As countries across Europe begin to lift lockdown restrictions, governments will be looking closely at the R number to monitor how quickly coronavirus is spreading in their country.

The R number, technically called R-nought (R0) or the basic reproductive number, is a measure of the contagiousness of a disease.

It refers to the average number of people a person infected with COVID-19 will pass the virus to, assuming no pre-existing immunity.

The R number varies in different populations, depending on factors such as age and how frequently people come into contact with each other.

This new R number is called the effective reproduction number (Re) or the real-time estimation of the reproduction number (Rt).

It is a measure of the disease’s transmission rate at any given moment, and it changes according to people’s behaviour and the level of immunity to the disease in the population.

If the number is above 1, the number of cases increases exponentially.

If the number is well below 1 then the disease will die out.

Researchers from the London School of Hygiene and Tropical Medicine are estimating the real-time transmission rates of COVID-19 across the world.

Analysis of their data shows that transmission rates have fallen in most European countries since the beginning of March, and that by mid-April the R number was approaching 1.

Data from the study also suggests transmissions rates have dropped by an average of one point in all UK regions since the beginning of March.

Transmission rates can be reduced by changes in the way we behave, but not all changes have the same impact.

A study from Imperial College London estimates lockdown can reduce the Rt number by up to 70% and avoiding public events can reduce the number by 50%.

Meanwhile other types of measures, such as school closures, have a more modest impact.

Voluntary measures can also be effective.

A recent publication from the German Robert Koch Institut suggests the infection rates in Germany might have decreased even before the lockdown, which could be because people were already choosing to change their behaviour.

Crucially transmission rates can go up as well as down, and changes in behaviour can also result in R increasing.

The study from the London School of Hygiene and Tropical Medicine also projects how the R number could be performing in more recent days.

Their projections show there is still a possibility that transmission rates could jump again to over 1.

This is why R will be monitored closed as countries try to work out how to end lockdown restrictions.

Calculating accurate R numbers is a complicated process, as it relies on the daily number of reported cases of the disease being correct and there are some limitations in the accuracy of this figure.

The first problem is that there is a lag in the number of daily cases reported, as it takes around 10 to 14 days for a person to catch the virus, develop the symptoms and be tested for COVID-19.

To correct for this gap between infection and when a patient tests positive, the researchers use a statistical model called nowcast.

This shows that in the UK the R number is decreasing, but that does not mean there won’t be new cases.

In fact, the study suggests the UK is among the six countries with the highest number of expected new cases, according to data up to 20 April.

The UK is the only European country in this group. The other five countries are Brazil, Ecuador, Turkey, Russia and the US.

The second problem with the official daily number of cases is that the figure is underreported.

Underreporting as such doesn’t affect the Rt estimates, as long as the proportion of underreported cases stays constant and affects the whole population equally, says Dr Sebastian Funk, the head of the research group at the London School of Hygiene and Tropical Medicine.

But changes in the testing criteria and practices can affect estimates of the reproduction number.

If cases are more likely to be reported in some parts of the population (if, for example, there is a focus on testing healthcare workers), then the reproduction number estimate will be biased towards this population and no longer reflective of the general population, Dr Funk says.

Furthermore if there is a ramp-up in testing, more cases might be reported even if the overall number of infections is in decline, he adds.

Professor Mike Tildesley from the University of Warwick says the effective reproduction number (Rt) is an extremely important quantity when it comes to determining whether lockdown measures can be lifted.

If Rt is above 1, this means that any epidemic will continue to grow and any relaxation of control will result in Rt growing further.

If Rt is significantly below one, this is an indication that a number of measures could be relaxed and Rt may remain below 1 meaning that cases will continue to decline.

However, if Rt is very close to 1, this does not give us much leeway in terms of lifting control measures. So it is important for us not just to establish whether Rt is below 1, but how far below 1 the value is.

(c) Sky News 2020: Coronavirus: How infection rates are changing across Europe and what it means for ending lockdown