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

normal.

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

vaccinated.

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.

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