WHY EXPERTS ARE CLOSELY WATCHING THE ‘R’ NUMBER
HE doesn’t sit on the Government’s expert committees on Covid-19 but Professor Sam McConkey has been modelling his way through the pandemic since January.
The professor, who heads the Department of International Health and Tropical Medicine at the RCSI and is one of the expert voices on the pandemic, has been keeping a close eye on the R number — that magic digit that measures the virus reproductive rate. It measures the average number of people whom one person with Covid-19 will infect and it’s an important predictor of how widespread a contagion may become. If R is above 1, an outbreak can grow exponentially. That is why the aim is to keep R below 1.
The Government, with the co-operation of the pubic, was managing that until recent weeks, when the gradual easing of restrictions led to greater movement, encouraged by the summer weather.
The National Public Health Advisory
Group (Nphet) puts the R number at between 1.4 and 1.8. McConkey says his calculations closely mirror those of Nphet — he puts the R number between 1.3 and 1.7 for the last five or six days. If the R number is 1.5, he says, then every five days the number of new cases is 1.5 what it was five days earlier. “We have 34 new cases on Friday, so that could translate to 51 new cases in five days’ time,” he says.
“That is worrying. If you project that on for 50 days, then you end up into the hundreds or thousands of cases. While the number of cases snowballs when the R number is more than one, it actually peters out when the R number is lower than one.”
The number is both a measure and a target: suppressing the reproduction number below 1 means that the social restrictions are working.