Coronavirus Q&A: What is the R value?

Person wearing a mask and visor
Support us and go ad-free

When scientists are faced with a new epidemic, they must act to understand how quickly it spreads.

They do this by estimating the basic reproduction number called the R0, or R nought, which reflects how infectious a virus is.

– What is the R0?

Read on...

Support us and go ad-free

It is defined as the average number of people an infected person can expect to pass the virus on to before any widespread immunity or attempts at immunisation are made.

For example, if one person develops an infection and transmits it to two other people, the number would be R2. The higher the number, the faster it progresses.

If it is greater than one, the infection will “spread exponentially”, but if it is lower than one, it will spread slowly and eventually die out, according to the Centre for Evidence-Based Medicine (CEBM) at Oxford University.

Coronavirus – Mon Apr 27, 2020
Chief Medical Officer Chris Whitty during a coronavirus briefing (Pippa Fowles/PA)

– What is the R0 for the new coronavirus?

The World Health Organisation (WHO) estimated at the start of March the R0 was between 2 and 2.5, but estimates from different studies vary widely.

According to nine studies in China and South Korea between December and March, the mean estimated R0 is 2.63, the CEBM said.

An Imperial College London study widely seen as influencing the government’s lockdown measures predicted around 490,000 deaths in the UK with a model using an R0 of 2.4.

By comparison, measles has an estimated value of more than R14 while smallpox is almost R6, the CEBM said.

– What is the rate of infection now?

Earlier this month, foreign secretary Dominic Raab said it was “almost certainly below one in the community”, as he announced at least three more weeks of lockdown.

Strict social distancing measures, a dramatic drop in use of public transport, and people leaving the house only for essential trips are all cited as effective measures at slowing transmission.

As fewer people come into contact with each other, there are fewer chances for the virus to spread.

“But overall we still don’t have the infection rate down as far as we need to”, Raab said, warning that lifting restrictions now would risk a dangerous second peak of Covid-19.

– How is the R value calculated?

There are many different mathematical and statistical models used to predict the R value.

One is dubbed the SIR model which accounts for three factors: the number of susceptible individuals, the number of infected people, and the rate of removal from the population, either by recovery or death.

But estimates between studies vary widely owing to the quality and reliability of data and different types of epidemiological models, the CEBM said.

Support us and go ad-free

We need your help to keep speaking the truth

Every story that you have come to us with; each injustice you have asked us to investigate; every campaign we have fought; each of your unheard voices we amplified; we do this for you. We are making a difference on your behalf.

Our fight is your fight. You’ve supported our collective struggle every time you gave us a like; and every time you shared our work across social media. Now we need you to support us with a monthly donation.

We have published nearly 2,000 articles and over 50 films in 2021. And we want to do this and more in 2022 but we don’t have enough money to go on at this pace. So, if you value our work and want us to continue then please join us and be part of The Canary family.

In return, you get:

* Advert free reading experience
* Quarterly group video call with the Editor-in-Chief
* Behind the scenes monthly e-newsletter
* 20% discount in our shop

Almost all of our spending goes to the people who make The Canary’s content. So your contribution directly supports our writers and enables us to continue to do what we do: speaking truth, powered by you. We have weathered many attempts to shut us down and silence our vital opposition to an increasingly fascist government and right-wing mainstream media.

With your help we can continue:

* Holding political and state power to account
* Advocating for the people the system marginalises
* Being a media outlet that upholds the highest standards
* Campaigning on the issues others won’t
* Putting your lives central to everything we do

We are a drop of truth in an ocean of deceit. But we can’t do this without your support. So please, can you help us continue the fight?

The Canary Support us