This is one of the fears circulating around the Pfizer / BioNTech anti-covid vaccine: that it will render vaccinated women sterile. This theory, which is spreading on social networks, affirms that the vaccine causes the immune system to attack syncytin-1, a protein involved in the development of the placenta. This would lead to infertility in vaccinated women who would therefore be unable to produce a placenta.

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Syncytin-1 “looks” like protein S

How would that be possible? The vaccine teaches the body to make antibodies against a protein in the virus, protein S (or Spike). According to this thesis, the antibodies thus produced could also attack syncytin-1 because it “looks” like the S protein. The two proteins indeed have in common a very short sequence of four amino acids, the basic components of protein.

A simple guess

Where did this idea come from? In one text dated December 1, 2020 addressed to the European Medicines Agency and spotted by AFP, Michael Yeadon, a former researcher at Pfizer who left the company in 2011, and a German doctor, Wolfang Wodarg, list the uncertainties around the Pfizer / BioNTech vaccine.

They indicate that if the anti-protein S antibodies attacked syncytin-1, “it would also prevent the formation of a placenta, which would result in the vaccinated women being infertile“. But they also state that it”there is no indication“going in this direction. It is therefore only a simple guess which is, according to several specialists interviewed by AFP, highly improbable.

“Very low” concern

Dansantila Golemi-Kotra, associate professor of microbiology at York University in Canada, first explains that “concern about the possibility that antibodies targeting these proteins [S, ndlr] can attack the syncytin-1 protein of the placenta“due to a small resemblance between the two”is very weak“.

And for good reason: the identification of protein S by the body “is rarely confined to a single short amino acid sequence“, explains the scientist.

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No demonstration

And although amino acid sequences are common between the two proteins, “these sequences reorganize themselves in the proteins (…) to give a kind of three-dimensional shape. It is this form that is recognized by antibodies“, explains Frédéric Altare, specialist in immunity and director of research at Inserm.

In the current state of knowledge, there is not a sufficient resemblance – and there is very little chance that there will be one – between the syncytin-1 protein and the spike.“, continues Frédéric Altare. He also notes that the authors of this theory “do not advance any demonstration that would show that anti-spike antibodies also target syncytin-1“.

Even if this is the case, since antibodies have a limited lifespan, any action would be transitory.“, excluding the idea of ​​lasting infertility, reassures the specialist.

No infertility in infected women

Finally, underlines Annette Beck-Sickinger, professor at the University of Leipzig in Germany, “if the syncytin argument were true, every woman infected with the virus would therefore have become infertile, but this is not the case“.

Because people infected with Sars-Cov-2 “all produced anti-spike antibodies and there is no evidence that this prevented women from getting pregnant“, abounds Frédéric Altare.”If it doesn’t happen naturally with the virus, there’s no reason it should with something else“, concludes the specialist.