Forget the R rate, it's the Covid-19 clusters we need to stop
Forget the R rate, it’s the Covid-19 clusters we need to stop: Revealing the pattern in the way coronavirus spreads is more useful, researchers say
During the coronavirus pandemic, the reproduction number — R for short — has been used as a key measure of whether the virus is under control.
This shows how many people, on average, one person with Covid-19 infects.
Trying to keep this number under one has been a major part of the Government’s response to the pandemic.
But researchers now claim a more useful tool for managing the virus is a measure known simply as K. This reveals the pattern in the way the virus spreads.
A K value of one or more means a disease spreads evenly throughout the population, as with flu (each person with flu generally passes it on to another).
During the coronavirus pandemic, the reproduction number — R for short — has been used as a key measure of whether the virus is under control. This shows how many people, on average, one person with Covid-19 infects
But if the K number is under one, this means a disease spreads in clusters.
This is significant because it provides key information about how we can keep the virus under control.
Covid-19 is estimated to have a K value of between 0.1 and 0.5 per cent, based on research by the London School of Hygiene and Tropical Medicine. This suggests that it spreads in clusters.
Indeed, scientists believe that between 10 and 20 per cent of people with Covid are responsible for passing on 80 per cent of infections — so-called super-spreaders — while most people with the virus won’t pass it on to anyone.
Dr Muge Cevik, a clinical lecturer in infectious diseases and medical virology at the University of St Andrews, explains: ‘While R shows the average of how many people are infected by one person, K shows the variation in transmission.
‘Research shows Covid-19 is a highly overdispersed pathogen, which means it tends to spread in clusters.’
As K estimates the way the virus is transmitted, it will not necessarily change over time.
However, using K rather than R to underpin our response to the virus would be a better way of tackling it, according to Dr Cevik, who recently co-authored a review on transmission conditions for Covid-19, published last month in the journal Clinical Infectious Diseases.
‘At the moment, our policy is to try to prevent every infection but, not only is that impossible, it’s not necessary, as most people who get Covid-19 won’t pass it on to others,’ she says.
It’s not about identifying super-spreaders, as that would not be feasible; rather, it is about tackling environments that encourage cluster spreading, she says.
‘We should concentrate on limiting the super-spreader environments — indoor, poorly ventilated environments and gatherings linked to the majority of infections, such as weddings and crowded restaurants and buses — and open up safer environments, such as well-ventilated indoor, or outdoor, meeting spaces.
‘People need spaces where they can safely interact with each other. K helps us come up with a far more nuanced and targeted approach.’
A K value of one or more means a disease spreads evenly throughout the population, as with flu (each person with flu generally passes it on to another). A woman is seen wearing a mask on the London Underground in February before social distancing came into effect
For a large cluster to occur, several things are needed: a highly infectious person, a crowded indoor event and prolonged contact with others, she adds.
However, the problem with Covid-19 is that people can be highly infectious but have no symptoms at all, or only mild symptoms, so may infect others unknowingly.
A person is highly infectious in the two days or so before symptoms appear, and then five days after.
‘That’s why self-isolation when you’re feeling ill is critical to prevent onward transmission,’ says Dr Cevik.
She adds that it’s important to avoid crowded, poorly ventilated indoor environments as transmission could occur even if you’re more than two metres apart. That’s because small droplets could linger longer due to the poor ventilation.
‘If you’re not able to avoid these environments, wearing a mask could help,’ she advises.
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