Coronavirus: What is the R number or R rate - and why does it matter?

Coronavirus might have meant big changes to people's lives over the last few months, but it's also brought some new words and phrases with it too, such as social distancing, self-isolation and lockdown.

Another one you've probably heard a lot recently is the 'R number' or 'R rate'.

It's actually really important for helping to stop the virus spreading and making plans for life to return to normal.

So what exactly is it and why should we care about it?

Here's everything you need to know.

What is the R rate?

The R rate means scientists can measure how quickly - or slowly - a disease is spreading.

The number can change - and has done - here in the UK and other countries over the last few months.

It basically refers to the number of people that one infected person will pass the virus on to, on average.

So if the R number is one, that means one person with the virus will on average infect just one other person.

However, if the R rate is higher - at say three - that means one person with the virus, passes it on to three other people.

A higher number means the virus is spreading quicker.

If the number is below one, for example 0.5, that means it takes two people to pass the virus on to one other person. The spread of the virus is much slower.

Scientists want the number to remain under one because this means the disease will eventually peter out, as not enough new people are being infected.

Dr Sebastian Funk, from the London School of Hygiene and Tropical Medicine, told the BBC: "A day above one doesn't concern me, a couple of weeks does."

How do we bring the R rate below one?

With viruses, each person who is infected can infect others by spreading their germs - for instance by things like coughing and sneezing, or by touching things without washing their hands.

This is why the UK, like countries all over the world, went into lockdown to try and limit the number of people coming into contact with each other and spreading the virus.

It's thought that before lockdown R was well above one. That's why schools were closed, shops and restaurants shut and only some people were still going to work.

The government - advised by scientists - believed steps like this could help push the R rate down.

Another way to reduce the R number is by having a vaccine. This would stop those who've had the jab from getting the virus.

For example, if one person had coronavirus and the R rate was at three (ie they will pass it on to three others), if a vaccine could protect two of them from infection, then the R number would fall from three to one.

How does the R rate affect lockdown?

The government has outlined five tests which affect whether or not lockdown rules are eased - the R number is one of them.

This means that measures, such as certain shops being closed and big events being cancelled, depend, in part, on how high or low the R is.

Once the R number dropped below one here in the UK - because people were staying at home more - the government has started to ease some rules.

Is the R rate the same across the UK?

The R number can change and isn't necessarily the same across the UK.

This can be for a number of reasons, for example the number of people in an area, such as big cities compared to villages in the countryside.

It also depends on when people first started getting the virus in an area - some places might be behind or ahead of others.

The way people behave can also have a big impact. For example if people don't follow guidance on social distancing or are unsure about what rules mean for them.

This difference in R means there has been some talk about the possibility of "local lockdowns". This could mean people living in areas where the R rate is higher, have stricter rules than those in places where it is lower.