Coronavirus: What is the K number and how could it help end lockdown?

As the UK lockdown begins to ease R will not be the only metric being closely monitored by scientists, the K number will also be important.

The R number is the average number of people each person infected with a disease will go on to infect.

But not every person with coronavirus passes it on to the same number of people.

Someone who self isolates soon after they catch COVID-19 might not infect anyone else at all.

Whereas someone who attends an event where they interact with lots of people could spread it to several of them without knowing.

The K number is scientists' way of measuring the variation in the number of people infected.

It allows them to see how uniformly the disease is spreading from person to person.

If K is high, for example around 5, then it means there isn't much variation in the number of people infected by each contagious person.

But if K is less than one, it means there is a lot of variation in how many people are being infected.

For example if one person with a disease infects one other person, while another person infected ten, then that the disease would have a low K number.

If a disease has a low K value then it suggests that so-called "super spreaders" could be behind a large proportion of cases.

Different diseases have different K values, says Dr Rosalind Eggo from the London School of Hygiene and Tropical Medicine (LSHTM).

"The estimated K value for SARS1 is 0.16, which is a 'low' value and means a lot of variation between people," she says.

"But for flu, it is maybe around 2, in that there is some but not a huge amount of difference in the number of infections by each person."

Research from the LSHTM suggests coronavirus has a K value of 0.1 when social distancing measures are not in place.

This means that 80% of transmissions of COVID-19 are being caused by fewer than 10% of infected individuals.

In other words, the K value tells us that, outside of lockdown, the majority of coronavirus cases are caused by "super spreaders".

Researchers from the LSHTM therefore think R could be "drastically reduced by preventing relatively rare super-spreading events".

This could prove helpful to the government as they look to further ease lockdown measures.

As it means that as long as events where super-spreading might take place are prevented, other areas of life could begin to return to normal.

A low K value could also aid track and trace efforts, as it suggest the majority of new cases will be caused by just a small number of individuals, which could help make new infections easier to identify.