If you had been born in the U.S. in the year
nineteen-hundred, when the three leading causes
of death were all infectious diseases - pneumonia
and flu, tuberculosis and gastrointestinal
infections - your life expectancy would have
been just over forty-seven years.
In the year twenty-ten, the leading causes
of death were chronic diseases like cancer
and heart disease.
We can thank improvements in sanitation practices
and medical advances like vaccines for drastically
reducing the impact of infectious diseases
and extending our life expectancy to nearly
seventy-nine years.
Before we learn exactly what a vaccine is
and how they’re made, let’s discover how
bacteria and viruses can make us sick in the
first place.
Both bacteria and viruses are microbes - tiny
microorganisms that are too small to see with
the naked eye.
Viruses are smaller than bacteria - most of
the largest viruses are smaller than the tiniest
bacteria.
Both viruses and bacteria are spread by contact
with infected people, animals, surfaces or
food, and both can cause diseases in humans
that produce similar symptoms, like digestive
upset, fever and chills, aches and pains,
and respiratory distress.
But despite these similarities, bacteria and
viruses could not be more different.
Bacteria are incredibly hardy living organisms
- they can survive in nearly any environment
and can reproduce on their own.
Most bacteria are harmless to us, and many
are actually helpful - trillions of bacteria
live all over our bodies, and scientists have
recently discovered that our bodies might
actually be only forty-three percent “human”
- the rest of us is made up of microscopic
organisms like bacteria.
Once a disease-causing bacteria has entered
our system, it begins to reproduce and give
off toxins, and it’s these toxins that make
us sick.
Bacterial infections can be treated with antibiotics,
which either kill the bacteria outright or
limit its ability to grow.
The Bubonic Plague, which killed sixty percent
of the population in Europe in the fourteenth
century, was caused by a bacteria.
The Spanish Flu epidemic of nineteen-eighteen,
on the other hand, was caused by an aggressive
strain of the influenza virus.
A virus needs a host to survive and reproduce.
Viruses make us ill by inserting their genetic
material into our own cells, causing those
cells to burst and die or to mutate into malignant
cells.
It’s nearly impossible to kill a virus once
it finds a host, we can only treat the symptoms.
The best thing we can do is try to prevent
viral infections in the first place, and that’s
where vaccines come in.
Before we can understand how vaccines work,
we need to know a little bit more about our
immune system.
Our innate immune system’s job is to monitor
the body for pathogens and act as the first
line of defense against infections.
For the record, a pathogen is anything that
can cause disease, like bacteria or viruses.
An antigen is any substance capable of stimulating
an immune response, which might be only a
part of a virus.
Many organs and tissues are involved in our
immune system.
Our bone marrow is where immune cells are
born and constantly replenished.
All immune cells start out as stem cells before
developing into specific immune cells.
Our lymph nodes are located strategically
throughout our bodies, usually near openings
where pathogens tend to enter, like our mouth
or genital areas.
Lymph nodes store immune cells to be deployed
during an infection - the swollen nodes you
feel when you’re sick are the result of
an increase in the number of cells in the
tissue as your body ramps up its efforts to
fight off intruders.
Our spleen filters pathogens from our blood
as it circulates through our body, and our
muscles contract to move air or liquids that
contain pathogens out of our body by means
of vomiting, diarrhea, coughing or sneezing.
Our skin acts as an important physical barrier
against pathogens.
The top layer of skin cells, called epithelial
cells, line the openings of our body, and
are coated with mucus, a thick, sticky solution
that makes it hard for pathogens to attach
to the cells.
The mucus coating also contains chemicals
that help your skin maintain a slightly acidic
pH, making it harder for viruses to survive.
Some cells also have microfibres called cilia
which help move mucus and pathogens along
the cell’s surface - our nose hairs work
similarly, filtering dust and other pathogens
as we breathe.
When a pathogen manages to sneak past our
innate immune system, our adaptive immune
system kicks in.
The adaptive immune system reacts in specific
ways to specific pathogens, whereas the innate
immune system responds the same way to all
threats.
Antigen-presenting cells, or APCs, are produced
in our bone marrow and monitor our bloodstreams
for pathogens.
When APCs encounter an unwanted intruder,
they phagocytose it, breaking it into pieces
and placing them on their surface before heading
to the nearest lymph node to present the antigen
signal and kickstart the full immune response.
The signals from the APCs activate T cells,
which oversee cytokine signalling.
Cytokines are small proteins that fit perfectly
with receptors on specific cells and give
that cell instructions to help regulate the
immune response.
They may tell cells to grow, change or reproduce,
or instruct them to kill other cells infected
by the virus.
B cells are a type of immune cell that multiplies
rapidly and secretes antibodies, or immunoglobulins
(Ig) which are proteins that target and kill
invading microbes.
Once the infection has been dealt with, all
that remains of your adaptive immune cells
are a small number of T and B memory cells.
After the primary immune response to the first
exposure to the illness - or to a vaccine
- these lingering memory cells will make your
secondary response to the same pathogen stronger
and more effective, meaning future infections
will be less severe, shorter, and may not
present any symptoms at all.
Now that we understand how our body fights
off pathogens on its own, let’s learn about
how vaccines leverage our immune response
to combat infections.
So, what exactly is a vaccine?
A vaccine is a compound that contains a killed
or weakened version of a pathogen that is
known to cause diseases in humans.
The vaccine won’t make you sick, but it
will stimulate an immune response.
Unlike medicines that treat the disease, vaccines
work by preventing the infection in the first
place.
The immune response triggered by the vaccine
produces the same memory cells that a true
infection would, which teaches your body how
to effectively deal with this pathogen in
the future, without the risks of having to
get the disease first.
Edward Jenner is credited with the first successful
use of a modern vaccine in 1796, when he used
cowpox material to inoculate a thirteen year
old boy from smallpox.
Louis Pasteur is another vaccine superstar
credited with creating the first successful
rabies vaccine in eighteen eighty-five.
He’s also famous for developing the process
of pasteurization, which kills disease-causing
microorganisms in food and beverages by heating
them to a certain temperature.
But, the history of vaccines actually goes
much farther back - evidence suggests that
the Chinese employed smallpox inoculation
as early as the year one-thousand!
Technically this was variolation, not true
inoculation, since their technique involved
smearing open tears in a patient’s skin
with true cowpox, rather than a weakened version
designed for vaccination.
By the 1930s, vaccines and antitoxins were
available for diphtheria, tetanus, anthrax,
cholera, plague, typhoid, tuberculosis, and
more.
By the later half of the twentieth century,
thanks to the success of vaccines and vaccination
programs, many previously deadly diseases
had been all but eradicated in much of the
world.
Although the way vaccines work may seem simple
in theory, creating a vaccine is actually
quite complicated.
First of all, the costs are astronomical - it
can cost upwards of one billion dollars and
take more than ten years to develop an effective
new vaccine that we can be reasonably sure
won’t cause the disease.
On top of that, viruses are constantly mutating,
and we’re constantly encountering new strains
of viruses like the flu - it’s almost impossible
to predict which strain will hit in a given
year, and by the time an outbreak hits there’s
usually not enough time to create a vaccine.
For viruses that are more stable over time,
there are many different methods for creating
a vaccine.
The method used depends on the pathogen being
targeted, whether it’s a virus or a bacteria,
and the way it causes disease in our bodies.
Live attenuated vaccines are created by weakening
the virus so that it reproduces more slowly
in our bodies, giving our immune system time
to learn how to respond to the intruder without
letting disease run rampant.
Normally viruses replicate thousands of times,
but attenuated viruses will only reproduce
about twenty times - enough to stimulate the
creation of memory cells without making you
sick.
The Measles-Mumps-Rubella, chicken pox and
some flu vaccines are all live attenuated
vaccines.
Live attenuated vaccines are made by passing
the virus through a series of cell cultures
or animal embryos up to two-hundred times.
With each pass, the virus gets better and
better at replicating in the culture or embryo,
and worse at replicating in humans.
The virus will still be recognizable to your
immune system, but won’t be able to replicate
well enough in your body to cause disease.
The advantage of live attenuated viruses is
that one or two doses is usually sufficient
to provide lifelong immunity, however, these
vaccines can’t be given to people with weak
immune systems.
There’s also a small risk that the virus
will mutate and revert back to its original
ability to replicate in humans.
For example, the oral polio vaccine has a
high risk of mutating and causing paralytic
polio, which is why the oral vaccine is no
longer used in the U.S.
Inactivated vaccines are made by killing or
deactivating the virus completely so that
it can’t reproduce at all, which means it
can’t cause disease.
The polio, rabies and Hepatitis A vaccines
are examples of inactivated vaccines.
The pathogen is inactivated using heat or
chemicals like formaldehyde, which destroys
the virus’s ability to replicate but leaves
it recognizable to your immune system.
Inactivated vaccines are advantageous because
the inactivated virus cannot cause even a
mild form of the disease, and these vaccines
can be given to people with compromised immune
systems.
However, it usually takes several doses to
achieve immunity, so additional booster shots
will be needed.
A Subunit or Conjugate vaccine involves removing
one part of the virus and using it to provide
immunity.
These types of vaccines can be created in
a few different ways.
One method involves isolating a specific protein
in the virus and presenting it on its own
as antigen.
These virus-like particles, or VLPs, contain
no genetic material and so can’t cause illness,
but still prompt an immune response which
triggers memory cells and provides future
protection from the virus.
Another method uses genetic engineering to
insert the genetic code from a disease-causing
virus into another harmless virus to create
what’s called a recombinant vaccine - your
immune system will recognize the genetic coding
of the virus and memory cells will provide
future immunity.
Both types of vaccines can be used on people
with weakened immune systems, and one or two
doses usually provides lifetime immunity.
The Hepatitis B and Human Papilloma Virus
- HPV - vaccines were both created through
genetic engineering.
Some diseases are caused by bacteria rather
than viruses.
Toxoid vaccines can be created to provide
immunity against bacterial infections that
are caused by the toxins released from the
bacteria.
Toxoid vaccines are created in a similar fashion
to inactivated viral vaccines - the toxin
is treated with heat or chemicals to create
a toxoid, which is similar enough to produce
an immune response, but cannot cause disease.
For example, tetanus infections are caused
by the Clostridium tetani bacteria, but the
symptoms are actually caused by the neurotoxin
Tetanospasmin, which is released as the bacteria
reproduces.
The Tetanus vaccine is a toxoid vaccine that
teaches your immune system how to respond
to the neurotoxin, rather than to the bacteria
itself.
Passive immunity is created when you inject
a person with antibodies which immediately
destroy pathogens.
While you don’t have to wait for your body
to generate its own antibodies, like with
most vaccines, the protection is usually only
temporary.
One of the most important parts of any vaccination
strategy is the concept of herd immunity.
When a significant portion of a population
is vaccinated, it makes it difficult for a
virus to travel from person to person, limiting
its spread.
Herd immunity is essential for protecting
vulnerable people who can’t be vaccinated,
like very young children, older individuals
and people with compromised immune systems.
These groups also tend to be most at risk
for complications from viral infections, so
they are depending on herd immunity to protect
them.
Thanks to vaccines and the concept of herd
immunity, smallpox has been completely eradicated,
we’ve almost eliminated polio, many other
diseases are held in check, and we’re well
on our way to creating vaccines for countless
other viruses.
Vaccines are truly one of the most important
medical advances in history.
If you enjoyed learning about how vaccines
are made, be sure and check out our other
videos, like this one called “What Made
the Black Death So Deadly”, or perhaps you’ll
like this other video.
As always, thanks for watching, and don’t
forget to like, share and subscribe!
See you next time!
