India had close to 8,400 positive cases on April 11, almost exactly double of what it was on April 5.

Just as India prepares to extend its 21-day lockdown period by two more weeks, the first indications have emerged that this measure could indeed be helping in slowing down the spread of COVID-19 disease in the country. Scientists have observed a slight, but “noticeable”, flattening of the growth curve starting April 6, an indication that the reduced contact between people as a result of the lockdown could be showing its desired effect.

If the trend continues, it could result in a significantly lower number of infections in the coming days, an analysis of disease data by Soumya Easwaran and Sitabhra Sinha at Chennai’s Institute of Mathematical Sciences shows. Sinha told The Indian Express his projections show that by April 20, less than 20,000 people would be infected by the disease. In the absence of any impact of the lockdown, this number could have been around 35,000.

India had close to 8,400 positive cases on April 11, almost exactly double of what it was on April 5. Sinha said as a result of this slowdown in the spread, the reproduction number for the disease between April 6 and April 11 was significantly lower than that for the entire period of the outbreak in India, starting March 4. Reproduction number refers to the average number of people who get infected by an already-infected person.

According to calculations by Sinha’s team, every infected person is estimated to have passed on the disease to another 1.83 persons on an average since the outbreak began. However, between April 6 and April 11, this number is estimated to be only 1.55.

“It is still too early to say this for sure, but it is possible that this reduction in the growth rate of the disease is the result of the lockdown,” Sinha said. The estimated slowdown could come as a justification for the government to extend the lockdown in a bid to further reduce the growth of the spread. The slowdown does not mean the beginning of the end of the disease in India. It does not even mean that the number of people testing positive for the virus would begin to fall immediately. All that the slowdown implies is the rate at which the virus was spreading would slow down.

Besides the reproduction number, there is another indicator that seems to be pointing in the same direction. The positivity rate — or the proportion of people testing positive against the total number of tests being conducted — has remained more or less static despite an increase in the number of tests.

Since April 3, which had seen an abnormal spike in the positivity rate possibly as a fallout of the Tablighi Jamaat event, this number has continued to hover around 4 per cent, meaning two in every 50 tests is positive. “Positivity rate can give you a sense of how widespread the disease is, assuming that a substantial number of people are being tested. We’ve now reached roughly 16,000 to 17,000 tests per day, which is pretty good. But we need to see where it stabilises on an average. If we are testing more and the positivity rate remains the same, it’s a clue that the infection is where it was, and is not expanding per se. If you see that it suddenly goes up, that’s something that could be worrisome,” said Tarun Bhatnagar, a scientist at ICMR and National Institute of Epidemiology, Chennai.

On April 9, ICMR had said that the positivity rate in testing over the last one to two months had not changed substantially, between 3 per cent and 5 per cent. An examination from March 18 to April 11 of the positivity rate – or the proportion of positive cases to the number of tests – shows that the rate has fluctuated in the range 1.1 per cent to 4.1 per cent.

April 3 was an exception when the rate was 4.68 per cent. That day, as reported by The Indian Express, the Tablighi Jamaat congregation in Delhi accounted for one-fifth of the COVID-19 positive cases nationwide.