‘Flattening the curve’, ‘community spread’ and ‘herd immunity’ are all terms that we’ve become familiar with due to the coronavirus pandemic.
You can now add ‘R’ value to the ever-growing number of buzz words associated with the virus.
Like testing, the R value is being touted as an important tool that countries can use to assess their level of readiness, as it relates to the reopening of the economy.
What is an R Value?
The R value or ‘reproductive number’ is a way of rating a disease’s ability to spread. The term, which is primarily used in epidemiology, is a model that is used to determine the average number of people that one infected person will pass the virus to.
The R value isn’t a fixed number, so it can be affected by a range of factors including location.
The R value for the coronavirus for example can vary from country to country, based on how densely populated the area is.
Given that Sars-CoV-2 – the proper name for the coronavirus– is a new pathogen scientist initially had to calculate its R0, pronounced ‘R nought’.
The R nought value assumes that a population is wholly susceptible at the beginning.
It further assumes that at the time there would be no suppression measures, and it also assumes that there are no individuals who are immune.
Early studies indicated that China may have initially had a R value of between 2 and 2.5
What is an ideal R value?
Increasingly, we’ve all heard countries like the UK and Germany make reference to an R value that is less than 1.
An R value of more than 1 indicates the possibility of an outbreak or pandemic.
A R value greater than 1 can mean a variety of things including a wave of new infections.
Given that R values are calculated daily and may often vary, the objective is to have an R value of below 1 for a sustained period of time, which could mean that the virus is on the decline and eventually die out.
An R value of 0.75 means that each sick person is transmitting the virus to less than one other person- this is a positive indication that could be used to make decisions about the reopening of the economy.
Is calculating an R value ideal for small island states like Jamaica?
While the R value can be an important piece of data that countries use to make decisions, the R value may not be useful for smaller Caribbean states, like Jamaica.
According to a local epidemiologist, the problem with calculating R values for smaller countries is the inputs needed to calculate the number.
The epidemiologist told BUZZ that a country would need a minimum of 60 new cases per day or a population of 3 to 5 million to get any sensible values.
Similarly, Chief Medical Officer for the Cayman Islands, Dr John Lee, said that because there are many different ways of calculating the R value, but with much of the math rooted in guesses, it may not be the most accurate measure.
“The R value also relates to whole populations and as you are aware, we have had a number of different clusters of cases and each of those clusters of cases will have its own reproductive number; which is different from the R value of the whole community. What people want is to know the R value for the whole community,” said Lee.
“I am saying that these clusters throw off the calculation and immediately lets you understand that when you look at a small population where there is not a lot of infection going on, it becomes a fairly meaningless figure,” added Lee.
The coronavirus has a median R value of 5.7, according to a study published online in an online medical journal.
Similarly, infectious diseases such as polio, smallpox and rubella have R nought values in the 5 to 7 range.
According to the Centers for Disease Control and Prevention polio and smallpox are among the nine deadliest viruses on earth.
Ebola which is widely considered to be exceptionally contagious, has an R value of only 1.5 to 2.