• The vaccines used in France against Covid-19 drastically reduce the risk of having a

    severe form of the disease.

    But what about the risk of contributing to the epidemic by contaminating around you?

  • For a long time, data was lacking to reassure on this point.

    But more and more studies and real-life confirmations show that vaccinated people have very few mild, asymptomatic forms of Sars-Cov-2.

    They protect themselves and therefore protect those around them.

    A message that is difficult to pass, according to some researchers.

  • Is this the case from the first dose?

    What percentage exactly?

    Are there any differences between vaccines?

    For how long ?

    Some questions remain unanswered for the moment.

Vaccination today rhymes with individual protection against serious forms of Covid-19.

But is this a citizen gesture, to avoid transmitting the coronavirus to relatives, colleagues, strangers in the metro, and soon on the terrace?

For a long time, doubts surrounded the risk, for people vaccinated with Pfizer, Moderna and AstraZeneca, of being infected and especially contagious.

But the data is accumulating and is very reassuring on this subject.

Why do we see positive PCR tests after vaccination?

The press echoes of vaccinated people who test positive for Covid-19 after one or two doses.

Proof that these vaccines do not protect against contamination?

“Vaccination cannot lead to positive nasopharyngeal tests, whether by antigen or PCR, reassures Nathan Peiffer-Smadja, infectious disease specialist at Bichat hospital (AP-HP) and researcher for Inserm.

However, in the fifteen days following the injection, the vaccination is not yet effective and you can become infected like unvaccinated people.

This is why we regularly see cases of Covid-19 in patients who have not had time to develop their protection after vaccination.

"

"In the history of medicine, there are very few vaccines that do not protect against contamination"

These cases should be put into perspective. During phase 3 of the clinical trials, neither Pfizer nor Moderna were interested in any asymptomatic post-vaccination contaminations. “We were in a rush. The important thing was to know if the vaccine protected against serious disease, recalls Sandrine Sarrazin, researcher at Inserm at the Marseille-Luminy immunology center. So we looked at how many of the people included in the clinical trial reported symptoms. To collect precise information on possible asymptomatic infections, the 40,000 volunteers would have had to be tested every week by PCR.

“From the start, we had good indications to say that vaccines protected against contamination, nuance Sandrine Sarrazin.

In the history of medicine, there are very few that do not protect against contamination.

There was no reason to believe, given the effectiveness of these vaccines, that they would not protect against transmission.

"

But that's not quite the message currently conveyed.

“This information was translated not into: 'We have not tested, so we have no certainty', but in: 'Vaccines do not protect against contamination, keep your masks on'.

"Too bad, according to this researcher, according to whom the message could be more reassuring ...

The vaccinated ten times less contagious than the unvaccinated

Especially since in the meantime, studies and evidence have accumulated to support this thesis. Moderna, after submitting its dossier for validation to the Food and Drug Administration (FDA), heard criticism of the lack of data. “When they called in the volunteers for the second injection, they tested them,” says Sandrine Sarrazin. Result: out of 14,000 volunteers in each of the two groups, 13 vaccinated people were positive in the PCR test, against 34 in the placebo group. "So there were people infected, but 60% less than without a vaccine," summarizes the researcher.

Now that millions of people are vaccinated around the world, real-life studies point in the same direction. In Israel, a true "guinea pig" country for Pfizer, the incidence curves have been seen to drop as vaccination progresses. But this correlation cannot be brandished as proof, some may argue that it would also be the effect of containment.

On the other hand, two studies confirm this link.

One is from the UK: after two injections of Pfizer, the risk of developing an infection that can be transmitted is reduced by 86%.

For the second study, from Israel, we are at 90%.

"According to the latest analyzes, when you are vaccinated and become infected, there is only a 10% risk of transmitting the disease", insists Sandrine Sarrazin.

A third study, American this time, is even more reassuring.

Out of 36,659 health professionals (highly exposed to the virus) vaccinated between December and February, only… 7 of them were positive and asymptomatic fifteen days after the second dose.

Depending on the vaccine ... and the variants

It remains to complete these encouraging data, in particular on the duration of individual and collective protection.

Which varies depending on the vaccine.

“But also depending on the type of variant,” adds Nathan Peiffer-Smadja.

These data are currently being compiled and analyzed, but numerous studies in the general population have found a reduction in the risk of asymptomatic infection between 60 and 90%.

"

What about AstraZeneca?

"We lack data," says Sandrine Sarrazin.

But this vaccine is effective, so there is no reason to believe that it does not protect against infections.

“Especially since a British study published at the end of April revealed that after a single dose, infected people have between 38 and 49% less risk of infecting a person in their home.

Barrier gestures, again and again

As of May 5, 16.7 million French people had received a first dose. Hence the growing temptation to go and kiss those close to them without a mask. But then why, if vaccination protects against even asymptomatic contaminations, do some doctors repeat that the barrier gestures must still be respected? "It is absolutely necessary to keep a maximum level of protection fifteen days after the first dose, it is as if there was no immune protection", justifies Nathan Peiffer-Smadja.

“Then, the risk of asymptomatic infection drops significantly and that of symptomatic disease is greatly reduced,” she continues.

However, at the scale of a population where the virus circulates a lot and where the majority of people have not yet been vaccinated, it is necessary to maintain barrier gestures if we want to control the epidemic.

The answer will be very different if we achieve less circulation of the virus and a high percentage of people vaccinated, as is the case in the United Kingdom or Israel, where the barrier measures are gradually starting to be lifted.

»A near horizon in France?

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