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From: Ian A. York
Subject: So You Want To Make A Vaccine! (Was: Re: Vaccines? Have your child's 
	Doctor sign this form!)
Date: Jun 02 1997

I've repeatedly pointed to Medline:

<>  or
for questions about vaccines (or, for that matter, about other medical
subjects).  However, Medline isn't all that good for fundamental
information.  In case anyone's interested, here's a primer on basic
vaccinology.  This is off the top of my head, and I'm not a vaccinologist,
so if you have doubts by all means check a library.  Introductory 
immunology texts are a good starting point.

                 So You Want To Make A Vaccine!

     Model trains lost their thrill?  Bird-watching too slow?  Your clones
          all grown up and moved away?  Why not make a vaccine? 

A while ago I wrote a general introduction to the principles of vaccines;
it's at <>.  If
you aren't familiar with the general principles of immunity, you might
take a quick look at that first.

Your first decision is whether you really should make a vaccine.  Is the
disease dangerous enough to merit a vaccine?  Or is it common enough? 
(There are a probably lot of people who would like a vaccine for the
common cold, even though it isn't particularly dangerous.  If you are
dealing with a disease that isn't lethal, then you have to be even more
sure that your vaccine is safe, of course.)  

Next, is the disease likely to be preventable with a vaccine?  The common
cold, again as an example, is likely *not* to be preventable: it's caused
by any one of over a hundred different viruses, so unless you can find a
common factor in all of them, the best you can hope for is preventing one
or two percent of colds--not worth the trouble, in this case.  Another
thing to look at is natural infection:  Does a natural infection lead to
long-lasting immunity?  Colds usually *don't*, many other diseases usually
do.  If the natural infection does not, then your vaccine is unlikely to
do so; few vaccines give as potent an immune response as does the natural
infection. Potentially worse, there's a small class of disease in which
immunity may make the disease *worse*:  for example, there are
circumstances under which Dengue infection is much more dangerous if
someone has natural immunity to the virus.  The same is true for feline
infectious peritonitis virus and a handful of others.  If that's the case,
you'd better be sure you understand the disease well enough to avoid that

Okay, let's say you've identified a disease that's common and/or severe
enough to justify a vaccine, and that you already know that natural
exposure leads to long-lasting immunity.  This disease is a candidate for
a vaccine.  Your next decision is the general type of vaccine.  Most
simply, you can make a live vaccine, or you can make one of several types
of killed vaccine.  Each has advantages and disadvantages.  Let's start
with the live vaccines.

Live vaccines tend to give very potent immunity: in many cases, comparable
to the natural infection. Generally, you'd like to have multiple types of
immunity--antibodies, Th lymphocytes, cytotoxic T lymphocytes--to protect
against a disease. You're much more likely to get the full spectrum of
immunity with a live vaccine, because the agent is going to follow the
natural life-cycle and induce the immunity in the appropriate sites.  This
is generally a good thing.

On the other hand, live vaccines are, well, alive.  That means that you're
dealing with an potentially infectious organism.  How do you ensure that
this isn't going to cause problems on its own?  There have been two
approaches historically, and a couple of others used recently, more

Historically, your two main choices were to use a virus which is related
to the disease, but different (and, obviously, less pathogenic).  The
other is to use the agent that causes the disease, and make it weaker.
The first approach was used in smallpox vaccines; the second, in polio

For smallpox, the original vaccine was cowpox.  (Well, much earlier than
that, the original vaccine was a weakened smallpox virus; the technique
was used in China and other places, but was rather risky and tricky.) More
recently, the vaccine is vaccinia virus, which is closely related to
cowpox but different.  It's much less virulent than smallpox, but it's
closely enough related that it confers a potent protection against
smallpox.  Of course, since vaccinia is not smallpox virus, it cannot
cause smallpox.  On the other hand, vaccinia is, as vaccines go, rather
risky: it led to complications (mostly fairly minor) at a rate of
something like 300 per million doses, and around 1/200,000 vaccinees had
serious problems. For a threat less fearful and common than smallpox,
vaccinia would probably not have been considered acceptable. 

I wrote more about the smallpox vaccine as an exemplar of vaccines in
general a while back; it's at

There's another problem in generalizing from this vaccine.  There just
aren't that many nice pairs of closely-related agents, one of which is
dangerous and one of which is relatively innocuous.  In fact, other than
vaccinia/smallpox, I can't think of any offhand.  So this approach, while
it worked very well in eliminating smallpox, isn't a generally useful
approach, and if you want to go with a live vaccine, you probably have to
make an attenuated version of the wild agent. 

A conceptual variant of the live-vaccine approach (though it's not usually
thought of as a vaccine) can be used with organisms which are dangerous to
adults but not children.  Chicken pox, for example, is reasonably
innocuous when a child gets it, but can be quite dangerous to adults. 
Many of you will be familiar with the ritual of deliberately exposing kids
to a peer with chicken pox.  This is, essentially, using a live vaccine to
prevent adult-onset chicken pox; in this case, the 'vaccine' is wild-type
chicken pox itself.  Similarly, mononucleosis is a disease of people who
first are exposed to Epstein-Barr virus when they're adults; most people
are 'vaccinated' against mononucleosis by being infected with EBV as

I don't have room to go into all the ways of generating attenuated
vaccines. In general, the approach has been to grow the agent in
conditions where it does not need to be virulent (that is, in tissue
culture, or in a species that it would not normally infect) and to pick
out versions that grow extra-well in those conditions.  Some of those are
likely to be better adapted to these simpler conditions, therefore less
well-adapted to the more rigorous conditions of natural infection, and
therefore less virulent.  In other words, make evolution work for you. 
(Historically, this has mostly been an empirical approach: test a lot of
variants and find one that is non-virulent, stable, and safe.  More
recently, there have been attempts at deliberately designing attenuated
agents, by identifying genes essential for virulence and deleting those.)

Attenuated vaccines are good for inducing potent immunity in the right
anatomic spot.  There is a theoretical and practical concern that they
might not be attenuated enough, and that they may revert back to a
virulent form; and they need to be monitored fairly carefully to be sure
this doesn't happen.  In practice, live attenuated vaccines have been
pretty safe: for example, the complication rate from live attenuated polio
vaccine (the Sabin polio vaccines) ranges between 1 in 1 million and 1 in
2.5 million doses.  Nevertheless, this is a real concern.

With killed vaccines, you have less concern about this effect.  A killed
vaccine is, theoretically, unable to cause the disease--that's what
"killed" means.  (Actually, I'm lumping together genuine killed vaccines
(where you start with the whole organism and actually inactivate it) and
things like subunit vaccines, where you take one small component of the
organism and apply that only.  So it isn't necessarily 'killed' so much
as 'never lived'.)

If you take the simplest route--grow up your organism, and inactivate
it--you have two big advantages.  You have reduced the risk of actually
causing the disease; and at the same time you have kept intact a wide
variety of antigens, because the organism is intact.  So your chance of an
effective immune response is pretty good.  On the other hand, killed
vaccines are rarely as effective as live vaccines: for one thing, the
killed organism doesn't replicate, and so the effective dose and duration
of exposure is lower.  Moreover, you're less likely to get the same range
of immune response as with a live vaccine; in general, inactivated
vaccines can be very good at inducing an antibody response, but are less
good at inducing a cellular (T helper lymphocyte and cytotoxic lymphocyte)
response.  Of course, for many diseases, antibodies alone are pretty good
at preventing infection (though they may not be all that good at clearing
an infection that has become established).  If you're interested, I wrote
a little blurb on how antibodies work, at

There's a way to partially circumvent the problem of weaker antigenicity. 
Most vaccines, other than live vaccines, use adjuvants.  These are agents
that boost the immune response to the vaccine. In the world of research,
there are many, many adjuvants in use.  In the whacky and relatively
unregulated world of veterinary vaccines, there are quite a few.  In human
vaccines. there are very few used, and they tend to be pretty mild: the
reason being that most adjuvants have not passed safety testing for
humans.  (Most of them haven't *failed* safety testing either; they simply
haven't been submitted.)  In the US and Canada (I don't know about other
countries)  you're pretty much limited to alum and related compounds,
which are about as weak as adjuvants can get, practically speaking. 

The other major concern with this kind of killed vaccine is that you
haven't inactivated the organism fully, and (since here you are probably
starting with a virulent organism) if there are any live ones left over
then you might have a big problem.  With killed vaccines, the key is to
monitor for any live organisms left after the inactivation process.  

Another simple way that can help boost the response to an inactivated
vaccine is to give multiple doses of the vaccine.  The immune system
responds much more aggressively to an agent it sees more than once.  

There are a couple of other concerns in inactivated vaccines, but overall,
inactivated vaccines are a good way to get an immune response while
reducing the risk of infection, so long as you have to keep a close eye on
a number of factors.  An example of a killed vaccine is the inactivated
polio vaccine: this is even safer than the Sabin (oral) vaccine, but it's
also slightly less effective.  This is a fairly typical tradeoff, and the
best option depends heavily on the risk of the actual disease.  Thus, when
wild polio was a great risk in North America, the oral vaccine was
preferable.  Now, with polio essentially eliminated in North America, the
recommendations for polio vaccine are shifting toward the safer but less
protective inactivated vaccine.

You can take the inactivated vaccine approach one step further.  If you
just use a single component of the infectious agent, and you don't include
any of the DNA, then the agent can't possibly cause the disease.  This
approach is called "the subunit vaccine", for obvious reasons.  Here you
have very little concern about the residual infectivity (although for
various reasons, and depending on how you produced your subunit, you may
still need to keep an eye on this). On the other hand, as you'd expect
from my comments on inactivated vaccines, you do have more concern about
getting adequate immunity.  An example of a subunit vaccine is the
hepatitis B vaccine that now used; it's derived from a recombinant
component of Hep B expressed in yeast (so there's no Hep B actually going
into the vaccine). 

That pretty much covers the basics of traditional vaccines.  There's a lot
of research on variants of these: combining the advantages of the live
vaccine with the safety of a subunit vaccine, for example, by genetically
splicing in the subunit from a dangerous organism into a safe one, and
then using the recombinant as a live vaccine.  Or, as I mentioned above,
specifically deleting genes that contribute to pathogenesis, so you end up
with an artificially attenuated vaccine.  And many other combinations and
permutations.  But the traditional vaccine approaches are still
responsible for virtually all the human vaccines, and most of the
veterinary ones.  

This is necessarily an extremely superficial overview, and there are a lot
of qualifiers and addenda I could add, but this is already a long post
(did anyone actually read this far?) so I'll leave them all out.  If
you're interested in learning more, a lot of immunology text cover the
basics of vaccination pretty well. 

And, as I say, you can always check Medline for specific questions:
<>  or

      Ian York   (  <>
      "-but as he was a York, I am rather inclined to suppose him a
       very respectable Man." -Jane Austen, The History of England

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