This seems to be based on perceived similarities between the coronavirus’s spike protein – which is the key part of the virus that the vaccines target – and a protein found in the placenta called syncytin-1. This has led to the unfounded theory that antibodies against the spike protein will attack syncytin-1, stopping it performing its important role in the placenta.
This is simply not true. The similarity between the proteins is insufficient for this to be of any concern.
All proteins are made up of long strings of individual building blocks called amino acids. SARS-CoV-2, the virus that causes Covid-19, is made up nearly 10 000 amino acids, of which around 1 300 are found in the spike protein. Syncytin-1 is made up of around 540 amino acids. Given that there are only 20 different types of amino acid, it isn’t surprising that many, many proteins share similarities.
To make a protein these long strings of amino acids are folded to form a 3D structure. 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).
No evidence of negative effects
Spike protein antibodies are made in response to both natural infections and vaccines. So women who have been infected naturally with SARS-CoV-2 during pregnancy also have antibodies to the spike protein.
Therefore, studies of the natural immune response of pregnant women to SARS-CoV-2 – and the effects of infection on pregnancy outcomes – can provide insight into the risk of spike protein antibodies to pregnancy.
Evidence shows that the risk of miscarriage is not increased in women who have had a SARS-CoV-2 infection in early pregnancy. This indicates that even if spike protein antibodies are present within the uterus, they don’t have a negative effect on implantation or early development of the placenta.
The mother’s blood supply is not fully connected to the placenta until the end of the first trimester. The passage of antibodies from mother to baby across the placenta then follows in the second and third trimesters.
This is a normal part of pregnancy and provides protection to the baby against infectious diseases. This is known as passive immunity and continues after the baby is born with antibodies being transferred via breast milk.
SARS-CoV-2 antibodies have been found in the newborns of women who have had Covid-19 during pregnancy and who continued to carry their pregnancy to term. (Typically, this is done by analysing umbilical cord blood collected when the baby is delivered.)
This shows that antibodies are passing across the placenta from mother to baby and are not having a detrimental effect on the pregnancy. This is despite cells expressing syncytin-1 being the first point of contact on the placenta for these antibodies.
Exclusion from trials not unusual
At the outset of the pandemic there was much concern about the potential harmful effects of SARS-CoV-2 on pregnancy and unborn children. Pregnant women were included in lists of vulnerable people as a sensible precaution.
This was based on experience with other viruses that cause more severe disease in pregnant women than the general population or can infect the placenta or the fetus and cause harm. This list of viruses includes influenza, hepatitis E, and Zika.
But the passage of time has shown us that we do not need to add SARS-CoV-2 to this list. Yes, pregnant women can develop severe Covid-19, which can increase the likelihood of being admitted to intensive care and giving birth preterm. Overall, however, pregnant women are less likely to experience symptoms of Covid-19 and are more likely to have milder symptoms when they do.
As a similar precaution, it’s normal practice to not include pregnant women in the testing of vaccines – or any other form of medicine – until their safety has been established.
Given the short timescale over which Covid-19 vaccines have been developed, there simply hasn’t been the time to gather the safety data needed to confirm that it’s safe to enrol pregnant women in clinical trials.
While pregnant women haven’t been recruited into these trials yet, due to these trials’ large size, there have been instances when women involved have become pregnant. In the cases where this has happened, there has been no adverse effect on the pregnancy.
Likewise, there hasn’t been time to gather enough data to allow pregnant women to be included in the priority groups for vaccination.
However, the safety data from the general population is sufficient for the Royal College of Obstetricians and Gynaecologists to advise that pregnant women who are clinically extremely vulnerable or frontline healthcare workers can consider being vaccinated and should discuss this with their doctor.
These women should not have any concerns that spike protein antibodies generated in response to the vaccine will have any harmful effects on their pregnancy.