COVID-19: WHAT USE IS 'R'?

We hear about the 'R' number regularly.  But what is it, and is it any use?

The simple answer is 'R' is of no use at all.  There are two reasons:

• It does not incorporate a time element, so it gives no indication of how quickly new infections rise or fall.  The daily change % is far more useful
• When the background infection level is at a very low level, any outbreak need only be considered by its size.  R just confuses the issue.

So what is 'R'?

According to the UK Government, "The reproduction number (R) is the average number of secondary infections produced by a single infected person."  The 'R' does not stand for 'rate'. 

"But we keep hearing that R must be kept below 1."  Well yes.  Above 1 and new infections increase, below 1 new infections fall, exponentially like this:

But saying R must not be above 1 is equivalent to saying the growth rate should not be positive.  It's the growth rate that actually matters.  The Government has been disclosing this alongside R for many weeks now.  This is the latest for the UK:

If you know the daily growth rate, you dont need to know R.  In fact R has to be back-estimated from the Daily Change growth rate:

Indeed because the growth rate is published to 2 decimal places, as a percentage, it is more accurate than R which is only to one decimal place.  Hence the growth rate range is not '0% to +3%'.

If R was 200, but transmission was ever so slow, the growth rate could be the same as an R of 1.1 but happening quickly.  It is the growth rate, not the 'R' that matters.

So why are we holding on to 'R'?  Gawd knows.  It was picked up by the media in the early days of the pandemic, when the growth rate wasn't published, and has become familiar.  Though poorly understood.

Now we should quietly forget 'R' and concentrate solely on the daily growth rate.


BUT WAIT.  THERE IS ONE USE FOR 'R'

It is worth understanding that the daily change rate is the product of R and the frequency.  So, for example, if an infectious person meets with a group and infects them, as happened to a pal at his choir rehearsal, then R will be high and the frequency will also be high.  The daily change rate will then be high.

That is why the 'Rule of 6' has been established, to limit the size of groups and therefore limit the chance of infections.


BUT CONVERSELY...

"As long as we keep R below 1" has been heard regularly over the last weeks and months.  That was basically the same as saying "We're happy with the current level of infection".  Which has been far too high, and got us into the position we are now with the test system overloaded, schools shedding pupils and staff, and cases rising further.

It should have been targeting a daily change rate of a significant negative % to get to #NearZero, using whatever means were necessary.