Recent posts on social media have claimed that the newly approved COVID-19 vaccines cause infertility. The biological mechanism by which the vaccines work, on top of data from animal and human studies, strongly suggest that this is not true.

When the World Health Organisation declared COVID-19 a pandemic in March 2020, few could anticipate how severely this virus would impact our daily life. Even before this announcement, research groups and pharmaceutical companies across the globe were embarking on the difficult process of developing a COVID-19 vaccine.

Their efforts have finally begun to pay off as vaccines have begun to be approved for public use. Unfortunately, each new vaccine is met with an explosion of misinformation, making it increasingly difficult for people to find reliable information to help them decide whether to get vaccinated.

We asked 8 experts in vaccinology and reproductive biology to clear up a recent claim – 'Do the COVID-19 vaccines cause infertility?' – here is what we found…

How do the COVID-19 vaccines work?

All of the COVID-19 vaccines work in the same way that all vaccines work, by activating the body's natural immune response. The vaccines expose a harmless version or a small part of the virus to the immune system, triggering it to make antibodies that can fight off future infection.

The differences between the vaccines lie in how they make the COVID-causing SARS-CoV-2 virus harmless or which part of the virus they use. Many of the COVID-19 vaccines have opted for using the spike protein as their main ingredient.

The spike protein is on the surface of the virus and is the target of the immune response during infection. The 'next generation' vaccines, such as those from Pfizer and Moderna, use genetic material, RNA, to code for the spike protein. The vaccine from AstraZeneca uses DNA.

Where did the concern about infertility come from?

Concerns that the COVID-19 vaccines cause infertility were raised through a series of social media posts, most of which have been removed after being flagged as misinformation.

Whilst most claims were vague in their explanation of how the vaccines caused infertility, there were many posts that connected the Pfizer vaccine to a protein found in the placenta called syncytin-1. These posts claimed either that the vaccine contained synctin-1 or that the spike protein that is part of the vaccine is similar to syncytin-1.

This raised the concern that the vaccine would train the immune system to attack the person's own placenta.

Is there a connection between the COVID-19 vaccines and syncytin-1?

As all vaccine ingredients are in the public domain, it is easy to check that syncytin-1 is not an ingredient. In terms of the similarity between syncytin-1 and the spike protein, this is not sufficient to cause any issues of an auto-immune response. All proteins are made of long strings of amino acids which are folded into 3D intricate shapes.

Professor Catherine Thornton from Swansea University explains, "For antibodies to mistakenly recognise syncytin-1 as SARS-CoV-2, there would have to be sufficient similarity of amino acids in these strings (which there isn't) and the critical amino acids would need to be clustered together in the 3D molecule in a sufficiently similar and accessible way (which they aren't)."

We know that antibodies against the spike protein of SARS-CoV-2 do not attack the placenta because SARS-CoV-2 antibodies have been found in new-born babies. These antibodies passed through the placenta from their mothers when they were infected during pregnancy.

Is the next-generation RNA technology a concern for fertility?

These next-generation vaccines use a piece of genetic code, RNA, instead of the spike protein itself to create an immune response. RNA is used by cells in the human body, in fact it is essential for all known forms of life!

Dr Lee Riley from the University of California adds, "pieces of RNA… rapidly get degraded at the site of injection after the RNA chain is translated into amino acids (building blocks of proteins). So there is no chance for the RNA to get anywhere else in the body for it to affect fertility."

What is the data on the COVID-19 vaccines and fertility?

All of the approved COVID-19 vaccines have passed through various animal testing steps, which have not found any effects on fertility. As of yet, there is no data from human clinical trials that specifically study the effect of COVID-19 vaccines on fertility.

The safety trials excluded pregnant women and participants were asked to avoid becoming pregnant.

Dr William Hausdorff an expert in vaccines explains that "The exclusion was not based on any particular theoretical safety concern, but rather due to a superabundance of caution that is generally seen in vaccine trials."

Despite these criteria, 53 pregnancies occurred in the clinical trials of the Pfizer, Moderna, and AstraZeneca vaccines. The outcomes of these pregnancies were no different in the participants who received the vaccines than those who didn't, indicating that these vaccines seem to have little effect on fertility or pregnancy.

The COVID-19 vaccines do not affect fertility – all 8 experts agreed.

Editor's Note (6 March 2021): This story originally grouped AstraZenca's vaccine with the Moderna and Pfizer vaccines as using RNA. This is incorrect as the AstraZeneca vaccine uses DNA. We have corrected the story here and informed Metafact of the error.

Article based on 8 expert answers to this question: Do the COVID-19 vaccines cause infertility?

This expert response was published in partnership with independent fact-checking platform Subscribe to their weekly newsletter here.