during a viral infection the virus
attaches itself to a human cell using
its spikes
following entry to the cell it uses the
host cell's machinery to replicate
producing viral proteins and genetic
material
these are then assembled into new viral
particles which are released as the host
cell dies
new viral particles again infect more
cells destroying body tissues producing
symptoms
infected cells alert the immune system
by displaying pieces of viral proteins
on their surface
by doing so
infected cells are said to present the
viral antigen to certain immune cells
namely cytotoxic t cells and thus
activate them
at the same time debris of dead cells
and viral particles themselves are
picked up by so-called professional
antigen presenting cells of which
dendritic cells are most effective
dendritic cells patrol body tissues
continuously sampling their environment
for intruders
after capturing the antigen dendritic
cells quickly leave the tissue for the
nearest lymph node where they present
the antigen to another group of immune
cells known as helper t cells
viral particles also activate b cells in
lymph nodes
these groups of immune cells work
together to mount two types of immunity
specific to the viral antigen
cell mediated immunity and antibody
mediated immunity the process however
may take up to a couple of weeks during
which time the person is sick and the
virus will have destroyed a vast number
of cells
vaccines deliver viral antigens to
trigger immune responses without causing
the disease
the events of a vaccine-induced immune
response are similar to that induced by
a natural infection although some types
of vaccines may induce only
antibody-mediated immunity and may
therefore be less effective
the immune process can sometimes produce
symptoms similar to a mild infection
even though there is none
as the lymph nodes near the injection
sites start producing antibodies
they may become swollen and tender for a
few days
these are signs that the vaccine is
working
many existing vaccines contain a
weakened or inactivated virus
because the whole virus is used
these vaccines require extensive safety
testing
attenuated vaccines may still cause
disease in people with compromised
immune systems
inactivated vaccines only induce
antibody-mediated immunity
subunit vaccines contain only part of
the virus usually a spike protein
these vaccines cannot cause disease but
they may not be seen as a threat to the
immune system and therefore
may not elicit the desired immune
response
for this reason
certain substances called adjuvants are
usually added to stimulate the antigen
presenting cells to pick up the vaccine
a subunit vaccine may also consist of
empty virus shells without genetic
material
having a typical size and shape of a
pathogen
these vaccines may not require adjuvants
to be perceived as danger but they can
be difficult to produce
nucleic acid vaccines contain genetic
information for making the viral antigen
instead of the antigen itself
dna vaccines introduce viral dna into
the cell's nucleus where it is
transcribed into mrna
the mrna is translated into viral spike
protein in the cytoplasm
the protein is then displayed on the
cell's surface just like with other
types of vaccines
naked dna vaccines require a special
delivery method to reach the cell's
nucleus
alternatively a harmless unrelated virus
may be used as a vehicle to deliver the
dna
in this case the vaccine is also known
as a viral vector vaccine
for example the oxford astrazeneca
covid19 vaccine uses a chimpanzee
adenovirus as a vector
the adenoviral genome is modified to
remove viral genes and the coronavirus
spike gene is added
this way the viral vector cannot
replicate or cause disease but it acts
as a vehicle to deliver the dna
the reason a non-human adenovirus is
used is because most people may have
been exposed to human adenovirus and
have immunity against it
the immunity could destroy the vehicle
before it can deliver the dna and thus
blunt the vaccine's effectiveness
dna vaccines have raised concerns about
the possibility of viral dna integration
into human genome
however studies in animal models have
shown that integration frequency is well
below the frequency of natural
spontaneous gene mutations
mrna vaccines introduce mrna that
contains information for making the
viral protein
mrna molecules are delivered within a
lipid covering that will fuse with the
cell membrane
once inside the cytoplasm the mrna is
translated into viral antigen which is
then displayed on the cell surface
unlike dna vaccines mrna vaccines are
extremely unlikely to integrate into
human genome but they are fragile and
require uninterrupted cold chain for
transportation and storage