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Suburban Diagnostics Dissects Reasons for Falling Trend in COVID-19 Cases in Mumbai

COVID-19 Cases in Mumbai

Suburban Diagnostics, in their attempt to objectify their diagnostic and management strategies, conducted a research on 83 patients with a known history of COVID-19. These patients were followed up on with serial antibody tests to study the sero-conversion and the trend of antibody response post infection. Out of the 83 patients, 64% (53 patients) had symptomatic COVID-19 infection and 36% (30 patients) had completely asymptomatic infection.

When followed up for 120-150 days post infection, 92% were positive for antibodies and 8% were still negative. Of the patients who completed 180 days post diagnosis, 90% were antibody positive. This figure is liable to change once all the patients being followed up complete 180 days and are tested.

When following up with antibody testing amongst the 83 patients, 88% showed a rise in titres at 60-90 days, while 4% cases showed a dip and 8% cases were still non-reactive. At 90-120 days, 55% cases showed rising antibody values, 35% showed a dip in these values while 2% cases showed a plateau in antibody values and 8% were still non-reactive. At 120-150 days, 37% cases showed rising antibody values, 55% showed a dip in these values and 8% were still non-reactive.

The table below shows that, from June to November, 2020 there was a significant rise in the percentage of patients who developed antibodies. Out of the total 37,646 tests conducted it was found that around 14,080 which is around 37.4% of the patients had developed COVID-19 antibodies.

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Month Number of
antibody tests
done

 

COVID Antibody
Positivity Rate
June 190 27.9%
July 6204 21.8%
August 6234 30.6%
September 10535 36.7%
October 9238 48.0%
November 5245 46.8%
Grand Total 37,646 37.4%

 

One more study conducted on a total of 200 patients by Suburban Diagnostics was to understand the concordance between the rapid antigen test and the RT-PCR test with different cycle threshold (CT) values. In this study, the patients were evaluated for the COVID-19 infection by the rapid antigen method and the RT-PCR method, both. The conclusion of the study is that the rapid antigen test was 100% specific with no false positives found and had a 60% sensitivity compared to the RT-PCR test. It showed a lower sensitivity of 32% in asymptomatic patients. The CT value correlation showed that 63% cases that the antigen test proved negative but the RTPCR test proved positive had borderline CT values corresponding to low viral titres. A RT-PCR analysis was also processed on more than 3.7 lac samples declared 70.7% results were negative and 29.3% were positive.

 

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The table below represents the number of RT-PCR tests carried out in each month between May and November, 2020. A significant fall can be seen in the percentage of patients who tested positive. Out of the total 4,06,780 tests conducted it was found that around 1,04,949 which is around 25.8% of the patients were COVID-19 positive.

Month Number of
diagnostic tests
done

 

SARS-CoV-2 virus
positivity rate
May 18044 29.3%
June 30161 35.0%
July 55543 35.1%
August 59943 27.8%
September 96201 30.6%
October 81802 19.6%
November 65086 11.3%
Grand Total 4,06,780 25.8%

 

In relation to antibody development, a rising trend has been seen since July in the city of Mumbai and a falling trend of COVID-19 virus positivity rate has been seen from July till November after a peak in July, 2020.

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