THE R rate, also known as the reproduction number, has been crucial to the Government's understanding of the threat Covid-19 poses.
But now experts have now revealed that another value is becoming increasingly important amid the coronavirus pandemic: the K number.
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And scientists believe it will be key for fighting a second wave of the disease – as the Government slowly begins easing lockdown measures.
But what exactly is the K value and what does it mean for Covid-19?
What is K?
While the R rate tells us the rate of transmission of coronavirus, the K value looks at how much variation there is in transmission.
It's a statistical value the can help shed light on different transmission rates.
The R rate is an average – for coronavirus at it's height each patient passed the bug on to an average of three other people. Now, after lockdown that number is between 0.7 and 0.9.
But the K value takes into account that not everyone will pass on the virus at the same speed, and not every environment poses a risk to mass spread of the bug.
If the K figure is close to one, as with influenza, a virus will move relatively evenly through the population without variation.
Dr Adam Kucharski, an expert in the dynamics of infectious diseases at the London School of Hygiene and Tropical Medicine (LSHTM), told The Guardian: “Once K is above about five or ten it tells you most people are generating pretty similar numbers [of secondary cases], you are not getting these super-spreading events.
"Once K is below one, you have got the potential for super-spreading.”
What does it mean for Covid-19?
In the case of Covid-19, the K is believed to be 0.1 to 0.5 per cent, according to Dr Kucharski and his team at LSHTM.
Their findings suggest the majority of secondary transmission may be caused by a very small fraction of individuals.
In other words, this could mean ten per cent of Covid patients pass on 80 per cent of infections – these people may be called 'super spreaders'.
The other 90 per cent of individuals don't actually spread it to many other people – and are behind the remaining 20 per cent of coronavirus transmission.
It's not just the average amount of transmission, it is also the amount of variation
This has huge implications for Governments trying to ease restrictions but avoid a second wave of infections.
“It could be that you have a lot of cases that crop up and don’t infect anybody and you have one case who goes to a particular environment, or particular workplace, and sparks a very large outbreak,” Dr Kucharski told The Times.
R, which the Government is trying to keep below one, was important when the pandemic was at its peak.
However, Dr Kucharski said now "it's not just the average amount of transmission, it is also the amount of variation."
To put it another way, super-spreading matters – a reality highlighted by reports including one from South Korea where one individual is thought to have infected dozens of others by attending church.
How might the relaxation of the lockdown affect K?
Lockdown lowers the risk of a single infectious person spreading the virus to other individuals.
Dr Kucharski said: “Obviously if you start to allow larger gatherings, have larger workplaces, if you have other types of interaction starting, then that does increase the chance that one infection could spread to more people than it would have been able to a couple of weeks ago.
"It could decrease the K, but it could also increase the R."
The effect of easing lockdown measures on K can be seen in other countries in Asia and Europe that have begun to open schools, shops, bars and restaurants.
Instead of widespread infection, they have had flare-ups where individuals spread it to large numbers of people during a single encounter, for example in a South Korean club or French, German or Dutch abattoirs.
Dr Kucharski said if a handful of events generate most of an outbreak, then if you can find a way of identifying where and when those events are happening then that reduces far more of your transmission than if you were just trying to follow up every case as if they were the same.
He added: "The difficulty is you have to predict where these ‘superspreading events’ happen."
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