We have heard the term ‘R number’ being used a lot in the news, but not everyone understands it. What is the R number, how is it calculated and when can it mean that we leave lockdown?

Boris Johnson has unveiled a new “conditional plan” to reopen society that has been widely criticised.

In his speech on Sunday evening, the PM talked about the R value of Covid-19 dropping to between 0.5 and 0.9.

This was used as a way to explain the gradual easing of lockdown measures, with people in England allowed to spend more time outside from Wednesday 13th April.

The term ‘R number’ or ‘R value’ has been said over and over again in the news, but many people don’t know what it means, how it is calculated or how it can be used as a way to lift lockdown.

What is the R number?

The R number or R0 number (short for the reproduction number) is a way of measuring a disease’s ability to spread. It is the average number of people that will be infected by one person with the virus.

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If the R number is higher than one, the number of new cases increases exponentially.

For example, if a disease has an R0 of 5, one infected person will transmit it to five people, who will each go onto infect another five people and so on.

The domino-effect of infection will continue until it reaches someone who has either been vaccinated or has the antibodies developed from recovery.

However, if the R0 is less than 1, then each infected person will cause fewer then one new infection (on average). This means that the disease will eventually fade away as not enough new people are being infected enough to sustain the outbreak.

If the R0 equals one, then each infection causes just one new infection. The disease remains stable within a population but there will be no outbreak or pandemic.

How is the R number calculated?

There isn’t a simple formula that we can follow to calculate the R number, but we can come to a judgement for the R number based on several factors:

According to the Centre for Evidence-Based Medicine, the R number is affected by:

  • The proportion of vulnerable people within a population, i.e. how many people are susceptible to the disease
  • The density of a population, i.e. how many people are within a small distance to each other
  • The diseases infectiousness, i.e. how infectious the disease is
  • The rate of disappearance, i.e. how quickly do people either recover or die from the virus

There are other factors that can also affect the R number, including the virus’s infectious period, contact rate and mode of transmission.

What R number is needed to lift lockdown?

Before the lockdown began in late March, the R number of Covid-19 was approximately 3.75. This meant that each person with the virus would infect, on average, 3.75 other people.

Thanks to the implemented measures, the current R number is estimated to be between 0.5 and 0.9.

However, in order to lift the lockdown, we need to keep the R number under one and make sure it doesn’t suddenly increase again.

Earlier in April, the R number in Germany fell to roughly the same level as it is here in the UK.

However, easing of lockdown guidelines increased their R number back up again, forcing measures to be reinstated.

The R number will be constantly evaluated by the British government and altered accordingly.

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