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.” Government guidance has now changed.

The Royal College of Obstetricians & Gynaecologists (RCOG) says that there is “no evidence” to suggest that the Covid-19 vaccines affect fertility. Similar statements have been issued by the British Fertility Association, the NHS and Public Health England

The RCOG also states that there is no biologically plausible mechanism by which the vaccine would cause fertility problems. 

The RCOG and the NHS recommend pregnant women get vaccinated.

We’ve written more about fertility and vaccines. 

The information included in this article contains the latest evidence and official guidance available at the time it was written. This is not a substitute for medical advice. If you require specific medical advice please consult your GP or midwife.

Update 26 August 2021

This story was updated to include the latest advice on vaccines and fertility and getting vaccinated during pregnancy.

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