As the pandemic continues the terms “the R0” (pronounced R nought) or the “R number” are being used in briefings and reports on the spread of COVID-19. We are being told we need to get the R0 number to less than one, but what does that actually mean?
The R0 is a mathematical term used to show how infectious a disease is. It is also sometimes called the reproduction number because as an infection spreads to other people it is reproducing itself.
The R0 indicates how many people will catch an infectious disease from one person with it. If the R0 is 2 one infectious person infects two others; if it is 20 then one infectious person will infect twenty others.
In simple terms, the higher the R0 the more infectious the disease is and the more people will get it. However the R0 applies to a population who have a) not had the disease, b) have not been vaccinated against the disease and c) where the disease is not being controlled.
It is possible to lower the R0 of a disease with measures such as vaccination and disease outbreak control.
(For the mathematicians and scientists - the R0 is actually subdivided into the "basic" RO of an infection which applies to a completely vulnerable population and the "effective" RO which takes into account the effects of vaccination and herd immunity. However, for simplicity, I will continue to use just the one term, the R0, as that is the term being used in government briefings and statements.)
Working out the R0 of a disease is vital to understanding the effects it is going to have.
When the R0 is less than 1, the disease will naturally die out because one new case leads to less than one more case. Extrapolating the numbers makes it clearer - if ten cases lead to a further seven cases which in turn lead to a further four and then none, the infection is gone.
When the R0 equals 1 each case causes a new case but there won’t be an outbreak or an epidemic.
When the R0 is more than 1 the disease will spread and this can lead to an epidemic, an outbreak within one community or, as in the case of COVID-19, a pandemic, a worldwide outbreak.
To give some examples of other diseases, the R0 of measles is in the range of 12 -18 in a non-vaccinated population, a high number because it is highly infectious. Before the measles vaccine became available the world saw regular measles epidemics. In a vaccinated society the R0 has dropped steeply as the virus cannot as easily find a vulnerable host to spread to.
In the SARS pandemic in 2003 the original R0 was just below 3. The intensive isolation and quarantining measures brought it down to below 1.
Occasionally, even when the R0 number is very low, occasionally one person can transmit the infection to hundreds of other people. This is the phenomenon known as super spreading and was one of the reasons SARS spread widely in Singapore, Hong Kong and Beijing.
So, do we know the R0 for the coronavirus causing COVID-19?
Imperial College initially estimated it to be between 1.5 and 3.5, it has since revised this to 2.5.
Studies on the passengers and crew on the Diamond Princess Cruise ship (a ship with 3000 people on board quarantined with a COVID-19 outbreak) suggested a R0 of 2.28.
There is uncertainty because we don’t yet know how many people may have had mild infections that don't result in symptoms but still spread the disease. We also don’t know how much herd immunity is being generated by those who have it and recover. Are they immune after one infection or can they catch it again? We don’t yet know.
And we don’t yet know the full effect lockdowns and restrictions are having on the spread of COVID-19 although early indications suggest such measures are slowing down the disease spread.
However if we hear that the R0 is falling that can only ever be good news.
https://labblog.uofmhealth.org/rounds/how-scientists-quantify-intensity-of-an-outbreak-like-covid-19