Anything has an advantage and a disadvantage,
and vaccines are no exception to that.
So what are the advantages and
disadvantages of live attenuated vaccines?
Well the advantages are that they mimic the natural
infection, and thus provide an appropriate immune response.
They stimulate the right type of immune response
for protection against that particular pathogen.
They can stimulate both humoral, in
other words the antibody mediated,
and the cell mediated responses
involving cytotoxic T-lymphocytes.
And they typically generate long term immunity
with a reduced need for booster immunizations.
I mean the Holy Grail if you like, of
vaccination is one shot lifelong protection.
And the less number of boosts you give--
need to give, the better obviously.
What about the disadvantages?
Well there is a slight potential
to revert back to virulent forms.
They’re still alive, they’ve been altered so they
are not nearly as pathogenic as the normal pathogen.
But they could undergo mutations that
would make them unacceptably pathogenic.
They often require refrigeration.
They have the potential to spread
from one vaccinee to another person.
So somebody being given this vaccine could then
pass this live organism on to somebody else.
And they are contraindicated in the immunocompromised
due to the risk of significant pathology.
In immunologically intact individuals,
these live organisms are dealt
with perfectly adequately, they
don’t cause significant pathology.
But it an individual with a compromised immune system,
for example somebody with an immunodeficiency,
or somebody being treated with immunosuppressive drugs,
perhaps because they have received a transplant.
Then these organisms can actually
cause significant pathology.
An important aspect of live attenuated vaccines--
is that they replicate at the site of infection.
So here we can see a live vaccine that
is replicating at the mucosal surface.
And this provides a really
appropriate type of immunity.
So a strong IgA response, exactly
where the protection is needed, at
the point that the pathogenic
organism would be first encountered.
Examples of live attenuated
vaccines include the intranasal form
of the influenza vaccine, the MMR
the oral polio vaccine, rotavirus, varicella zoster virus that
protects against chickenpox and
shingles and the yellow fever vaccine.
These are all vaccines
There are also examples of live
attenuated vaccines against bacterial
diseases - the oral live typhoid vaccine and the BCG vaccine.
What about inactivated vaccines?
What are the advantages and
disadvantages of those?
Well they are relatively
easy to manufacture.
There’s no possibility of reversion to virulent
pathogens because they’re inactivated, they’re dead.
And therefore they’re also safe
to use in the immunocompromised.
The disadvantages are that
adjuvants are usually required.
Typically they require an initial two to three
immunizations and then relatively frequent boosts.
And the immunity can be fairly short-lived
with predominantly humoral, in other
words antibody mediated responses, and
rather poor cell-mediated immunity.
Examples of inactivated vaccines are
the Hepatitis A virus vaccine, the
influenza vaccine (that’s the inactivated
version rather than the intranasal
version which is live that we just
mentioned), Japanese encephalitis virus,
the inactivated polio virus, rabies
and tick borne encephalitis virus.