Explained: Rate of COVID-19 spread in India, world

Karishma Mehrotra
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On March 19, every positive case in India was transmitting the virus to 1.7 people on average.

Transmission of COVID-19 in India has increased to an extent over the past week, yet the rate remains significantly lower than that the global rate.

On March 19, every positive case in India was transmitting the virus to 1.7 people on average. By March 26, that number had risen to 1.81, but remained significantly lower than in countries such as Iran or Italy, according to Institute of Mathematical Sciences in Chennai scientist Sitabhra Sinha.

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For comparison, a study in The Lancet estimates the global transmission rate of COVID-19 — the number of persons infected by every positive case — to be between 2 and 3.

“By the end of the month about 1,500 live infections,” Sinha estimated, speaking on March 26. “By April 5, just upward of 3,000, and in the worst case, will just cross 5,000. There has been a slight upward shift of the slope of the exponential curve after March 16. However, the effect of the lockdown should also bring down the growth rate somewhat, but we’ll only know by how much after a week.”

30 days, six countries

With this low growth rate, it took one month for India’s COVID-19 case numbers to climb from three to one thousand. Comparing this month’s case growth to South Korea’s first month of cases shows that India’s curve has recently become flatter than that of the other country at a corresponding stage. The death rate has remained flatter as well.

In fact, a comparison of the first 30 days of six different countries’ case counts shows that India has had a lower growth than South Korea, Spain, Iran, and Italy, but not Singapore.

*India’s case count is for Day 29 and death count for Day 18, both based on WHO situation report for March 30. These reports often reflect the counts reported on the previous day (May 29 in this case).

It is important to note, however, that due to testing discrepancies, these country-reported numbers taken from the World Health Organization are dependent on the amount of conducted tests. Also, these numbers are subject to change depending on a variety of factors, such as the effectiveness of India’s 21-day quarantine.

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Week by week

Looking at week-long periods, India’s cases grew from 3 to 43 to 114 to 415 to 1,071 on the 29th day.

South Korea climbed from 4 to 23 to 28 to 104 to 1,766 on the 29th day (1,776 on the 30th). Reports show South Korea experienced an anomaly; a “Patient 31” was a super-spreader, transmitting the virus to an unusual number of people before testing positive.

Weekly, Singapore climbed from 4 to 18 to 43 to 75 to 90 on day 29, and 91 on Day 30. Spain climbed from 2 to 151 to 1,639 to 11,178 to 39,673, then over 47,000 a day later. Italy climbed from 3 to 650 to 3,858 to 15,113 to 41,035 (29th day) and 47,021 (30th). Iran climbed from 2 to 141 to 2,922 to 9,000 to 17,361 (29th) and 18,407 (30th).

Deaths across the six countries showcase a relatively low growth in India thus far.

In the two weeks after each country’s first coronavirus death, India’s numbers grew from 1 to 4 to 17 from one week to the next. South Korea’s rose from 1 to 13 to 35. Spain climbed from 1 to 48 to 598. Iran went from 2 to 22 to 92. Italy soared from 2 to 29 to 234.

Rate of transmission

Compare India’s low transmission rate (1.81) to that of Italy - between 2.76 and 3.25, according to the European Centre for Disease Prevention and Control.

This number is called a “reproduction number” or R0, used to describe the contagiousness of a disease. When the R0 less than one, meaning not every infected person spreads the virus to another, the disease ceases to become an epidemic. Above one, and the country witnesses exponential growth in cases. Epidemiologists study outbreaks by watching this exponential growth.

Epidemiologists have championed “social distancing” in order to “flatten the curve.” This means that by preventing people from interacting, each infected person is likely to transmit the virus to less people, slowing the speed of its spread and allowing healthcare systems to handle the influx of patients.

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