What do we know about the Covid-19 vaccines crossing the placenta?

We have been asked on WhatsApp about what is known on the Covid-19 vaccines crossing the placenta. Can a baby get any protection from their mother’s vaccination, and does mRNA —which is present in some of the vaccines— go across the placenta and pass from the mother to the baby?

Most of the research about the Covid-19 vaccines in pregnancy has focussed on evidence of any harms to mother and baby, and the effectiveness of the vaccines for pregnant women, rather than looking directly at what crosses the placenta. We have written more about this previously. 

Both of the Covid-19 vaccines recommended in pregnancy at present are mRNA vaccines (Moderna and Pfizer). mRNA vaccines work by using a lipid (fatty) membrane to deliver the genetic code (mRNA) for a protein specific to the pathogen’s surface (in the case of the Covid-19 mRNA vaccines, the SARS-CoV-2 spike protein). Cells in the body then use this mRNA to build copies of these proteins which the immune system responds to by producing antibodies. This provides protection if the person catches the real disease later. 

AstraZeneca is a viral vector vaccine, which uses a virus that has been modified to be harmless to deliver the genetic code of the SARS-CoV-2 spike protein to the body’s cells, allowing the cells to produce the spike protein and prepare an immune response. Some pregnant women may receive AstraZeneca as their second Covid-19 vaccine dose, if they already received a first dose of AstraZeneca vaccine prior to pregnancy. 

The UK Teratology Information Service, the provider of evidence-based information on fetal risk following pharmacological and other potentially toxic pregnancy exposures, told Full Fact that the Covid-19 vaccines cannot cause infection in either the pregnant woman or her fetus. This is because they don’t contain live SARS-CoV-2 virus, nor viral vectors which are capable of replication.

Dr Victoria Male, Lecturer in Reproductive Immunology at Imperial College London, told us that the mRNA in the Pfizer and Moderna vaccines mostly stays at the injection site, and a small amount travels onwards, mainly to the liver.

mRNA also breaks down very quickly after injection, which makes it “extremely unlikely” that it could survive passage through the placenta, in the view of Dr Sarah Stock, reader and honorary consultant in maternal and fetal medicine at the University of Edinburgh. 

A spokesperson for The Royal College of Obstetricians and Gynaecologists (RCOG) also told Full Fact that “as the vaccine is injected into a muscle and not the bloodstream and the mRNA in the vaccine is rapidly broken down, it's extremely unlikely that the mRNA would cross the placenta”. 

They added: “The other ingredients in the vaccine include salts, water and lipids (fats) to stabilise the vaccine and none of these are known to be harmful to the baby (many of these are commonly found in the foods we eat).”

Scientists are starting to study this more specifically with pregnant women and babies. Currently, there is no information from animal studies which can tell us directly about the transfer of mRNA in the vaccines across the placenta. Dr Stock told Full Fact that animal studies are unlikely to be able to directly “prove” that mRNA does not cross the placenta, but instead we can learn indirectly about this through antibody studies. Antibodies are some of the key proteins in your body that make up part of the immune system.

There are now a few antibody studies in humans which have examined the antibodies present in babies born to mothers who received the vaccine during pregnancy. 

What do the antibody studies say about mRNA in the vaccines crossing the placenta?

Information around this is still emerging, but the data so far suggests that mRNA in the vaccines doesn't cross the placenta. Exactly why that's the case is complicated - so here's a quick explainer.

There are five main different types of antibodies. The two important ones for looking at immunity in mothers and babies are called IgM and IgG. Your body will create particular antibodies, such as IgM and IgG, in response to antigens that you are exposed to. 

An antigen is something that causes the body to create an immune response, for example, by creating antibodies. Pathogens like bacteria and viruses have particular proteins which act as antigens. These antigens allow the body to recognise the pathogens and produce the appropriate immune response. 

Vaccines work by providing the body with a similar antigen to trigger the immune response, so that when you meet the real virus your body already has a defence in place.

IgM and IgG antibodies are useful in maternal and fetal studies, because IgM isn’t known to cross the placenta, but IgG can. They can therefore give us information on what types of antigens crossed the placenta during pregnancy. If there is an immune response from the fetus, the antigen has likely crossed the placenta. If there is only an immune response in the mother, it is likely the antigen didn’t cross the placenta. 

Dr Stock explained that the studies so far (described below) on the mRNA vaccines have not detected IgM in the blood of any babies whose mothers were vaccinated during pregnancy. This suggests that the babies were not exposed to the vaccine antigen in the womb. If they were, they would have produced their own IgM antibodies. 

Dr Stock explained that the studies have shown, however, that the babies do have IgG antibodies. This suggests that the IgG antibodies that the mother created in response to the vaccine antigen can be passed across the placenta to the fetus. 

She added that IgG antibodies are likely to provide protection from Covid-19 in the first few weeks after birth, although this has not been confirmed in research studies yet.

One study that Dr Male highlighted, published in May 2021, looked at results from 86 women who were vaccinated during pregnancy and 65 women who had Covid-19 infection during pregnancy, as well as a control group who had neither. 

As we’ve said, no babies from the group whose mothers had been vaccinated during pregnancy produced IgM antibodies, suggesting that they were not exposed to the vaccine antigen in the womb and it did not cross the placenta. By contrast, five babies born to mothers who had Covid-19 infection during pregnancy did have IgM antibodies, suggesting that the antigen for Covid-19 had crossed the placenta during maternal infection and the babies had been exposed while in the womb.

The study also showed that the babies of both the vaccinated mothers and the mothers who had Covid-19 in pregnancy had IgG antibodies (as expected, likely passed from their mothers). 

Another study, published in August 2021 examined the antibodies in 27 vaccinated women and their babies. Again, the majority of the babies had IgG antibodies, and none had IgM antibodies. 

The two studies mentioned above are the only studies outlined by our experts for information on both IgM and IgG antibodies in babies born to vaccinated mothers. It is this type of study that can allow us to understand more about whether the vaccine antigens cross the placenta.

There are other antibody studies which have looked at IgG antibodies (and other factors) in mothers and babies after Covid-19 vaccination. These have also shown that babies born to mothers who had the Covid-19 vaccines had IgG antibodies, again demonstrating that IgG antibodies that the mother created in response to the vaccine can be passed across the placenta to the fetus. As described by Dr Stock, these antibodies are likely to provide protection from Covid-19 in the first few weeks after birth, although this has not been confirmed in research studies yet.

Together, with what is known about how mRNA works in the mother’s body, these studies are being used to contribute to a growing evidence base on the Covid-19 vaccines in pregnancy. 

Do the Covid-19 vaccines cause damage to the placenta? 

Since the beginning of the vaccine roll out, there have been false rumours that the Pfizer vaccine could cause damage to the placenta. There is no evidence to support this. We have written more about this previously. 

We have also written more about the Covid-19 vaccines and pregnancy.

We would like to thank Dr Victoria Male and Dr. Sarah Stock for their help in reviewing this article. Full Fact is responsible for the final text.

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.