Amsterdam—A new study demonstrated why pharmacists should avoid administering the Johnson & Johnson/Janssen COVID-19 vaccine to new mothers who are lactating, instead opting for one of the mRNA vaccines.

The report in the Journal of the American Medical Association Pediatrics noted that—while COVID-19 usually has a mild course in children—newborns and infants are much more susceptible to severe disease and, thus far, ineligible to be vaccine in the United States. That is why protection from the mother is so important, the Dutch researchers pointed out.

"Human milk is suggested to play an important role to protect against infections, mostly owing to disease-specific antibodies," according to the article. "Antibodies against SARS-CoV-2 are present in the human milk of previously infected women, as well as following vaccination with a SARS-CoV-2 vaccine and are capable of neutralizing the virus. Because maternal vaccination during lactation may protect not only the mother but also her breastfed infant, knowledge of its effect is important to guide health care workers and lactating women in decision-making regarding SARS-CoV-2 vaccination."

That led to the study to compare the antibody response in human milk after vaccination with mRNA-based and vector-based vaccines. Four vaccines are available in the Netherlands: two mRNA-based vaccines (BNT162b2 developed by Pfizer-BioNTech and mRNA-1273 developed by Moderna) and two vector-based vaccines (AZD1222 developed by Oxford/AstraZeneca and Ad26.COV2.S developed by Johnson & Johnson/Janssen).

Eligible to participate in the longitudinal study were lactating women who received full vaccination with one of those vaccines; the 124 participating lactating mothers each collected 17 human milk samples over a period of 100 days during the study period of January 2021 through July 2021.

The researchers used an enzyme-linked immunosorbent assay with the SARS-CoV-2 spike protein to assess IgA and IgG antibodies in human milk.

Based on the 1,650 human milk samples in the final analysis, researchers determined that almost all participants who received an mRNA-based vaccine showed detectable IgA in their milk, with 25 of 26 (96%) and 37 of 38 (97%) after the BNT162b2 and MRNA-1273 vaccines, respectively.

"For participants who received a vector-based vaccine, this was remarkably lower, as only 13 of 33 (39%) and 10 of 21 (48%) showed detectable IgA after the AZD1222 and Ad26.COV2.S vaccines, respectively," according to the researchers, who added, "After both doses of BNT162b2, MRNA-1273, and AZD1222 vaccines, all participants showed detectable IgG. However, after the mRNA-based vaccines, this was on day 23 and day 32 after the first dose, while after the AZD1222 vaccine, all participants showed IgG at day 94. After vaccination with Ad26.COV2.S, which is only 1 dose, 6 of 23 (28%) participants had detectable IgG in their milk."

The authors noted that their findings demonstrated "that SARS-CoV-2–specific IgA in human milk was present more frequently after vaccination with an mRNA-based vaccine compared with a vector-based vaccine. Additionally, IgG was present in all participants after receiving 2 vaccine doses, independent of vaccine type. However, IgG was detectable earlier after vaccination with either of the mRNA vaccines, which can be explained by timing of the second dose."
The researchers advised that based on the data, an mRNA-based vaccine "is the optimal choice for lactating women when they want to transfer antibodies to their infants."

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