R value: What does number mean for coronavirus infections and easing lockdown?

The government has set out five key tests which ministers say must be met before the UK’s coronavirus lockdown measures are significantly eased.

The third and arguably most crucial test involves ensuring the rate of transmission is decreasing to “manageable levels”.

Key to understanding this is a number known as the ‘R’ value, or the “effective reproductive number”.

Boris Johnson has said reducing the R number is our “collective endeavour”, and the UK’s top epidemiologists will be continually trying to calculate it as the government mulls easing measures.

So what is R?

The R number is a measure of the extent to which a disease is spreading throughout the population.

Put simply, it is the calculation of how many people an infected person is thought to transmit the virus to on average.

If the R value is one, each carrier passes the virus on to one other person, meaning the prevalence of the virus will remain at the same level, or – if it falls below one – start to diminish.

However, if the R is two, each infected person will transmit the virus to another two people. Even if the R is only slightly above one, the virus will continue to spread exponentially.

For example, German chancellor Angela Merkel explained in April that if Germany’s R remained at 1.1, her country’s health system would be overwhelmed by October. This would happen in July at 1.2, or in June at 1.3, she said.

What is the R for coronavirus?

Epidemiologists stress that estimations of R – which rely on predicting human behaviour as much as the ability of a disease to transmit – are imperfect and are forever being revised.

In early March, the World Health Organisation placed the R for the novel coronavirus between 2 and 2.5, in the setting of a population with no immunity or social distancing measures.

However, this greatly varies from place to place, with one US government study putting R as high as 5.7 in the early days of Wuhan’s outbreak. Imperial College London modelling in early April suggested R may have been between 3 and 4.6 in Europe before lockdowns were introduced.

This compares with around 1.3 for seasonal flu, and 2 to 4 for Sars. Measles is often cited as being between 12 and 18.

How is it calculated?

There are many different ways of calculating R, with much of the mathematics reliant upon educated guesses and often extremely variable estimates.

The chief scientific adviser Sir Patrick Vallance has previously said UK scientists are “looking at all sorts of things including contacts, looking at genomics, looking at data from ambulances, hospital admissions, and so on”.

Tracking the true scale of the virus’ spread throughout the country has likely been impeded somewhat by the decision to abandon widespread contact-tracing and testing in mid-March.

The UK is now seeking to significantly ramp up its strategy of “test, track and trace”, which will likely help epidemiologists in their bid to monitor the country’s reproductive number as accurately as possible.

In late April, Matt Hancock unveiled a vast new programme which could see 300,000 people regularly tested for Covid-19, which is aimed at helping scientists understand the rate of transmission and possible immunity across the nation.

What does R need to be to ease lockdown measures?

Governments across the world have said the R needs to be considerably below one for lockdown measures to be eased in any significant fashion.

On 7 May, as Dominic Raab paved the way for the prime minister to announce the slight easing of some restrictions, he suggested the current estimates placed R between 0.5 and 0.9.

The foreign secretary said that any changes would be made in line with the government five key tests and scientific advice, warning: “If people abandon the social distancing, if people forget the sacrifices that were made to get us through the peak, to get us to this point, the virus will grow again at an exponential rate.

“That would lead to a second peak which would threaten the NHS, which would trigger another lockdown, which would prolong the economic pain.”

He added: ”If people don’t follow the rules or if we see that the R level goes back up, we will tighten the restrictions again, we will always retain the option to do so.”

However, England’s chief statistician Professor Ian Diamond said moments later that he believed R was actually rising, mainly as a result of transmissions in care homes.

But despite the R number teetering slightly above one in some regional estimates, the government has been satisfied that it can continue to ease lockdown measures - reducing its Covid-alert level from four to three on 19 June.