>> Hi, my name's Paul Offit.
I'm talking to you today from the Vaccine Education Center here at the Children's Hospital
of Philadelphia. And what I thought I'd talk about is influenza vaccine, or flu vaccine,
which is a little tricky.
It's been hard to make a vaccine that uniformly protects children for long periods of time
after just a single dose.
So why is that?
Let's go through it.
The first is that influenza virus changes so much from one year to the next that natural
infection or immunization the previous year doesn't protect you the following year.
And the reason primarily is because of one protein that sits on the surface of that virus
called the hemagglutinin, which is responsible for the virus attaching to cells and then
infecting them.
That particular protein, the hemagglutinin, mutates or changes so much so from one year
to the next that we need a yearly vaccine.
Now to make this yearly vaccine, what we do is we look at the strains that are circulating
in South America because those strains typically then come up into the United States and affect
people in the United States.
What happens, however, sometimes is that when we take those strains that are circulating
in South America and then make a vaccine, during the period of ... monthslong period that
we're making that vaccine, the virus actually mutates just traveling up from South America
into the United States.
And that happened a few years ago when we had essentially a bad match between the strains
that were in the vaccine and the strains of influenza that were circulating.
In fact, protective efficacy that year, which was a few years ago, was only about 13 percent,
which is abysmal, and that's because of that change.
The second thing that makes influenza vaccine difficult to make is usually the vaccine viruses
are made in eggs; and when you're making them in eggs, the virus reproduces itself again
and again.
Sometimes just that the mutations that occur while the virus is being manufactured can
cause the virus to drift, if you will, far enough away from the strains that are circulating
so that the protective efficacy isn't as good as it could have been.
The third possibility, and this is I think probably the hardest to understand, is
something that immunologists refer to as original antigenic sin.
So, by antigens what I mean are the protein or those proteins on the surface of the virus
that induce an immune response — that's an antigen.
When you're a baby and you're first exposed to influenza or you're first exposed to an
influenza vaccine, your memory is best against those viruses, those viruses that you were
first exposed to.
So, if later in life you're exposed to a very similarly virus but slightly different, you'll
respond as if you were infected with that original virus and not recognized that subtle
difference that comes about later.
Now, a fourth possibility, and I think people who are concerned about vaccines often raise
this, although I think this is not as important or frankly important at all, is the concern
that by immunizing people that you're creating mutant strains.
In other words, that by immunizing people those people will make antibodies, those antibodies
will then arguably change the virus to create what people would worry would be super strains,
or what immunologists would refer to as escape mutants.
That does not appear to be important and why it is that we aren't able to have a high level
of efficacy with the influenza vaccine.
Now last year for example, the protective efficacy against influenza vaccine overall
was about 43 percent.
For children between 6 months and 8 years of age, it was about 61 percent. And people
could reasonably argue that's not great, wouldn't it be better to have a vaccine that's 90 or
100 percent effective.
Yes, it would be.
But again when you consider that tens of thousands of people die every year from influenza often
anywhere between 75 to150 children, many of whom were previously healthy die every year
from influenza.
Forty-three percent or 50 percent or 60 percent protective efficacy is much better than zero
percent.
Although influenza vaccine is an imperfect tool, it's clearly the best tool we have to
prevent this disease, and therefore should be used routinely.