question re:vaccination

mdoherty mdoherty at atlas.niaid.nih.gov
Wed Jan 14 11:15:29 EST 1998


In article <e62c691b.34bc32b6 at aol.com>, HavaByte at AOL.COM (HavaByte) wrote:

> how does a vaccination work? what exactly happens when, say, you get a measles
> mumps & rubella , or hepatitis B shot in the arm? whats a vaccine made up of?

Vaccines can be made of many things!  Since I doubt this question comes
from an immunologist, I'll explain the basic principles (immunologists will
have to forgive some over-simplification).  It'll be fairly long
explanation, I'm afraid.

To take the questions a step at a time, vaccines are based on the
observation that many diseases are a once-in-a-lifetime thing.  Long before
we knew anything about how the body handles disease, people had noticed
that if you got some really obvious disease (smallpox, say) than you didn't
get it ever again.  As people started looking at this, the same was found
to be true of less obvious diseases.  Every parent today knows that
measles, mumps, chicken pox, etc are diseases that you can safely ignore
once your child has been exposed, and older parents will know the same is
true for diseases that are rare now in developed countries, such as polio,
whooping cough etc.

In fact, it's true for most diseases.  The reason you never seem to develop
immunity to some things (like the commmon cold for instance) is that there
are hundreds of different viruses that can cause cold-like symptoms.  You
might be immune to one, or even a hundred - but that still leaves plenty
more to cause winter misery.

Now we know that the body has large numbers of cells specialised to kill
off different sort of infections, and it responds to an invading infection
by causing the cells best tailored to that type of disease to multiply and
become ready to do their work.  That lets your body remove the offending
infection.  However, once the disease is over, most - but not all - of
these activated cells are gradually removed.  The ones that remain are the
basis of what is called "immunological memory" - and they mean that if you
are ever exposed to the same disease again, then your immune system doesn't
have to start off from scratch.  The second (or third, or fourth etc) time,
the response is faster and stronger - and often the disease is wiped out
before it can cause you any significant illness.  You might feel
"off-colour" for a day or two, or you may not even notice it.

However, people also noticed that you didn't have to get a specific disease
to be protected - in some cases, a similar disease would help.  The classic
example is smallpox.  Traditionally milkmaids were famous for their nice
complexions and somebody (Edward Jenner usually gets the credit) worked out
that this is because they didn't get smallpox very much - a disease which
often left the face scarred and pitted.  It turns out that they DID get
cowpox - a fairly minor disease in humans but one which causes a small sore
like the ones caused by smallpox.  Not only that, but they usually got the
cowpox sore on their hands (the part of their body that usually came most
in contact with the cow), where it wasn't so noticeable.

So people realised that having one disease could protect you from a related
one - if the relationship was close enough.  This is the basis of all
modern vaccine technology.

Nowdays we know a great deal more about how the body recognises foreign
"invaders".  Your cells can't tell the different between a disease-causing
item and a safe one.  But it can tell the difference between something
which is normal to your body and something that is not - and it responds to
something that is not normal by mounting an immune response.

So, the last stage in vaccine theory came when we realised that you didn't
have to have an *infection* to be protected from a disease.  Your body just
had to be exposed to the proteins that comprised that infection.

So today vaccines fall into three loose groups:
Live vaccines
Killed vaccines
Subunit vaccines.

The first one is a derivative of the "cowpox-smallpox" paradigm.  This
aproach gives you a fairly harmless infection to protect you from its much
more dangerous cousin.  A good example of this is vaccinia - fairly
harmless virus, which is related to the extremely dangerous smallpox virus. 
If you get one, you're immune to the other - for life.  Scientists have
also taken dangerous infections and weakened them by growing them outside
the body for a long time or genetically altering them.  These weakened
diseases are called "attenuated strains" and they induce the same disease
that the original one does - but just a much weaker version of it. 
Obviously they are closely related, so they also confer protection from the
nastier version of the disease.  The live (oral) polio vaccine is an
example of an attenuated vaccine.

Some diseases can't be safely attenuated, or are simply too dangerous to be
treated like this.  So, another option is to take the agent that causes the
disease and simply kill it, and then inject the killed material.  Killed
vaccines are pretty safe, but unlike a live vaccine - which normally grows
a little bit in the body and behaves fairly naturally - the killed vaccine
can't.  So, as a result, killed vacines tend to be less effective than live
ones (although much less likely to cause side effects).

A side note here - one point to keep in mind is that vaccines act to
enhance your body's immune response.  There's nothing special about the way
they act.  Different people react to vaccines different, much as they would
to the same disease.  So most people respond to vaccination very well, and
get full, solid immunity.  Most of the rest respond to a vaccine by getting
partial immunity - which means they can still be infected by the disease in
question, but they get a much milder case.  A few people won't respond to a
vaccine well - and are not protected.  How a vaccine performs in these
three categories defines how effective it is.  The same is true of disease
- when a cold goes through youroffice, some people get sick for a week,
some people only for a day or two, and some people not all.  This also
reflects (among other things) how effectively their body handles that
particular infection.

The third type of vaccines are based on the principle of the killed
vaccines.  Since your body normally responds to the proteins out of which
an infectious agent is made, even a dead infectious agent can offer some
protection.  So, the thinking went, why not work out what proteins are most
commonly recognised by the immune system and just inject those one?  This
would mean you could boost only the immune response you wanted, with little
or no risk of side effects.  Safe, simple, clean.  The only problem so far
has been that most subunit vaccines don't work very well - they're totally
safe, but not as effective.  Still, as more work is done on this field,
more promising candidates are coming up.  Most modern vaccines are being
developed along these lines.

One final note.  There are a few people - especially prominent on the
internet - who are anti-vaccinologists.  Basically they're ignorant fools,
but there are a few with slightly more sinister motive - one who posted to
this forum for several months using a couple of false names ran a mail
order business and his pitch was essentially "vaccines are dangerous and
don't work, buy my (expensive and untested) natural health care remedies
instead."

Lets be clear about this.  Vaccines DO work.  Where improved sanitation had
been only partially successful, the polio vaccine reduced paralytic polio
in the US from tens of thousands of cases a year to... none in the last two
years and a total of a couple of dozen in the 10 years before that. 
Smallpox vaccination totally wiped out smallpox, from literally hundreds of
thousands of cases a year to none in the last 12 years.  There are dozens
of other cases, where diseases which were rampant in the early part of this
century have largely disappeared in the developed countries - and in those
developing countries where strong vaccine programs are in effect.  Even
today, where vaccine rates fall,the disease rates rise.

Still, some vaccines CAN induce side effects - sometimes very serious - in
people.  Remember that a vaccine works by mimicking - in a reduced way - a
disease, and people vary a lot in their response to disease.  However the
first consideration for a vaccine is safety.  Most vaccines with
significant side effects induce them a thousand times or a million times
less frequently than the disease that they are designed to protect against. 
There are no vaccines with significant side effects against trivial
diseases.

Right, that hopefully should have answered your question :-)

Cheers, Mark



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