In a vaccination center in Lyon.



  • Vaccination has accelerated, with the opening on January 18 of injections to people over 75 and to people who suffer from a pathology that has made them vulnerable to Covid-19 since.

  • Two vaccines are currently available in France: that of Pfizer-BioNTech and that of Moderna.

  • 20 Minutes

    invited its readers to ask their questions about vaccination, and here are some answers.

Every day, new information ... and new questions about vaccination against Covid-19.

For some, it's the glow at the end of the tunnel.

For others, the subject of all suspicion.

To be closer to its readers,

20 Minutes

 invited them to ask their questions on the subject.

And today responds to some of them.

Françoise: "If we received a dose of vaccine X, the 2nd dose must be of the same vaccine or whatever?


No, it will not be possible in France to mix the doses and receive, for example, a first injection of the vaccine from Pfizer and a second from Moderna.

Why ?

"For reasons of monitoring side effects," explains Marie Lachâtre, infectious disease specialist at the Cochin Pasteur Clinical Investigation Center in Vaccinology at Cochin Hospital (AP-HP).

Indeed, if you ever have two different vaccines and you develop side effects, how do you know which one caused them?

Other Internet users wonder about the duration between the two injections.

"Today, we are trying in France to respect the 21-day interval," said the infectious disease specialist.

In the vast majority of cases, the second appointment is made during the first injection.

If, for X reasons, this second injection should be postponed (shortages, health problem, etc.), it can be postponed until D42 [42 days after the first injection].

In clinical trials, people who had to space the two injections did not appear to be at greater risk of contracting Covid-19.


This is why Olivier Véran announced on January 14 that the three-week deadline could be extended.

“From the first dose, we can talk about an immune response to the vaccine from D10 or D14, explains Marie Lachâtre.

It is best to meet the three-week deadline for Pfizer and four weeks for Moderna.

But in a constrained context, it does not seem unreasonable to extend this period to six weeks.

In order to vaccinate as many vulnerable people as possible.


Christian: “My daughter-in-law, who had Covid-19, is no longer immune after six months.

If so, will there be a vaccine booster every six months?


The question of the duration of immunity is particularly interesting and complex.

“Today, we do not yet have enough hindsight to know the duration of vaccine immunity,” explains the infectious disease specialist.

Anyone vaccinated will be monitored and studies will be set up to define this duration.

“Indeed, clinical trials having lasted a maximum of three months, it is impossible to say for the moment whether people are protected for years ...

“After phase 3 of the clinical trials, vaccine surveillance continues with the implementation of phase 4 [post-marketing].

This step will make it possible in particular to collect pharmacovigilance data and on the duration of immunity, she continues.

This is the usual process.

Today, we know that these two vaccines have a fairly incredible effectiveness [around 95%] in protecting the most vulnerable populations from the disease.

This is why it seemed urgent to vaccinate as quickly as possible those who risked developing severe Covid-19 ”.

Frédérico: "I would like to know if the various variants compromise the current vaccines"

This is the concern of many researchers and citizens.

Preliminary studies show that the Pfizer vaccine is effective against the variant discovered in England.

This is not certain for the variants discovered in South Africa and Brazil.

“The advantage, with messenger RNA vaccines [Pfizer and Moderna], is that this technique allows rapid production of new vaccines if necessary, reassures Marie Lachâtre.

As soon as the genetic sequence of a new variant is known, one can imagine quickly producing a new suitable and effective vaccine.

“Within weeks, Pfizer assured.

Emeline: "Can you choose your vaccine?


For the moment, the question does not really arise, because there are, as we have seen, only two vaccines available in France: that of Pfizer-BioNTech and that of Moderna.

The European health agency is studying the application for marketing a third vaccine, Astrazeneca.

It is therefore conceivable that in February, a third vaccine will be offered in France.

Remember that the European Union has pre-ordered six different vaccines.

"When we have to vaccinate such large numbers on a global scale, it does not seem possible to allow everyone to choose, answers the infectious disease specialist.

But the Haute Autorité de Santé could establish strategies: such and such a population group would require such a vaccine according to scientific data.

If, for example, Astrazeneca were effective in suppressing the transmission of the coronavirus, but not very useful in preventing severe forms, it would probably be more appropriate to prescribe it to young people.

These possible choices, made by the health authorities, should take into account the effectiveness of vaccines, their safety, practical access, their cost, etc.

Dan: "I am positive for the virus, can I get vaccinated while I am sick?


No, people who are infected should not be vaccinated.

Hence the interest of the pre-vaccination consultation, during which the doctor will ask you if you have had Covid-19 and when.

“After having been infected, beyond three months, the benefit / risk balance is in favor of vaccinating people, including asymptomatic people,” continues the infectious disease specialist.

Before three months, post-disease immunity is enough to protect them.


There is therefore no need to make an appointment if you have had symptoms for less than three months or if you have the Covid currently, the doctor will refuse to vaccinate you.

And for contact cases?

“We are waiting for the PCR (test result) on D7.

If he comes back positive, we wait three months to get the vaccine.

If it is positive, we can be vaccinated ”.


Coronavirus: Why are there more and more variants?


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