There’s no evidence the Pfizer vaccine interferes with the placenta

22 December 2020
What was claimed

The Pfizer vaccine contains syncytin-1 which is vital for the formation of human placenta, so could lead to infertility.

Our verdict

The vaccine contains instructions on how to build a spike protein, a very small part of which resembles syncytin-1. Realistically, there is very little chance the vaccine would cause the body to attack syncytin or cause infertility.

We’ve seen a number of claims on Facebook claiming that the Covid-19 vaccine will cause fertility problems. There’s no evidence that this is the case.


The vaccine contains a spike protein called syncytin-1,

vital for the formation of human placenta in women.

If the vaccine works so that we form an immune response AGAINST the spike protein, we are also training the female body to attack syncytin-1, which could lead to infertility in women of an unspecified duration!”

As we’ve written before, this is a complete misunderstanding of the facts. 

There is a protein on the surface of the SARS-CoV-2 virus (the virus which causes Covid-19), called a spike protein. This spike protein helps the virus enter cells and is also one of the ways the human body recognises a virus and knows to let its immune cells attack it. 

The Pfizer vaccine works by giving the body instructions on how to make the spike protein, so that if the person is later infected, their immune system can generate a response that attacks the virus (via the spike protein) faster and more effectively.

A small part of this spike protein resembles a part of another protein vital for the formation of the placenta, called syncytin-1. The building blocks of proteins are called amino acids, and it’s sequences of those that make up different proteins. But the sequence of amino acids that are similar in syncytin-1 and the SARS-CoV-2 spike protein is quite short.

Only two very small parts of these proteins look the same, so the body is extremely unlikely to confuse the two and also attack syncytin-1 after getting the vaccine.

Virology professor Ian Jones at the University of Reading told Full Fact, via the Science Media Centre, that syncytin-1 is “completely unrelated to the SARS [spike] protein” and the risk of infertility is “therefore essentially fictitious”.

Professor Jonathan Stoye, Virologist at the Francis Crick Institute, told Full Fact: “I would never say never, but the possibility is vanishingly small.”

This spike protein is not syncytin-1, as some Facebook posts have claimed. It is also not true that the vaccine contains syncytin-1.

Some of the Facebook posts link to UK government-published guidance on this vaccine, that says: “It is unknown whether COVID-19 mRNA Vaccine BNT162b2 has an impact on fertility.”

Although we haven’t had years to watch what happens after people get the vaccine, this doesn’t mean there’s any evidence this vaccine might impact fertility. It just means that hasn’t been explicitly studied. At the moment, the government advises that until more information is available, anyone pregnant should not receive the Pfizer vaccine.

This article is part of our work fact checking potentially false pictures, videos and stories on Facebook. You can read more about this—and find out how to report Facebook content—here. For the purposes of that scheme, we’ve rated this claim as partly false because two small parts of the spike protein and syncytin-1 are similar, but there is no evidence this means the Pfizer vaccine can affect fertility.

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