What do we know about the safety of COVID-19 vaccine mRNA in breast milk?

Introduction

A study by Hanna et al. published in September 2023 reported the presence of mRNA from COVID-19 mRNA vaccines in the breast milk of some lactating women^[1]. This study gave rise to many social media posts questioning whether vaccine mRNA in breast milk would lead to adverse health effects in breastfeeding babies.

Because pregnant and lactating mothers were initially excluded from the clinical trials that led to the COVID-19 vaccines’ authorization, the safety of vaccines for that group of persons provided fertile ground for vaccine misinformation, as numerous Health Feedback reviews on the subject can attest to.

In this Insight article, we’ll review what is known about the presence of mRNAs from COVID-19 vaccines in breast milk. We’ll discuss the study by Hanna et al., and explain why its findings don’t mean health concerns for babies.

Studies indicate vaccine mRNA mostly remains at the injection site, though small amounts disseminate throughout the body

Vaccines using the mRNA technology are made of single strands of RNA inside small lipid nanoparticles (LNPs). The vaccine mRNA carries information coding for producing the SARS-CoV-2 spike protein. After injection, the LNPs can enter the recipient’s cells, which use their molecular machinery to translate the spike mRNA into the spike protein. The presence of spike then triggers an immune response that will train the body to better respond against infection by the virus SARS-CoV-2.

Data on the biodistribution of the spike mRNA from COVID-19 vaccines in the body are limited. However, preexisting results from other studies suggested that it was plausible that at least some mRNA from the vaccine reached the mammary glands.

The European Medicines Agency (EMA) provided information on the biodistribution of LNPs similar to those used in COVID-19 vaccines after intramuscular injection in rats. The data showed LNPs could be detected throughout the body in the few hours following injections.

The LNPs concentrated most at the injection sites, followed by local lymph nodes, the liver, and the spleen. Following injection, the level of LNP in tissues progressively decreased. In its assessment reports of the Moderna COVID-19 vaccines, the EMA wrote:

“Only a relatively small fraction of the administered mRNA-1647 dose distributed to distant tissues, and the mRNA constructs did not persist past 1 to 3 days in tissues other than the injection site, lymph nodes, and spleen”.

A 2017 study on a prototype of mRNA flu vaccine also found that the vaccine mostly accumulated at the injection site, with lower concentrations in tissues such as lymph nodes or the liver. The vaccine mRNA could also be detected in other tissues at concentrations that were one hundred to one thousand times lower^[2].

There is also evidence that the mRNA from COVID-19 vaccines reaches the blood: researchers observed spike mRNA from the Pfizer-BioNTech vaccine in the blood of vaccinated people up to fifteen days after vaccination^[3].

Therefore, the available data indicate that mRNA from vaccines mostly remains at the site of injection, although much smaller amounts can reach the blood and other organs. In light of these data, the presence of some vaccine mRNA in mammary glands and possibly in breast milk is plausible.

COVID-19 vaccine mRNA is transiently excreted into breast milk

Although initial results suggested that the vaccine mRNA didn’t reach breast milk^[4], later studies reported limited presence of the spike mRNA from vaccines in the milk of lactating women.

In 2021, Low et al. detected spike mRNA in the breast milk of three out of ten (30%) lactating mothers who received the Pfizer-BioNTech vaccine^[5]. A 2022 study by Yeo et al. also reported mRNA from the Pfizer-BioNTech vaccine in 13% of lactating mothers^[6] (4 out of 31).

Similarly, Hanna et al. published two studies identifying vaccine mRNA in breast milk. The first study detected mRNA up to 45 hours after vaccination. More specifically, they found that the mRNA accumulated in breast milk extracellular vesicles, which are natural lipid nanoparticles produced by the body^[7]. The same team later confirmed its discovery by detecting vaccine mRNA after 50% of doses given to 13 women^[1], but only during the first 48 hours after vaccination.

In summary, data on this subject has been available since 2021, indicating that the mRNA from COVID-19 vaccines can occasionally pass into breast milk. Thus, the study by Hanna et al. from 2023 didn’t report a bombshell discovery nor did it uncover a medical scandal, contrary to what some social media users implied (more on that below).

However, results by Hanna et al. also suggest that the vaccine mRNA present in breast milk may have no biological effects on lactating mothers or breastfeeding babies, as we will explain in the next section.

mRNA vaccines in milk are in trace amounts and not functional

Because we know now that the mRNA from vaccines can pass into breast milk, it is reasonable to ask ourselves whether this could bear any consequences on breastfeeding babies and whether we should be worried about it. However, data also suggest that this mRNA may have no biological effect.

First, the vaccine mRNA can be found in breast milk in only very low amounts. Low et al. explained in their study, “We detected negligible amounts of BNT162b2 mRNA in a minority of human milk samples using a very sensitive assay”^[5]. Yeo et al. said that the level of mRNA detected in their study was “low”^[6] and Hanna et al. said they only found “trace amounts”^[7]. Hanna et al. doubled down on that statement in their follow-up study by concluding “the mRNA was only occasionally detected in BM [breast milk], in trace amounts”^[1].

Hanna et al. also carried out a more in-depth analysis of the vaccine mRNA found in breast milk. First, they analyzed whether the mRNA strands were still in their native, full-length form, as they were in the vaccine. The study found that most of the vaccine mRNA in the milk had degraded into fragments. Only 12% to 25% of it was still intact.

Second, Hanna et al. determined whether the vaccine mRNA in breast milk still retained the ability to induce spike protein production in cells. To do so, the authors exposed lab-grown gut cells to vaccine mRNA-containing breast milk or to the vaccine itself. Although the decision to use gut cells for this experiment wasn’t explained in the article, it was most likely done to simulate what happens when babies drink and digest breast milk.

If the gut cells did produce the spike protein when directly exposed to the vaccine, it would confirm that the choice of these cells is appropriate because they have the inherent capability to respond to the vaccine—this is what is called a “positive control”. If the cells express the spike protein when exposed to breast milk, it would indicate that the vaccine mRNA from the milk is still functional.

The results showed that the cells expressed spike when directly exposed to the vaccine but not when exposed to the mRNA harvested from breast milk. This is an important result because it indicates that the vaccine mRNA in breast milk isn’t functional. This means that even if ingested during breastfeeding, it wouldn’t lead to spike protein production in the baby and an immune response.

COVID-19 vaccines are beneficial for pregnant or lactating women

Pregnant women are at greater risk of developing severe COVID-19 than the general population. Getting COVID-19 while pregnant is also associated with a higher risk of negative pregnancy outcomes, like miscarriage and stillbirth^[8-11].

Even though clinical data about vaccine safety specifically for pregnant women were lacking at first, the vulnerability of this group to COVID-19 and the dire consequences it could have on their health shifted the risk-benefit assessment, leading public health agencies to recommend vaccination.

Later on, clinical studies confirmed that getting vaccinated during pregnancy was safe for the mother and the baby^[12]. Similarly, clinical data available so far indicate that COVID-19 vaccines are safe for breastfeeding mothers and breastfed children with no associated severe reactions^[13,14].

Furthermore, breast milk from vaccinated mothers could actually provide protection against COVID-19. Indeed, breast milk is known to contain antibodies that can help fight infection and pass to the baby during feeding. Numerous studies showed that breast milk from vaccinated mothers contains antibodies that can neutralize SARS-CoV-2 and can pass to the baby during feeding. This could help protect babies from COVID-19, particularly those who are too young to be vaccinated against COVID-19^{[6, 15-22]}.

Claims exaggerated the significance of study on vaccine mRNA in breast milk

Reacting to the publication of Hanna et al. in 2023, some people implied that the study had revealed results contradicting official guidance by public health agencies that the vaccine mRNA would only remain at the site of injection.

For instance, YouTuber and former nurse instructor John Campbell presented the study as “definitive proof” that the mRNA from the vaccines “goes everywhere” in the body, contrary to what “we were initially told”. French YouTuber Idriss Aberkane claimed, “the mRNA from “vaccines” goes into breast milk…although we were told it would never go there”.

However, this misrepresents reality. The fact that the mRNA mostly accumulates at the injection site doesn’t mean that there is absolutely no vaccine mRNA elsewhere. Rather, it means that the large majority of it can be found near the injection site.

As we explained in the previous section, the presence of very low amounts of vaccine mRNA in tissues other than the injection site is consistent with the biodistribution data collected and published by health agencies prior to the vaccines’ authorization. Furthermore, the occasional presence of vaccine mRNA in breast milk has been known since 2021. Thus, Campbell and Aberkane’s statements exaggerated the novelty of the 2023 study by Hanna et al.

Campbell also implied that the presence of vaccine mRNA in milk could be dangerous to breastfeeding babies. He called for “large-scale epidemiological surveys” to find out if breastfeeding post-vaccination was safe.

Others were more explicit. Retsef Levi, a professor at the Operations Management Group of the Sloan School of Management at MIT, claimed that Hanna et al. “raised concerns of babies’ exposure to toxic mRNA via breastfeeding moms”. Both Campbell and Levi are known for propagating COVID-19 vaccine misinformation.

But as we explained earlier, the study had found that vaccine mRNA could only be detected during a short period of time and at a very low level in breast milk. The study also found that this mRNA is mostly degraded into pieces and is not functional.

Campbell actually dismissed these results, arguing that researchers may not have used the right type of cells to determine whether vaccine mRNA was biologically active. However, positive control data presented in the study (Figure 2 from Hanna et al. 2023) clearly show that these cells are capable of responding to vaccine mRNA, contradicting Campbell.

We also cited clinical and epidemiological studies showing that babies from mothers vaccinated during pregnancy or breastfeeding didn’t have specific adverse reactions^[12-14].

Thus, data available so far don’t provide a plausible basis for claims that vaccine mRNA occasionally found in breast milk could pose a threat to babies or their mothers.

Conclusion

In summary, all data available so far indicate that the vaccine mRNA has little to no effect on breastfed babies. Indeed, it is only detected on occasion, and in very low amounts, in breast milk. Furthermore, data indicate that most of this mRNA is degraded. The presence of trace amounts of mRNA is also compatible with what was already known. Biodistribution data indicate that the mRNA transiently disseminates in the body before being mostly cleared out for tissues away from the injection site.

Therefore, the data made available in 2023 about the presence of vaccine mRNA is breast milk aren’t likely to fragilize the body of evidence already available showing that mRNA COVID-19 vaccines are safe and effective for pregnant or lactating women and, in fact, are particularly useful to this group.

REFERENCES

• 1 – Hanna e al. (2023) Biodistribution of mRNA COVID-19 vaccines in human breast milk. eBioMedicine.
• 2 – Bahl et al. (2017) Preclinical and Clinical Demonstration of Immunogenicity by mRNA Vaccines against H10N8 and H7N9 Influenza Viruses. Molecular Therapy.
• 3 – Fertig et al. (2022) Vaccine mRNA Can Be Detected in Blood at 15 Days Post-Vaccination. Biomedicines.
• 4 – Golan e al. (2021) Evaluation of Messenger RNA From COVID-19 BTN162b2 and mRNA-1273 Vaccines in Human Milk. JAMA Pediatrics.
• 5 – Low et al. (2021) Codominant IgG and IgA expression with minimal vaccine mRNA in milk of BNT162b2 vaccinees. NPJ Vaccines.
• 6 – Yeo et al. (2022) Neutralizing Activity and SARS-CoV-2 Vaccine mRNA Persistence in Serum and Breastmilk After BNT162b2 Vaccination in Lactating Women. Frontiers in immunology.
• 7 – Hanna et al. (2022) Detection of Messenger RNA COVID-19 Vaccines in Human Breast Milk. JAMA Pediatrics.
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