Human viruses are highly adapted to growing in human cells.
These viruses reproduce thousands of times in human cells
gaining access using specific receptors that match molecules on cell surfaces.
Like athletes, they get stronger and stronger
by continually training under the same conditions.
And yet, it’s this very strength that makes it possible to weaken the virus.
This weakening is the key to making live vaccines.
How do scientists make this kind of vaccine?
By forcing the virus to grow in different kinds of cells
in the lab.
In these chick embryo cells,
most of the virus particles
that were so good at growing in humans
can’t access the chick cells.
But with sheer numbers on their side,
a few virus particles are able to unlock the chick cells,
and begin adapting to this new environment.
The virus adapts by modifying its genes.
But, there's a price to pay.
As the virus gets better and better at growing in the chick cells,
it gets worse and worse at growing in our cells.
Now, when re-introduced to the human body as a vaccine,
these modified or weakened viruses
have a hard time infecting human cells.
The virus multiplies less efficiently
giving the immune system the time it needs
to gather information about the virus
and mount an antibody response.
The vaccine provides immunity to natural infection
without making us pay the price of natural infection.