They get there the first vaccines against covid 19 an effective vaccine
90% which would be an extraordinary rate 90% efficiency
after Pfizer after the Russians of gamaleya these are again
modern Americans who may have made history so much 2020 several
laboratories announce results promising for vaccines against
coc fast 10th some of them would be more than 90% effective which
is a priori good news but is this the guarantee of returning to a life
In 2021 we will be able to end the coville epidemic 19
well it’s not sure at all to illustrate it’s taking a population
of its people exposed to an epidemic of covid 19 years young these are the
infected people who can transmit the disease to others people
for the moment there is no effective treatment to treat sick
the hope of stemming the epidemic lies.
So on vaccination here in blue ideally the facts are
then double on the one hand the vaccine has a direct protective effect, i.e.
that once vaccinate people exposed to the virus rarely gets sick
and therefore there are less severe forms of disease
on the other hand, there is an indirect effect patients can no longer be infected
new people and the virus does can no longer circulate.
The question that arises then is to know how many people it takes
vaccinate so that these two effects put end to the epidemic.
To answer these scientists take into account a parameter
essential is zero the number of basic reproduction in other words it is
the total number of people each manage.
will contaminate over the entire duration of his infection in the case of sart skov 2 at the start of the epidemic and beyond any measure distancing
scientists estimated the r0 to be 2.5 that is to say that each patient will
infect two to three new ones people if this 1 0 is also important it is
because thanks to him we can calculate an essential thing the number of people
that it takes immunity to stop the epidemic here is the peace equation
of the population to be immunized must be greater than 1 – 1/06 eur 0 is equal to
2.5this percentage is then 60.
we could therefore say that in our fictitious population 200 people
only need to vaccinate 60 to stop the spread of sart skov 2 except that
it’s a little more complicated than that for that the vaccination works it is
essential to take into account many parameters.
first there to the question of efficiency vaccines, for example in the case of
vaccine effective 90 percent a in ten people vaccinated will not be
immune and she may still be contaminated in our equation
efficiency comes into play here we see while it is no longer 60 people
that must be vaccinated but 67 in our population it is therefore necessary to vaccinate 67 people to be sure that in the end 60 of them are indeed immune
and that the epidemic is contained and in if the vaccine is effective only 60%.
100% of the population should be vaccinated population to reach the same threshold
which seems ambitious but another parameter plays in our
favor as the epidemic has already started many people have already fallen
sick a large part of them seasoned and was able to develop immunity
they can therefore no longer be a priori contaminated and do not need to be
for the moment in France end 2020 we can envisage that at the scale
national about 10% of the population and in this case, it represents ten
people in our population always in the case of a vaccine
90% efficient with 1 0 of 2.5 it is then necessary to vaccinate that
56 people 50 will be immunized thanks to to the vaccine
to which are added the ten others already naturally immune we find
then an overall immunity of 60% which helps end the epidemic.
obviously, this only works if the vaccination targets people who do not
are not already immune and this implies therefore to test beforehand the whole
people to be vaccinated so far nothing impossible but there is another element still wrong known who could slow down the fight against the epidemic
some vaccines prevent you from developing disease but not to be contagious
in other words even if the people vaccinated can no longer get sick
they can continue to circulate the virus is called a vaccine
imperfect there are very few for humans but it is a possibility
take into account anyway that doesn’t mean that such a vaccine would be useless.
Contrary if to deploy them widely it would still protect the
people vaccinated by avoiding severe forms of the disease
finally, there is one last crucial aspect to be taken into account in the calculation
for how long do people vaccinated remain immune
because it has a direct impact on the roof vaccinations needed in
considering that the deployment of vaccination is spread over a year and the
hero is 2.5 researchers have summarized in this graph the number of people he
will need to be vaccinated during this.
first year we see that by using a vaccine that is 90% effective
and in the case of a long immunity of three years the proportion of the population
to be vaccinated during the first year is 60% but if the immunity only lasts a year
this proportion increases to 90% it is 90% would indeed be necessary
to compensate overtime for vaccinated people who lose their
immunity and thus maintain immunity constant 60%
the duration of the immunity and therefore a key data to establish the strategy
vaccine vaccinate 90% and 60% of the population in a year is not the
same challenge with everything we have seen one thing is
sure it will be necessary to vaccinate many people
to end the epidemic or vaccination campaigns that
takes time so who should be vaccinated first for this the strategy is twofold.
first, we can start by targeting by priority populations at risk
we know that the elderly are particularly vulnerable to covid 19 but there are other factors risk such as obesity more .
We protect them quickly the more mortality has chances of going down but there are two conditions for this the vaccine must be safe for these people
fragile and the vaccine must also works on this type of population this
which should be checked during large-scale test phases
and then we can also aim at another category of people those who are
most likely to transmit the virus.
for this, we must return to the numbers of basic reproduction 0
because it must be understood that the figure of 2.5 used so far is a
average in reality within the population the r0 is not the same for
each individual for example, a person who lives alone
backing up has very little chance of crossing and infect people are 1 0 is
so close to zero on the other hand, a baker who works
every day crosses a lot of people are r 0 can largely exceeded 2.5
it is also possible that people are more or less contagious for
biological reasons but it remains still a lot of unknowns on this point
in any case, we can vaccinate as a priority people with an air 0.
potentially high due to their activities and it would be quite efficient
since years they have this type of person scientists have calculated that the
proportion of the population to be immunized could be only 43% instead of 60
to effectively vaccinate the precise knowledge of 1 0 is therefore a
decisive ally but not only to the individual scale because it can also
change at the collective level.
For exemple in France with the application of barrier gestures and
social distancing the average r0 has not exceeded 1.5 between the two confino
with a mask and maintaining his distances a carrier person has less
likely to infect others in these circumstances if we resume our
equation with an air 0.2 1.5 and a 90% effective vaccine providing
long-lasting immunity it would suffice to vaccinate 37% of the
population to contain the epidemic which makes significantly fewer people
to be vaccinated.
But for that, it would be necessary maintain distancing measures
time that more of the population is immune to all these
parameters we understand the vaccine is not a miracle solution but in
additional health measures he will be a precious allies to put
end to covid 19 epidemic based of its effectiveness of the number of people
that we vaccinate with Kiel we vaccinate with duration of immunity and maintenance or no distancing measures on overall success of vaccination will not be
not at all the same.