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Explained: What is the term R0, used to measure coronavirus transmission?

Coronavirus: The mathematical figure ‘R0’ indicates how fast an infection is spreading.

Explained: What is the term R0, used to measure coronavirus transmission? Construction workers at Sector 127 in Mohali during the coronavirus lockdown. (Express Photo: Jasbir Malhi)

While billions of people around the world are currently living under a lockdown due to the coronavirus pandemic, researchers are still in the process of understanding the nature of its transmission.

Numerous studies are being done to understand how contagious the virus is. This is important as it helps governments make more informed decisions on how to lift lockdown measures while safeguarding lives.

In these studies, the mathematical figure ‘R0’, which indicates how fast an infection is spreading, is considered essential.

Coronavirus: What is ‘R0’, or reproduction number?

R0 (pronounced “R-naught”) is the rate at which a virus is transmitted.

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Also called the basic reproduction number, it indicates the average number of people who will contract the virus from a person who has already been infected, in a population that does not have immunity for the said disease.

R0 is the division of the number of new infections by the number of existing infections, or the average number of new infections over an infectious period (R0= new infections/existing infections).

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The infectious period depends on the virus — this can be a few weeks, months, or even decades.

The virus is assumed to be introduced to a population that does not have herd immunity or vaccination against it.

To illustrate with an example, if the number of people diagnosed with a disease increases from four to eight in two days, it is likely to increase to 16 two days later, or in four days from the start. This makes the R0 two.

The higher the R0, the more contagious the infection.


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Explained: What is the term R0, used to measure coronavirus transmission? Healthcare workers at Sapthagiri Institute of Medical Sciences (SIMS), Bengaluru, cheer as 47 patients are discharged after testing negative for COVID-19. (Express Photo)

When the R0 is 1, it means the number of infected persons in a population is constant. For every person who is cured of the disease or dies due to it, there will be one fresh case in the population.

By this logic, the ideal scenario is when the R0 is below 1. This means the infection is transmitted to fewer people. When such a rate is maintained over a considerable period of time, the disease is eradicated.

Disease transmission is not fixed, and the rate can be reduced by pharmaceutical interventions (like vaccines) or non-pharmaceutical interventions (screenings, lockdowns), or both.

Once such measures are put in place, contagiousness is measured in terms of Rt, or the actual rate of transmission at time t.


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The same strategy has been used in the past for eliminating diseases such as polio and smallpox.

Explained: What is the term R0, used to measure coronavirus transmission? The higher the R0, the more contagious the infection. (File Photo)

What is the R0 for the novel coronavirus?

While many studies have differed in their conclusions, the World Health Organization (WHO) has estimated the R0 for coronavirus at 1.4 to 2.5.

In China’s Wuhan, where the disease originated, lockdown measures brought the Rt (effective rate) down to 0.3, thus significantly slowing the number of new infections. The restrictions were eased on Wednesday.


Governments around the world which have enforced such measures are hoping to follow the same path, although experts fear a second wave of infections once lockdowns are lifted.

Here’s a quick Coronavirus guide from Express Explained to keep you updated: What can cause a COVID-19 patient to relapse after recovery? | COVID-19 lockdown has cleaned up the air, but this may not be good news. Here’s why | Can alternative medicine work against the coronavirus? | A five-minute test for COVID-19 has been readied, India may get it too | How India is building up defence during lockdown | Why only a fraction of those with coronavirus suffer acutely | How do healthcare workers protect themselves from getting infected? | What does it take to set up isolation wards?

First published on: 09-04-2020 at 11:50 IST
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