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What to Expect from a COVID-19 Vaccine When You’re Expecting or Breastfeeding

When pregnancy or breastfeeding bumps up against COVID-19 vaccines, evidence is lacking but guidance is needed.

What should public health bodies do in the absence of data? And, even more importantly, what should the people left to make an important health decision do when no data is available?

I have seen a lot of anxiety over the question of whether or not people who are pregnant or breastfeeding should get the COVID-19 vaccine. These very same people were prevented from participating in the large clinical trials in which these vaccines were put to the test. Therefore, we don’t have trial data on these groups of people, but they must now make a decision based on seemingly no data at all.

The various committees that make vaccine recommendations all over the world had to deal with this dearth of information and they did so in sometimes contradictory and always evolving ways. A paper penned by Italian researchers summarizes the changes in recommendations for people who are either pregnant or breastfeeding and it is a headache, with the World Health Organization retooling its advice several times over the course of a single month. You would be forgiven for being confused.

Guidance in these situations tends to lean on the precautionary principle, especially where fetuses and newborns are concerned, but it would be wrong to say there is zero data on the safety of the COVID-19 vaccines in pregnant or breastfeeding people. While we do not have optimal data, we do have some information that paints a reassuring picture.

What is not a risk with the vaccine

Our knowledge of biology can already answer an urgent question: the COVID-19 vaccines cannot give people COVID-19. None of the approved vaccines are live attenuated vaccines. A live attenuated vaccine contains a weakened form of the microbe itself. The COVID-19 vaccines contain the instructions to make a small part of the coronavirus, its spike protein, so they cannot cause the disease they are trying to prevent, just like a car cannot be recreated merely from its tires.

Vaccinated people also do not shed the virus, so pregnant people do not need to stay away from their vaccinated friends and colleagues, as has been suggested by many misinformed people on social media.

Moreover, for anyone worried about preservatives or adjuvants in vaccines (and there is a lot of unsubstantiated fearmongering about those), there are none in the currently approved COVID-19 vaccines made by Pfizer-BioNTech, Moderna, Janssen Johnson & Johnson, and AstraZeneca-Oxford.

What does not appear to be a risk with the vaccine based on preliminary evidence

Animal studies conducted by Pfizer for its vaccine showed no fertility or reproductive toxicity, and preliminary data from the other three companies on their animal models likewise does not reveal any harmful effect in pregnancy.

What about humans? While the clinical trials that tested these vaccines in human volunteers specifically excluded people who knew they were pregnant, it just so happens that you can accidentally become pregnant while enrolled in a clinical trial! This did happen during the COVID-19 vaccine trials (at least for Pfizer’s and Moderna’s) and there was no difference in the rates of miscarriage between those who got the vaccine and those who received a placebo injection.

Moreover, now that the vaccines are being rolled out to the population at large, many pregnant individuals are choosing to take the vaccine. Some of them install the v-safe after vaccination health checker on their mobile device, which is an app made by the Centers for Disease Control and Prevention (CDC). They can let the app know if they are pregnant and they get added to the v-safe pregnancy registry. (Alternatives exist in Canada.) These real-world data, along with reports made to the Vaccine Adverse Event Reporting System (VAERS), were recently analyzed by a team from the CDC, taking into account a grand total of 35,691 pregnant people.

The side effect profile from the vaccine was quite similar between pregnant people and non-pregnant women, and any reported issue with the pregnancy was comparable in frequency to what has been described in the academic literature for pregnancies. The authors concluded that in this preliminary and still small data set, there was no obvious signal that the vaccine was unsafe for people who were pregnant.

As for people who are breastfeeding, the concern that something harmful might get transmitted to the infant via the breast milk is so far unsupported. The safety of breastfeeding is not affected by the vast majority of vaccines. The only potential issue is with the smallpox and yellow fever vaccines, both of which are live attenuated vaccines and none of the COVID-19 vaccines are of this type.

There is no plausible mechanism for how an intact and functional coronavirus spike protein could find its way into breast milk after vaccination, according to the Drugs and Lactation Database. What about the RNA encoding the spike protein? In a couple of very small studies, mothers volunteered their breast milk before and after receiving their COVID-19 vaccine and researchers extracted all of the RNA molecules present in it to see if any SARS-CoV-2 coronavirus RNA could be found. None was detected.

Finally, there is the concern that the fever sometimes caused by the vaccine might be worrisome during pregnancy. This concern seems to stem mainly from pregnant laboratory animals, in which high body temperatures are associated with malformations of the fetus or death. However, a large Danish study in humans investigated this from the get-go to avoid people misremembering after the fact and did not find any association between fever during pregnancy and the risk of congenital anomalies. It turns out that not getting enough folic acid can make it appear as if a fever during pregnancy increases the risk for a specific congenital anomaly, but when that is taken into account, the association disappears.

What is or could be a risk with the vaccine

Both AstraZeneca’s and Janssen’s vaccines are associated with a very rare risk of a type of blood clot associated with low levels of blood platelets.

Also, there are “unknown unknowns.” Even though the preliminary real-world data we have on the safety of the vaccines in pregnant and breastfeeding people is encouraging, these groups were not recruited for the clinical trials. Therefore, there is always the possibility of a complication because the issue has not been rigorously studied, especially if the complication is rare.

What could be a benefit of the vaccine

A choice like whether or not to get the vaccine should not solely rest on the potential risks. It should include the likely benefits as well. Obviously, getting the vaccine means gaining a solid protection against COVID-19, an important benefit in the middle of a pandemic; but it may also end up offering some protection to the newborn as well.

A number of studies (summarized here) have reported that the protective antibodies the mother makes in response to the vaccine can be found in both the placenta and in her breast milk. This is a phenomenon known as “passive immunity:” the baby gains protection not from being directly vaccinated but from accessing its mother’s own antibodies. The claim that babies are now protected from COVID-19 in this way has not been proven, but the fact that protective antibodies can be detected in the placenta (for pregnant individuals) and in breast milk (for lactating individuals) is very encouraging.

What is an increased risk for pregnant people: COVID-19

The risks and benefits of an intervention should also be weighed against the risks and benefits of refusing or delaying the intervention, and COVID-19 poses a special risk to pregnant people.

In general, being pregnant seems to put the person at a higher risk for complications and death from respiratory infections, and this risk translates to COVID-19. Pregnancy has been linked to a higher risk of severe COVID-19, meaning a higher risk of hospital admission, of stay in the intensive care unit, of need for mechanical ventilation, and of death from the illness, although it is possible pregnant patients are in part treated more aggressively when they have COVID so as not to take a chance.

Protection by exclusion has to stop

All of this piecemeal information is the result of a process denounced as “protection by exclusion.” In wanting to protect fetuses, babies and newborns at all cost, researchers routinely exclude people who are pregnant or breastfeeding from participating in clinical research, a practice which traces its roots to the discovery in the 1950s that certain drugs taken while pregnant could lead to birth defects. The practice, however, creates a vicious cycle. Pregnant people must be protected from a new drug, for example, because it hasn’t been tested in their group and could be dangerous, but they must also be protected when the drug is being researched. This is a closed loop that never generates good data. Meanwhile, members of the opposite sex who are looking to conceive are rarely treated in the same way. As Helen Hare and Kate Womersley wrote in the British Medical Journal, “Men who are trying to conceive have not been cautioned about the [COVID-19] vaccine, even though no data exist about possible effects on spermatogenesis,” the process by which sperm is made inside the body.

Thankfully, Pfizer has announced a clinical trial of its vaccine in 4,000 pregnant people, and Janssen has also announced a similar trial in 400 people.

As medical societies and public health bodies all over the world update their recommendations regarding the COVID-19 vaccine in pregnant and breastfeeding people, they are converging on the idea that the vaccine should be offered (as opposed to recommended) and that the decision can be informed by a discussion with a healthcare provider. (Recommendations from Canada’s National Advisory Committee on Immunization can be accessed here, while those of the Society of Obstetricians and Gynaecologists of Canada are available here.)

Coming to a decision is a mental juggling act that must cope with general risks and benefits, as well as the person’s own health situation, their values, and the severity of the pandemic where they live and work. But at least, pregnant and breastfeeding people are allowed to participate following informed consent.

It’s a shame the same is rarely offered to them when clinical trials start recruiting.

Take-home message:
-Although people who are pregnant or breastfeeding were excluded from the clinical trials that tested the COVID-19 vaccines in humans, real-world data is so far showing no obvious concern about the safety of these vaccines in these individuals
-There is no plausible way the spike protein encoded by the vaccines could be transmitted to a newborn via breast milk, but protective antibodies against the coronavirus generated after vaccination have been detected in breast milk and they could theoretically protect the newborn
-Many advisory bodies say the COVID-19 vaccine should be offered (as opposed to recommended) to people who are pregnant or breastfeeding, and that the decision to take it, delay it, or refuse it depends on risks, benefits, and the person’s values.


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