India’s ‘R’ for Covid remains almost unchanged

The effective reproduction number ‘R’ for Covid-19 — a key parameter to measure the rate of infection — this week remained almost unchanged at 1.12. Last week, the value stood at 1.13.

Sitabhra Sinha, a researcher at the Institute of Mathematical Sciences (IMSc) in Chennai, said at this ‘R’ value, active cases are likely to cross 2.5 lakh by 4 July.

Last week, the ‘R’ value reduced to 1.13 after holding steady at 1.22 for about a month.

An ‘R’ of 1.22 would have translated to 3.5 lakh active cases by 4 July. ‘R’ must be driven below 1 to arrest the epidemic.

As of Thursday, India has 1,86,514 active cases. The total number of cases recorded so far stands at 4,73,105, which include 2,71,697 patients, who have recovered/discharged, and 14,894 deaths.

Difference between ‘R’ and ‘R0’
‘R’ is different to the basic reproduction number ‘R0’, though both denote the number of people one person can infect.

R0 is calculated at the beginning of the epidemic, when the entire population is assumed to be susceptible to the disease.

‘R’, which changes with time, takes into account that some individuals are protected from the disease either because they have developed immunity or because of social distancing and other measures.

Without lockdown measures, Sinha had estimated the basic reproduction number (R0) of Covid-19 in India to be 1.83, a value that was relatively lower than other regions where there have been major outbreaks.

‘R’ is a product of three factors — the probability of infection when a susceptible person comes in contact with an infected individual, the average rate of contact between susceptible and infected individuals, and the duration during which an individual spreads the infection.

It is calculated through mathematical epidemiological models that compute how fast the virus spreads in the population.

‘R’ value of US, Germany
‘R’ value can increase again if the population stops following social distancing measures or if travel restrictions are lifted.

For example in Germany, the value of ‘R’ shot up from 1.06 on 19 June to 2.88 on 22 June. This sharp increase over just a few days was due to large local outbreaks, particularly in four areas in Germany — Gütersloh, Warendorf, Magdeburg and Neukölln.

According to estimates by London School of Hygiene & Tropical Medicine, the ‘R’ value for Covid-19 in the US has increased to 1.1 — similar to that of India — this week after having briefly dipped below 1 in the first week of May. – The Print