The reason given for this is that the vaccine works by igniting an immune response to a spike protein on the Covid-19 virus’ surface, and that this immune response could also attack similar proteins that make up the placenta, and therefore reduce fertility in women.
But there’s no evidence this is the case.
Where did this claim come from?
The origins of this claim stems from 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 the instructions on how to make this spike protein, so it can generate an immune response that attacks the virus via the spike protein faster and more effectively if it is later infected.
The building blocks of proteins are called amino acids, and it’s sequences of those that make up different proteins.
A small part of this spike protein resembles a part of another protein vital for the formation of the placenta, called syncytin-1. But the sequence of amino acids that are similar in syncytin-1 and the SARS-CoV-2 spike protein is quite short.
But only two very small parts of these proteins look the same—it’s not the whole protein—and therefore realistically the body’s immune system is not likely to confuse the two, and attack syncytin-1 rather than the spike protein on SARS-CoV-2 and stop a placenta forming.
What do the experts say?
Virology professor Ian Jones at the University of Reading told Full Fact, via the Science Media Centre, that the spike protein and syncytin-1 don’t look similar enough for the same antibodies that the body would create as an immune response to the vaccine, to bind to and attack syncytin.
He said 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 these proteins are not identical, as some have claimed.
On whether the vaccine could cause an immune reaction to the syncytin-1 protein vital for placenta formation, he said: “I would never say never, but the possibility is vanishingly small.”
What did the trials show?
There also isn’t any evidence from trials of the Pfizer vaccine that it affects fertility.
As the vaccine stimulates an immune response to the spike protein, if it did affect fertility we might also expect to see Covid-19 infections affecting this too, as the body should produce a similar immune response if infected. But we don’t. Although it has been suggested that Covid-19 cases seem to be more severe in pregnant women, there doesn’t seem to be evidence that Covid-19 causes women to lose their pregnancy, or struggle to get pregnant later.
The government was updated on 31 Dec 2020 to say that “if you are pregnant you should not be vaccinated unless you are at high risk – you can be vaccinated after your pregnancy is over” and “if you are pregnant but think you are at high risk, you should discuss having or completing vaccination with your doctor or nurse.”
A number of Facebook posts point to UK government-published guidance on this vaccine, that originally said: “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.
Update 31 December 2020
This fact check was updated to reflect updated guidance for pregnant women from Public Health England.