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

Alice Udale-Smith, news reporter
Not everyone infected with the coronavirus infects the same number of people

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.

There are concerns the Cheltenham Festival in mid March may have accelerated the spread of the virus.

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.