In this session, we're going to
talk about the common numbers
that you will have seen to
describe and predict the behavior
of the COVID-19 pandemic caused
by the SARS-Coronavirus II.
The most familiar number probably
is the basic reproduction number,
known as the “R naught.”
This is the average number of secondary
infections produced by an index case
or a typical first case of
an infection in a population,
where everyone is susceptible.
It is used to measure the
transmission potential of the disease,
in this case of COVID-19.
If R naught, is greater than 1,
then the number of infected people
will likely increase exponentially
and an epidemic or pandemic can ensue.
If the R naught is less than one, then
the number of those secondary cases
from the index case, will not be
sufficient to sustain an outbreak
and it will likely recede on its own.
The problem of course as
with any of these indices,
is that the R naught, is not sufficiently valid,
to definitively forecast an outbreak.
It certainly can be affected by other factors,
but it is very important as an
early warning system, sort of,
giving the indication of the possibility,
of an epidemic or a pandemic.
The R naught then can be used
even to compare and contrast,
different types of epidemics or pandemics,
even such as we see here, comparing
the current COVID-19 pandemic,
with an annual influenza epidemic.
So, on the left COVID-19 pandemic, R
naught values have been from 2 to 2.5,
certainly, in the early days of the pandemic,
compared to a typical year with an influenza,
where the on R naught has ranged
anywhere from 0.9 up to 2.1.
Those years where the R
naught was 0.9 for influenza,
were largely fizzles, that there were not
significant numbers of secondary cases
and the epidemic was not extreme.
Versus those years of influenza
where the R naught was 2.1,
where there was a rip-roaring
epidemic with tons of secondary cases.
So, the R naught, can vary widely
and in fact, those ranges you see
both for COVID-19 and for influenza,
the R naught varies widely because
there are other host and viral factors,
which can affect it.
Certainly, if you think about it are not
can be affected by interventions, right,
so, as we have seen with COVID-19
and as we hope to see every year with influenza
intervening with public
health interventions such as,
wearing masks, social distancing,
physical distancing, closing the schools,
avoiding social gatherings, all
those can have a major impact,
on the exposures for the virus,
both for SARS-Coronavirus II
and for influenza and therefore
on the numbers of secondary cases.
After those public health measures are initiated,
then we can look and see what
happens and that number is called,
the “Effective reproductive number.”
As you would hope this is typically
much lower than the R naught,
meaning that we've had an
effective cluster of interventions.
So, the effective reproductive
number, if it's greater than 1,
means that we were not very
good at our interventions
and that the outbreak is likely going to
overwhelm further healthcare resources,
to overwhelm the healthcare system.
In if for example you can see this playing
out in lifetime in February of 2021,
the R naught for COVID-19 in the United
Kingdom was 0.6 to 0.9, so less than 1,
meaning that there was going to be a successful
or predicted successful diminishing of
the outbreak numbers in the United Kingdom
and that in fact was what
was seen after a very strict
lockdown measures were implemented.
So again, diminishing spread with the R naught.
Unfortunately, things change and variance
and in this case, we're talking
about virus factors now.
Variants of the SARS-Coronavirus II,
have shown increasing,
differing levels of infectivity
and are of major concern.
So, despite the United Kingdom
having an initial response,
it's both R naught and its effective
reproductive numbers below 1,
unfortunately, the delta variant the B.1.617.2,
first reported in India in late 2020,
entered into the United Kingdom and
changed its epidemic picture completely.
The R naught and the effective
reproductive number both rose,
to be above 1.
This is played out in other
parts of the world as well.
So, here's where viral
factors can negatively affect,
the R naught and the effective reproductive number
and how does that happen?
Well, it likely has to do
with variants or mutations,
which occur in the SARS-Coronavirus II itself.
This virus itself has shown itself
quite able to undergo mutations,
especially in the spike protein,
which will increase its infectivity
or its transmissibility.
Of course, it's very difficult to know
exactly what effect a single mutation
or a cluster of mutations might have,
in the transmissibility of the virus.
But, by using the R naught and
the effective reproductive number,
then we can at least follow
the small changes and predict,
that there's going to be a change
in the behavior of the pandemic.