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EXPLAINED: As India's R-Naught Inches Upwards, Here's What It Is And Why It Matters In Covid Fight

A crowded weekly market in Delhi. NITI Aayog member Dr VK Paul has said that India's R value is now at 1.22, which means that an infected individual is passing on the virus to more than one person. (AP)

A crowded weekly market in Delhi. NITI Aayog member Dr VK Paul has said that India's R value is now at 1.22, which means that an infected individual is passing on the virus to more than one person. (AP)

The R-naught value indicates the rate of spread of an infection and can point to the trajectory that the case load might take

If it goes beyond 1, that’s a sign that infections are spreading at a fast rate, which is why public health experts in a pandemic keep a sharp eye on the R number, or R naught. Amid fresh surges across the world and the rise of the Omicron variant, India, too, has started seeing an increase in the count of daily infections. Now, experts have said India’s R number has started to move upwards in what would be a cause for worry for the authorities. But what is R-naught, and how is it calculated?

What’s India’s R Number?

According to NITI Aayog expert VK Paul, India’s R-naught, or R0, is at 1.22 amid a significant rise in cases, which have breached a more than a 30-day high in the last week of 2021. The daily rise in Covid-19 cases crossed the 13,000 mark after around 49 days, says data reported by the Centre.

Authorities have noted that Maharashtra, West Bengal, Tamil Nadu, Delhi, Karnataka and Gujarat are emerging as states and UTs of concern on the basis of weekly infections and the test positivity rate, which is the number of positive Covid-19 cases as a percentage of all tests conducted.

“We believe on the scientific basis R0 is 1.22 as per the available data… so the cases are now increasing, not shrinking. As the scenario emerges, we believe what we are witnessing could be part of the global rise in cases pushed by the Omicron variant… we are already aware that this variant is highly transmissible and that perhaps explains the speed with which it is rising in the world," Paul is reported to have said at a press briefing on December 30.

A Union health ministry official said that eight districts in India are reporting more than 10 per cent Covid-19 weekly positivity, while 14 districts have shown a weekly positivity ratio of between 5-10 per cent.

What Is R0?

It is considered to be an early warning signal that can suggest what trajectory Covid-19 cases can take with even minor variations holding major significance.

“Even a 0.1 change can make a big difference in terms of how many active cases there will be," Dr Sitabhra Sinha of the Chennai-based Institute of Mathematical Sciences had said earlier this year after a study on the R rate.

The ‘R’ rate refers to the “effective reproduction" number for any infectious disease. What it indicates is the number of people on average to whom a single infected person can pass on the disease. So, the higher the R rate, the worse it is for the outlook on how the disease is likely to spread.

When we talk of an R rate of 0.89, it means that a single person will spread the disease to less than one person. Or, seen in another way, that every 10 infected persons are passing the disease on to about nine others.

The key threshold for the R number is 1. If the R number is below 1 then, experts say, the disease is liable to die out. But if the R is even slightly higher than 1, then authorities may be looking at a rapid spread. Consider this: when a disease has an R number of 1.2, it means that every 10 people are passing the disease on to 12 others, who in turn will pass it on to more than 14 others and so on. When we talk of big populations, it is easy to see how an R number of more than 1 can quickly lead to a large case load.

Now, if the R factor is less than 1, say 0.75. It means that 10 people will give the disease to about eight others and they in turn would pass it on to 6 others from whom it would jump to 5 others.

How Is The R Factor Calculated?

Several factors can have a bearing on the R number. Among the key ones are how infectious the disease itself is — which assumes importance amid the rise of new variants of the novel coronavirus that are said to be able to spread more easily among people — and how a population behaves. Which means that the sights of crowds at holiday destinations and public spaces does not bode well as India gears up to battle a third wave of Covid-19 amid the emergence of the Omicron variant, which is seen to be more transmissible than the earlier avatars of the novel coronavirus.

Prior immunity in a population — either due to exposure to the disease or vaccination — also plays a part in determining the R number. Per government data, 90 per cent of India’s adult population has received at least one shot of a Covid vaccine with more than 63 per cent having received both doses. Vaccinations has now been opened for 15-18 year olds while a “precaution dose" of a third jab has been cleared for health and frontline workers and senior citizens with comorbidities.

The BBC says that a number of factors are tracked for scientists to arrive at an infection’s R number, including the rate of deaths, hospitalisation and positive results from tests.

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first published:December 31, 2021, 14:10 IST