00:01
Now, the attack rates are
the highest in the young,
but the mortality is highest
in older individuals.
00:14
So what then are other risk factors for
the excess mortality and morbidity?
Well, people with cardiovascular
and pulmonary conditions,
if they get influenza, you can imagine
why they would have a problem.
00:33
Patients with chronic metabolic
disease like diabetics,
patients with renal dysfunction
who already have problems
with, for example, pulmonary edema,
patients who are
immunosuppressed.
00:50
They can be infected by
common and unusual organisms.
00:55
Pregnant women are particularly prone
if they get influenza during pregnancy
to having problems with
influenza pneumonia,
not just the respiratory tree,
but actually the lung parenchyma
in the second and
third trimesters.
01:13
And I’m not sure we
clearly understand why,
but it may be because the diaphragm
is being elevated by the growing baby
and that may compress the lungs and
produce some normal atelectasis.
01:29
And then obese persons are at risk for
severe complications of influenza.
01:37
Now, epidemic disease would be some
kind of outbreak in one location,
a city, town, or a country.
01:46
And we usually find that it’s the
single strain doing all this damage
and it starts abruptly.
01:54
In temperate climates we
primarily see it in the winter.
01:58
And these mini
influenza epidemics,
they peak in two to three weeks and
they last about five to six weeks
where doctors are diagnosing
the flu right and left.
02:10
Now you would think that someone
who has not been vaccinated
against the current strain
would be very susceptible.
02:17
Well, they are,
but it only affects about 10%
to 20% of unvaccinated persons.
02:25
Now, the vaccine efficacy
ranges between 50% and 90%.
02:32
It’s not as effective
for example
in patients who have immunodeficiency
or in elderly individuals.
02:42
So let’s try to explain a little bit
about these epidemics and pandemics
that occur with influenza.
02:50
There are several
outbreaks every few years
that we don’t seem to
have much immunity to.
02:57
And that’s usually the
result of antigenic drift.
03:02
And what we’re talking
about are these spikes.
03:05
Remember, the virus is
helpless without these spikes,
so it would be minor changes
in the hemagglutinin spikes
or minor changes in the
neuraminidase spikes.
03:19
And if we have good antibodies to both,
we’re going to be pretty
much immune to influenza.
03:24
But if they change, either one,
we can be susceptible.
03:30
We might not have a severe flu,
but we may be susceptible.
03:36
So what’s happening with these spikes
is they get new amino acids in them,
new antigenic sites.
03:48
Now, we can get a very
serious change in influenza
with something called
antigenic shift.
03:55
This would be some kind of radical change
in the hemagglutinin spikes,
neuraminidase spikes, or both.
04:04
And when there’s a radical change that
we haven’t seen in years, for example,
then think of the world.
04:11
Most of the world hasn’t
seen a virus like this,
there are very few people, for example,
that are still alive
from the 1918 epidemic.
04:23
So if the population has
little or no prior immunity,
then that virus is going to
wreak havoc on the population
and there are going to be
much higher attack rates
and it’s going to begin abruptly,
maybe at a time of the year
when you don’t expect it.
04:40
So, once again, antigenic
shift is the emergence
of another antigenically
variant influenza virus
which results in
worldwide disease.
04:51
And of interest, it
may involve waterfowl
because, believe it or not,
waterfowl can get influenza.
04:59
They don’t get human influenza, but
there’s a distinct type of bird flu.
05:05
It may go to other animals,
and then to humans.
05:09
So first you see, for example,
human influenza strain and
a bird influenza strain.
05:17
Now, they are similar viruses
but the bird flu wouldn’t
necessarily infect a human.
05:26
However, if there’s an intermediate host,
such as swine,
these viruses mix and the
genes from these viruses mix
and out of the swine comes
a new human strain,
and it may be one
that no one has seen
and that’s why it spreads
to the general population.
05:54
Alternatively,
the human exposed to bird flu
may mix with their own influenza strain
and come up with a new virus strain
and that can spread to
the general population.
06:14
So antigenic drift is a minor
change in one of these spikes,
an antigenic shift is some major
change that no one has seen in years.