In the last one month, cases in seven large Indian cities have reduced from 59% to 50% and deaths from 46% to 35%. Drop in cases seen in a few metros cities accompanied by rise in cases in smaller cities shows the new foci of local expanding circulation.

Even as the number of novel coronavirus cases across the country continues to rise rather exponentially, the share of novel coronavirus cases in seven large cities — Ahmedabad, Bengaluru, Chennai, Hyderabad, Kolkata, Mumbai, and New Delhi — has reduced from 59% to 50% in the last one month. Similarly, the share of deaths too has dropped from 46% to 35% during the same period.

Tweeting this, Prof. Giridhara Babu Head-Lifecourse Epidemiology, Indian Institute of Public Health, PHFI, Bengaluru and a member of Karnataka COVID-19 Technical Advisory Committee says: “The fall in cases seen in a few large metros such as Mumbai and Delhi is accompanied by rise in cases in other metro and tier-2/tier-3 cities. This shows new foci of local expanding circulation.”

He then adds: “This will be the case in future too. The shift in cases from mega cities to tier-2 and tier-3 cities depends on travel and enforcement of preventive measures such as wearing a mask.”

Dr Rijo John, health economist and consultant, concurs with Prof. Babu. “The reduction in cases in the major cities is a sign of COVID-19 spreading to smaller cities and towns. It’s a sign of the spread reaching far and wide,” he says. He had earlier in the day tweeted: “…Only 56.7% of the total cases and 73% of deaths are now accounted by top 20 districts as against 65% cases and 81% deaths a month ago.”

Prof. Babu says that overall, the burden in large cities has come down in comparison with cases reported from across the country. “Cities which reported high burden are witnessing a decreasing trend. Mega cities like Ahmedabad, Mumbai and Delhi seem to have peaked in terms of number of daily fresh cases,” he says. But there are concerns about daily reported cases from Ahmedabad.

Concerns with Delhi data

It is not clear if the reduction in daily fresh cases seen in Ahmedabad, Hyderabad, and Kolkata reflect the ground truth. Even as the sero survey conducted in Delhi found that 23.48% (nearly 4.5 million) have been infected as against nearly 0.12 million detected so far, over-reliance on rapid antigen tests appears to have not served its purpose of detecting more cases.

Rapid antigen tests have low sensitivity but high specificity. While a positive test result from rapid antigen test surely indicates that the person is infected, a negative test does not completely rule out infection. Hence every negative test from rapid antigen test should be reconfirmed with RT-PCR, the gold-standard test.

On June 14, ICMR issued guidelines that recommended the use of rapid antigen tests to increase the number of tests carried out daily particularly in containment zone or hotspot, and healthcare settings. These tests were to be offered to people with influenza-like illness (ILI) and certain categories of asymptomatic patients. But ICMR had clearly mentioned that rapid antigen testing should be used in combination with RT-PCR, the gold-standard test and stressed that negative test from rapid antigen test should be reconfirmed using RT-PCR. The guidelines say: “Suspected individuals who test negative for COVID-19 by rapid antigen test should be definitely tested sequentially by RT-PCR to rule out infection, whereas a positive test should be considered as a true positive and does not need reconfirmation by RT-PCR test.”

But in Delhi, just about 0.5% of people who tested negative with rapid antigen test was reconfirmed with RT-PCR.

Test positivity rate

But even as the number of fresh cases in the large cities continues to reduce, the test positivity rate has been rising. “In one week, the average test positivity rate in mega cities has shot up from 6.4% to 9%. This is a strong indicator that virus continues to spread at a faster pace,” he tweeted. Any increase in test positivity rate only reflects that the number of daily tests done in each city has to increase to detect a large pool of undetected cases. With surveillance not being commendable, a high test positivity rate reflects wider spread of the virus in the community.