Explained: Why Rising SARS-CoV-2 Reproduction Number Is Raising Concerns

Data from July show that across India, 10 patients are infecting nine others. But in eight states, one infected patient is spreading the disease to more people than the all-India average. A further increase could put India back on the path of exponential growth, and another wave

Covid-19: Rising SARS-CoV-2 Reproduction Number Is Raising Concerns

Jaipur: Since June 20, 2021, the Covid-19 virus' effective reproduction number or R, which is the number of people one Covid-19 patient infects on average, has moved closer to one, rising from 0.78 to 0.88, calculations by Sitabhra Sinha, a professor of computational biology and theoretical physics at the Institute of Mathematical Sciences in Chennai, show. This is dangerous as good pandemic management requires that fewer and fewer people catch the infection, as we reported in May 2020.

An R lower than 1 means that one patient is not even infecting one more person, on average, and if the number keeps falling, there will be fewer and fewer cases going forward. If every Covid-19 patient is infecting one more person, the spread remains slow, yet many people contract the disease. If R is more than 1, the infection can spiral out of control.

For instance, at the current R of 0.88, 10 patients would have infected about nine people. If the R were 1, they would have infected 10 more people. The R in Kerala is currently 1.1, which means that 10 patients infect 11 more people.

Since India's R is still lower than 1, active cases will be reducing but the rate of this decline has slowed, which is "worrying", said Sinha.

Between May 15 and June 26, new Covid-19 cases reduced at an average rate of 4% daily while between June 20 and July 7, they reduced at an average rate of 0.5%, data from Covid19India.org show.

When everyone in the population is equally susceptible--that is, when no one has developed immunity against the disease from prior infection or vaccination--and there is regular contact between people without any quarantine or containment measures, the disease would spread at the basic reproduction number (R0 or naught), which is the infectivity of the virus itself. Between March 14 and April 5, 2020, at the onset of the pandemic when almost every person was susceptible to the virus, every Covid-19 patient would infect 2.5 other patients, Sinha said.

As time passes and some people recover from the infection or get vaccinated and become immune, and measures are taken for containment, such as isolation, masking and physical distancing, the disease would be restricted from its natural transmission, and researchers calculate its effective reproduction number, R.

State-level trends

Even though India's R is less than 1, several states' R is greater than 1 at present. "The Northeast is a region of great concern," said Sinha. Arunachal Pradesh and Tripura have an R greater than 1, while Meghalaya has an R of 0.92, Sikkim of 0.88 and Mizoram of 0.86, show data from the four states for which R numbers are available until the first week of July. Manipur's R between June 27 and June 30 was 1.07.

Covid-19's R in Uttarakhand is 0.94. In Odisha and Tamil Nadu, it is 0.90. "The other states have R<1 but not that much lower than 1," Sinha said.

These small numbers can make a lot of difference to the increase in Covid-19 in the state. For instance, 100 people in Arunachal Pradesh, at an R of 1.14, would infect 114 other people. At West Bengal's R of 0.89, 100 people would infect 89 others, reducing the number of active cases over time. Similarly, at Karnataka's Covid-19 R of 0.56, 100 people would infect 56 others, leading to a rapid decrease in cases over time.

(Pragathi Ravi, an intern with IndiaSpend, contributed to this article.)

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