00:01
Now, let's talk about Herd Immunity.
00:04
Herd Immunity is the idea
where if enough people are infected
or rather immune in a population,
they're able to protect the people
who are not yet immune
because the disease
isn't able to access those people
who are not yet immune
for herd that protect them.
00:23
In other words,
when it's a significant proportion
of the population is immune,
then we have herd immunity.
00:33
We tend to get herd immunity
from vaccination.
00:37
But it's possible to have a
component of herd immunity
being offered by natural immunity
recovering from a disease.
00:46
The herd immunity threshold
is the percentage of the population
that needs to be immune,
in order for there to be
herd immunity.
00:55
And to compute that number,
we need the base
reproduction number,
the R nought.
01:01
That's given by this
formula here.
01:03
The herd immunity threshold is
R0 minus 1 divided by R0.
01:07
Or if you rearrange the numbers,
you also get 1 over R0.
01:13
So, in the case of COVID-19,
given what we observed
earlier in China,
in terms of what we think
the R0 is,
we think 50 to 60% of the population
needs to be immune
before the entire population
is immune.
01:29
But that's probably
an underestimate,
as more data comes in,
that number is going higher.
01:35
So again, all of these estimates
are based on
the quality of the
existing information,
which is always
subject to change.
01:47
So, you can have a lower threshold
of herd immunity,
if we have lesser contact rates.
01:55
Let's say you can only vaccinate
40 or 50% of the population,
you won't reach herd immunity,
but you can artificially reach it,
if you intervene with
non-pharmaceutical interventions
in a population.
02:10
Mask wearing,
or social distancing,
or preventing people
from gathering more so
then you artificially lower
the reproduction number,
and therefore,
you artificially lower
the herd immunity threshold.
02:25
And you do that until you're able to
immunize more people
and then you can lift
these restrictions
because you will achieve a higher
herd immunity threshold.
02:35
Here are some examples
of herd immunity thresholds
for well known diseases.
02:39
So, measles, which is transmitted
mostly by airborne pathway
has a very high R0,
extremely high.
02:48
12 to 18 people can be infected
by an existing infected individual
very quickly.
02:54
Therefore, we need a high
herd immunity threshold.
02:57
Up to 95% of people need to be
inoculated against measles
for there to be herd effects
observed at the population level.
03:06
That's why for measles,
it doesn't take a lot
for there to be
a crisis in a population.
03:13
So, with the
anti-vaccination movement,
gaining ground
in some countries,
and some children not getting their
obligatory measles vaccinations,
it doesn't take a large number of
such individuals to cause outbreaks
to actually happen
in the population.
03:31
We cannot tolerate
more than 5% of people
not getting inoculated
by measles.
03:37
Smallpox, we thought
had an R0 of between 5 to 7.
03:42
Therefore, we needed a pretty high
herd immunity threshold
about 86% of the high end.
03:48
We achieve that number through
vigorous global inoculation.
03:54
And that enabled us to eradicate
smallpox from the face of the earth.
03:58
That was due entirely
to inoculation,
reaching the computed
herd immunity levels.
04:07
Ebola need possibly up to 60%
of the population to be immune.
04:12
Influenza.
Every year in many countries,
there is a problem with the
influenza flu vaccine uptake.
04:23
And in countries like
Canada and the USA,
we're lucky if we get 40% of the
population accepting the vaccine.
04:31
It's lucky then that we need
about that same level
to get herd effects each year
when the flu does its global dance.
04:42
SARS was highly infectious.
04:46
An R0 of somewhere between
2 and 5.
04:48
Therefore, a very high herd
immunity threshold would be required
if that disease had not gone away
on its own.
04:54
For COVID-19.
04:56
We think given
what we've observed
about the transmissability
of a disease to this point,
we'll need above the 70%
of a given population
to be inoculated before
we achieve herd immunity.
05:13
If a mutation occurs
and the R0 goes up,
then the herd immunity threshold
will also go up.
05:20
So, the target for vaccination
will increase.