Coronavirus cases in Ahmedabad: Ideally, more testing should lead to more infections—in Tamil Nadu, the positivity rate or new infections upon new tests has stayed constant in the 10-11% range—in Ahmedabad positivity has rapidly declined with more testing.
Coronavirus cases in Gujarat: While Ahmedabad was one of the few cities where testing data and a sero-survey were showing similar results—on July 11 the sero-survey showed a 17.7% infection, whereas the case infection rate was 15.1%–a change in testing strategy has meant a stark divergence in results. Increased testing in the city has led to the infection rate falling to 3.4% on September 13.
Ideally, more testing should lead to more infections—in Tamil Nadu, the positivity rate or new infections upon new tests has stayed constant in the 10-11% range—in Ahmedabad positivity has rapidly declined with more testing. So, in the case of Ahmedabad, the converse is true. As the city tests more, it finds fewer and fewer infections.
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On July 13, Ahmedabad had a positivity of 12.1%. The city found 336 infections by conducting 2,789 tests, but this dipped to 0.8% on September 13. On Friday, Ahmedabad conducted over 19,000 tests and discovered only 153 were infected, which means a positivity rate of 0.8%.
Although the city administration does not provide data for rapid antigen tests, most of this decline may be attributed to RAT testing. In fact, an analysis of Mumbai shows that as RAT testing in the city increased, the positivity declined. While RAT accounted for 8% of tests in the week of July 7-14, the positivity during that week was 22.4%. In the first week of August, when RAT accounted for 48% of the tests, the positivity had halved to 11.1%. As the ratio of RAT tests in subsequent weeks declined, positivity increased again.