by
Les May
WHILST talking on
Sky News about the rallies being held in London today the
presenter managed to convey the impression, inadvertently I hope,
that we should vary our behaviour in accordance with the ‘r’
number, the average number of people that a person infected with the
virus causing Covid 19 will themselves go on to infect.
This is a classic case of
‘putting the cart before the horse’ because it is OUR
behaviour which will influence the ‘r’ value. It
is what WE do which will determine whether the number of infections
will continue to fall or grows exponentially. Exponential growth
will follow even if the ‘r’ number only just creeps
above 1.0.
For example if the ‘r’
number is only very slightly higher at 1.01 over a period of two
months 1,000 infected people will result in more than eleven thousand
new infections, but if it is very slightly lower at 0.99 the number
of new infections each week will decline. If you have difficulty in
appreciating how small is the difference between these two numbers
think of having 99p in your pocket and having £1.01p. Smaller ‘r’
values will result in fewer new infections and a more rapid decline
in the numbers. In the north-west of England we are balanced on such
a knife edge because the ‘r’ value is estimated to
be about 1.01.
Based on an analysis of about
20,000 people in 9,000 households it is estimated that in the two
week period 17-30th May, one in one thousand people (0.1%)
in the non-hospitalised population were infected with the virus and
potentially able to infect others. In the previous fourteen day
period 3-16th May the estimate was 0.25% of the
population. We can interpret these figures to mean that if we meet
one thousand people we can expect at least one of them to be
infectious. But there is a ‘gotcha’ in viewing it like
this. We do not know if the infected person will be the first,
second…. person we meet, or if we are that one infected
person.
Keeping the ‘r’
number below 1.0, and preferably well below this figure, is a job for
us. It cannot be palmed off onto Boris Johnson,
Dominic Cummings, Matt Hancock or anyone else, as
someone who was billed as the shadow minister for health tried to
imply in an interview with Sky News.
So what can we do to get and
keep the reinfection rate below 1.0? Quite a lot if few are willing
to make the effort. For the moment ‘making the effort’
means
not just sanitising hands and
surfaces regularly, but also ensuring that we meet up with as few
people as possible. That means anyone who we do not share a house
with. If we are forced to come into contact with people we don’t
live with then we can physically distance ourselves from them so that
any spittle that comes from the mouth as they talk will not land on
us and we can avoid eating, drinking or sharing utensils with them.
Just in case we are the ‘one in a thousand’ who is
infected and shedding virus particles we can wear a face covering.
Even a home made mask will be effective in preventing your spittle
reaching anyone nearby. To steal a phrase I first heard used by the
biologist Jared Diamond, we need to behave with ‘constructive
paranoia’ in mind.
https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-to-make-cloth-face-covering.html
Step by step instructions for
making a cloth face mask can be found here:
The survey results for 17-30th
May, and earlier, can be found at:
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