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“I’ve read that the Pfizer vaccine causes female infertility by blocking a protein that is key in the formation of the placenta in mammals. Is there any shred of truth to this?”

Families Uncertainty and Misinformation Vaccines

A: No. There is no truth to this rumor. There is nothing in the human genome that is similar to the spike protein that each of the vaccines are targeting to prevent COVID-19.

💉First of all, rumors about similarity between the placenta proteins and spike protein just do not pass the sniff test. Remember: extraordinary claims require extraordinary evidence. The articles being shared making this false claim are written in a very scientific fashion with big words that are hard to understand and difficult to search, and are not peer-reviewed, or even written by scientists. Many of the websites that are sharing this information are not legitimate sources, and appear to be inventing this story out of whole cloth. See the link at bottom of post on evaluating sources for yourself.

💉Secondly, all of the vaccines in development are aiming to produce an immune response to the spike protein which is part of the virus. When we get COVID-19, we also make antibodies against this protein. If immune responses against a part of the SARS-CoV-2 virus caused problems with the placenta, this would also be the case with immune response to natural SARS-CoV-2 infection–and that is not what we see happening. There is no evidence that immune responses to SARS-CoV-2 infection causes immune response against placental proteins or female sterility.

💉The rumors that the spike protein of SARS-CoV-2 is similar to syncytin-1, the placental protein, are simply not true. When we compare the protein (amino acid) sequence of viral SARS-CoV2 spike protein with the protein sequences in humans, there is very little overlap. Therefore, it is very unlikely that an immune response against the SARS-CoV2 spike protein would cause immune responses to any human protein, including the proteins in the placenta.

For drug development, pregnancy is considered a “special condition”. That means pregnant individuals are usually excluded from clinical trials because of the unique health status, risks, and needs of this population. Drugs are not generally tested in pregnant people, and certainly not before they have been demonstrated to be safe in healthy non-pregnant adults. Most of what is known about the safety of vaccines or other therapies in pregnant women is learned outside of a clinical trial–we get information when a woman gets a vaccine or therapy and then is later found out to be pregnant. When this accidental exposure in pregnancy occurs, it is reported to the FDA and the vaccine company so they can follow up and learn whether there were any adverse effects. Once many of these events have occurred and some safety data in pregnancy is available, a drug may be considered for clinical trials that include pregnant women and/or approval for use during pregnancy. The exclusion of pregnant women from the trials and from receiving the new vaccine are in line with standard drug development processes.

We do know that COVID19 infections cause negative outcomes in pregnant women. Part of that may be due to pregnancy reducing your immune response and makes it harder to fight infections in general. Regardless, pregnant women will have to take additional precautions to prevent SARS-CoV2 infection, especially because the vaccine will not be immediately available to currently pregnant women.

In general, there is very little similarity (or homology) for SARS-CoV2 spike protein with any protein in humans. It is very unlikely that an immune response to the spike protein of SARS-CoV2 would cause an immune response to placental proteins. Frankly, the rumor that SARS-CoV2 vaccines would impact fertility is a dangerous scare tactic and is completely unfounded.

Evaluating sources

More information about how there is no overlap between placental proteins and SARS-CoV2 spike protein:

AP News

Full Fact

SARS-CoV2 infection in pregnant women

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