Antibodies in Human Milk After COVID-19 Vaccine
by Anne Eglash MD, IBCLC, FABM
This is probably a harder question to answer than whether antibodies to the vaccine are found in the milk of lactating individuals who receive the mRNA COVID-19 vaccine.
At this moment, 2 studies have demonstrated antibodies in the milk of lactating individuals who have received either the Pfizer or Moderna mRNA COVID- 19 vaccine. One of the studies documented antibody responses in the milk of 6 women and has not been peer reviewed yet, so today’s CQW will focus on the study published this week online in the American Journal of Obstetrics and Gynecology.
The study enrolled 84 pregnant, 31 lactating, and 16 non-pregnant women, most of whom were white, non-Hispanic healthcare workers. Among the lactating women, blood and breastmilk samples were collected at the time of the first and second vaccine, and 2-6 weeks after the second vaccine.
The study participants who received their vaccines during pregnancy had their umbilical cord blood collected at delivery, but not their breastmilk samples.
They found IgA and IgG antibodies in the milk of lactating women who received the COVID-19 vaccine with the bulk of the antibodies being IgG. In contrast, IgA make up the bulk of the antibodies found in milk among lactating women with natural SARS-CoV-2 infection.
- The increases in IgA, IgG, and IgM antibodies in breastmilk reflected the increased levels of these antibodies in the mothers’ blood.
- The antibody responses to the mRNA vaccines in all subjects were higher than what has been measured in pregnant women with natural infection.
- Giving the second vaccine did not increase the antibody response beyond the levels measured after the first vaccine.
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Pregnant and lactating women were excluded from initial COVID-19 vaccine 88 trials; thus, data to guide vaccine decision-making are lacking.
To evaluate the immunogenicity and reactogenicity of COVID-19 mRNA 91 vaccination in pregnant and lactating women compared to: (1) non-pregnant controls and (2) 92 natural COVID-19 infection in pregnancy.
131 reproductive-age vaccine recipients (84 pregnant, 31 lactating, and 16 non-pregnant) were enrolled in a prospective cohort study at two academic medical centers. Titers of SARS-CoV-2 Spike and RBD IgG, IgA and IgM were quantified in participant sera (N=131) and breastmilk (N=31) at baseline, second vaccine dose, 2-6 weeks post second vaccine, and at delivery by Luminex. Umbilical cord sera (N=10) titers were assessed at delivery. Titers were compared to those of pregnant women 4-12 weeks from natural infection (N=37) by ELISA. A pseudovirus neutralization assay was used to quantify neutralizing antibody titers for the subset of women who delivered during the study period. Post-vaccination symptoms were assessed via questionnaire. Kruskal-Wallis tests and a mixed effects model, with correction for multiple comparisons, were used to assess differences between groups.
Vaccine-induced antibody titers were equivalent in pregnant and lactating compared to non-pregnant women (median [IQR] 5.59 [4.68-5.89] pregnant, 5.74 [5.06-6.22] lactating, 5.62 [4.77-5.98] non-pregnant, p = 0.24). All titers were significantly higher than those induced by SARS-CoV-2 infection during pregnancy (p < 0.0001). Vaccine-generated antibodies were present in all umbilical cord blood and breastmilk samples. Neutralizing antibody titers were lower in umbilical cord compared to maternal sera, although this finding did not achieve statistical significance (median [IQR] 104.7 [61.2-188.2] maternal sera, 52.3 [11.7-69.6] cord sera, p=0.05). The second vaccine dose (boost dose) increased SARS-CoV-2-specific IgG, but not IgA, in maternal blood and breastmilk. No differences were noted in reactogenicity across the groups.
COVID-19 mRNA vaccines generated robust humoral immunity in pregnant and lactating women, with immunogenicity and reactogenicity similar to that observed in non-pregnant women. Vaccine-induced immune responses were significantly greater than the response to natural infection. Immune transfer to neonates occurred via placenta and breastmilk.
The first dose of the mRNA COVID-19 vaccine increased IgA in breastmilk, and the second (booster) vaccine did not increase the IgA levels further but did significantly increase the IgG levels. The levels of IgA and IgG in breastmilk reflected the balance of immunoglobulins in the blood stream.
So, what does this mean for the immune protection of the infant who is either nursing or receiving human milk from the vaccinated parent? We don’t know!
We do know that when a lactating parent becomes ill from a virus or bacteria, IgA is generated from their immune system in the gut and the IgA transfers into the breastmilk. The IgA is swallowed by the nursing or human milk-fed infant, coating the inner linings (mucosa) of organs such as the gut, eustachian tubes, nose, lungs, throat, bladder, etc. IgA is a strong mucosal defender, working along with other bioactive factors in human milk to prevent pathogens from invading into the infant’s organs, hence preventing infection. The immune response to the vaccine is largely IgG. Although the IgG is found in the breastmilk, we don’t know to what extent IgG acts at the mucosal lining. Will it protect the infant from infection, like IgA does? We don’t have an answer to that question.