A healthcare worker tests a man for Covid-19 in Ahmedabad | Photo: Praveen Jain | ThePrint
A healthcare worker collects a man's swab samples for Covid-19 test (representational image) | Photo: Praveen Jain | ThePrint
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New Delhi: India’s effective reproduction number ‘R’ for Covid-19 — a key parameter to measure the rate of an infection — has reduced significantly from 1.16 last week to 1.05 this week, with Tamil Nadu’s R value dipping slightly below one.

The country’s R value had gone up to 1.19 on 7 July, from 1.11 on 26 June, a week after ‘Unlock 2’ kicked in. Around 13 July, the R value decreased to 1.11 and then on 20 July it had increased again to 1.17.

However, this week — with many worst-affected states showing a downward trend — India’s overall R value has reduced.

All the calculations have been made by Sitabhra Sinha, a researcher at the Institute of Mathematical Sciences in Chennai.

An epidemic is considered to be under control when the R value remains below 1 consistently. As of Wednesday, India recorded 6,43,948 active Covid-19 cases and over 46,091 deaths.

Also read: Why stopping ‘super-spreading’ events may be key to arresting the Covid pandemic

Most worst-affected states showing downward trend

Both R and R0 (basic reproduction number) are an estimate of the number of people a patient can infect.

While R0 is calculated at the beginning of an epidemic when the entire population is assumed to be susceptible to a disease, R changes with time and takes into account the fact that some individuals are protected from the illness — either because they have developed immunity or due to social distancing measures.

For India, the R0 was calculated to be about 1.83 in April.

The R0 of a disease depends on 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 an infection.

Sinha had also calculated the R value for states with the highest number of active Covid-19 cases. Most of these states have shown a downward trend.

For Tamil Nadu, the R value has gone down to 0.92 this week from 1.06 last week.

Andhra Pradesh’s R value was at 1.14, a reduction from 1.48 around 4 August, when it had the highest R among 12 worst-affected states.

Bihar’s R value was close to 1.62 around 22 July, the highest at the time among the worst-affected states. This had declined to 1.32 last week, and has further reduced to 1.22 this week.

For Rajasthan, R value has fallen from 1.19 last week to 1.18 this week. Around 4 August, the value was reported to be about 1.21.

Both Uttar Pradesh and Telangana have witnessed a decrease in R values, after an increase in the week before.

While UP’s R value was at 1.28 around 22 July, it had increased to 1.33 last week. This week, the value decreased to 1.27.

Telangana’s R value was estimated to be around 0.9 around 22 July, although Sinha told ThePrint that this estimate was not robust due to fluctuations in data. Last week, Telangana’s R had increased to 1.18, but this week it has reduced to 1.13.

West Bengal’s R value, which had remained steady at 1.34 since 22 July, has now reduced to 1.14.

Gujarat and Maharashtra have shown small decreases in R values since last week. While Gujarat’s R value declined from 1.09 to 1.08, Maharashtra’s R value reduced to 1.12 this week from last week’s 1.14.

Also read: Exposure to common cold viruses could train body to identify and fight Covid, study says


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