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from when are we effectively protected against Covid-19? And at what point?

From when are you protected against the SARS-CoV-2 virus? This is a question which after one month of the launch of the vaccination campaign in Belgium deserves some clarification. A question that arose in particular after the discovery of 18 positive cases out of 90 residents in a home where the first dose had been administered 10 days earlier. A discovery that is not abnormal: we explain.

►►► Read also : At least 18 out of 90 residents of a home in Visé positive for Covid shortly after receiving the first dose of vaccine: nothing abnormal

How vaccines work

In Belgium, two vaccines are currently in use. The first to be authorized was that of Pfizer / BioNtech, then came that of the firm Moderna. Last Friday, the European Medicines Agency announced its recommendations for AstraZeneca’s vaccine.

The first two are messenger RNA vaccines. That of the AstraZenca laboratory, which has not yet been administered in Belgium, is a viral vector vaccine.

►►► Read also : The Moderna vaccine and the Pfizer vaccine: what are the differences?

AstraZeneca vaccine candidate: should we focus on more effective vaccines for people over 65?

Remember that the principle of vaccines is to administer a small modified part of a microbe (virus or bacteria), incapable on its own of causing disease to expose it to the cells of our immune system. We cells immune will activate in the face of potential danger that they believe to detect, multiply and produce items (as antibodies) which will allow to fight against this foreign organization if we actually meet him afterwards.

Optimal protection one to two weeks after the second dose

It all takes a while. “It takes a minimum of 7 to 10 days for you to start seeing these antibodies. Then it will amplify. And especially when, after 3 to 4 weeks, we will administer the second dose of the same vaccine, we will give our immune system a boost. And there, we will produce a large amount of antibodies“, explains Sophie Lucas, immunologist and president of the Duve institute at UCLouvain.

In summary, after the first dose of the vaccine (not all vaccines require two injections : editor’s note) a person is already partially protected after 14 days, but this protection is not yet optimal.

As for the protection obtained after the second dose, for the Pfizer vaccine, for example, it will be optimal after two weeks. This protection is therefore gradually being put in place, specifies the specialist. Note that the protection obtained is never total. In the case of the two messenger RNA vaccines, the efficacy was measured and observed to exceed 90% in the case of the phase 3 studies. “Efficiency of more than 90% is excellent, it is among the best vaccines that we know“.

let’s remember that ce measures the effectiveness of a vaccine, so it is not the number of people who develop the disease, but it is “risk reduction” develop the disease with a dose of vaccine compared to a population that has not received it.

Read also: Pfizer: What does that mean, a “90% effective” vaccine? How do you calculate this number?


Beyond what we have learned from the various phase 3 studies of vaccines, observations continue to verify the effectiveness and side effects when administered under real conditions and to a larger part of the population. This is particularly the case in Israel, where millions of people have been vaccinated.

The results of these scientific studies have yet to be published and peer-reviewed, but health officials have started reporting data on this: “which seem extremely encouraging“, Sophie Lucas tells us.

But she also remains very cautious: “It is not yet published […] We would observe rates of protection by the Pfizer vaccine equivalent to those measured in phase 3 clinical studies, but this time in a “real” situation on a very large number of subjects.“.

How long are we protected?

It all goes very quickly. The virus only appeared about a year ago, and vaccines more recently.

But for now and with the hindsight we have: “on the basis of the estimates that have been made […] we think this protection will last several months […], around 8 to 12 months minimum […] and probably more than that“, explains Sophie Lucas.

But to confirm all this it will take time: the time that must necessarily elapse beyond the 12 months before we can measure and to say with certainty that we are protected beyond this minimum“.

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