronavirus antibodies testing in India: Nearly one-fourth of Mumbai’s people have already been exposed to coronavirus! According to the data from two private testing laboratories in Mumbai, of the 9,590 samples tested for the presence of antibodies against coronavirus, 24.3% tested positive, according to a report by IE. This means that the people who tested positive in this test had been exposed to coronavirus and had also generated antibodies against it. These samples were collected from the general public of the city, including the salon staff, businessmen, health workers and office goers, the report added.

As against this, the latest data of sero surveillance in Delhi has also suggested that the people of the national capital are more exposed to the virus than those in the Maximum City. The IE report stated that the sero survey data of the National Centre for Disease Control (NCDC) combined with the data of samples tested by Thyrocare showed a positivity rate of 25.10%. Individually, the NCDC conducted the IgG antibody test of 21,387 people across 11 districts of the national capital, and found that of them, 5,022 had developed antibodies against COVID-19. This indicates a figure of 23.48%. Meanwhile, private lab Thyrocare found antibodies in 1,340 of the 3,956 samples it tested, indicating a positivity rate of 33.8%.

Thyrocare was also one of the two private labs that conducted the tests in Mumbai, and it had tested 5,485 samples in the city, finding that 1,501 or 27.3% of the samples had antibodies. Apart from this, Suburban Diagnostics lab also found that 830 or 20.2% of the 4,105 people it tested had developed antibodies against COVID-19.

The report quoted Sion Hospital’s Head of Microbiology Dr Sujata Baveja as saying that these numbers are good news, since they indicate that a large number of people who had been exposed to the novel coronavirus did not develop any symptoms and built immunity instead.

An IgG antibodies test is used to detect whether antibodies, and therefore immunity, against an infection has been developed in the person being tested. A COVID-19 patient requires two weeks for antibodies against the infection to develop. Moreover, the antibodies test is only a tool for surveillance to assess the macro level of exposure to the virus and it cannot be used for diagnosis. Whether a person is COVID-19 positive or not is tested mostly using an RT-PCR test, which detects the virus using nasal and saliva swabs.

The report stated that Mumbai’s positivity rate using the RT-PCR test was 23.3%, lower than the percentage of people that tested positive for antibodies. The report stated that according to experts, this showed that while the transmission of the virus was high, it also meant that several more people had been exposed to it but did not develop any symptoms and instead, recovered on their own.

It also quoted Head of Operations, Thyrocare, Dr Caesar Sen Gupta as saying that the rate of positivity for antibodies test showed that a larger percentage of the population had been infected but never realised since no symptoms emerged.

Meanwhile, Chief Microbiologist at Suburban Diagnostics Dr Anupa Dixit said that the antibodies test they conducted represented the general population.

Dr Pradeep Vyas, Maharashtra Health Secretary, was quoted by the report as saying that since RT-PCR test is targeted at those who symptomatic or close contacts, while the number of exposed people is more, the positivity rate of IgG antibody testing would always remain higher than that of RT-PCR. However, he said that the latest findings showed that immunity is lost within a few months, so the positivity rate of antibody tests might not mean much in the long run.