A simple description of what’s a more complicated formula is contained in a book called ‘The Rules of Contagion’ by Adam Kucharski, an epidemiologist and mathematician in the London School of Hygiene and Tropical Medicine. He says there’s four factors that go into the r0; Duration, Opportunity, Transmission and Susceptibility, or DOTS for short.
– Duration is the length of time a person who gets the virus is infectious, with Covid-19 it’s thought that could be up to 14 days.
– Opportunity is the amount of social interactions a typical infectious person has that could result in the virus passing to another person.
– Transmission is a value for how likely an interaction between an infectious and uninfected person will result in the virus infecting the second person. If every interaction resulted in a new infection this would be enormous, if half of interactions resulted in the virus passing then the probability would be 0.5.
– Susceptibility is about how much immunity – or ability to fight back against the disease – the population has, with Covid-19 we are all susceptible to getting it.
In simple terms, put the numbers for those four factors into a specific formula and you get the r0.
‘How do we bring down the r0?’ is the obvious next question. The answer: as a society, we find ways to bring down the values for the DOTS that make up the r0. As Professor Horgan, Dr Virginie Gautier, and Dr Fiachra Humphries detailed during the making of the latest Explained by Prime Time video episode, the tools for doing this aren’t complicated.
The duration of time we’re infectious we can’t change, so scratch ‘duration’. With other virus strains susceptibility could be lowered in the population through flu shots, or other programmes designed to prepare our immune systems for the disease, that’s not possible with Covid-19. That means we can’t change the value for susceptibility either, we are all considered susceptible.
So we’re left with addressing opportunity and transmission. These are values we do have control over. For diseases that more research has been done into, opportunity values can be specific to societies. Professor Horgan describes them as ‘semi-cultural’. In places where people tend to be more socially active or physically affectionate, the opportunity or transmission numbers are higher, and vice versa.
To reduce the opportunity value isn’t complicated. You make sure the virus has less chances to spread to other people, by seeing less of them. When you’re social distancing it might feel like you’re doing nothing, but according to everything the world’s best scientists know about epidemics, you’re actively lowering the r-nought. You’re actively changing the value.
The same with transmission. How do you reduce the chance during an interaction that the disease will pass to you – or from you to – to another person? Stay two metres away, cough into your elbow. Wash your hands regularly, so during the next interaction that you have the likelihood is lower that you’re carrying the virus.
As the HSE’s clinical director Colm Henry put it on Prime Time last week, these behaviours “will help to save many lives”. It might feel like you’re quite literally doing nothing, but as far the maths and science works, when you are, you’re helping to bring down the r-nought towards that key figure to stop the spread – one.
Credit: Mark Coughlan, Prime Time