This is a summary, written by members of the CITF Secretariat, of:

Ismail S, Unger S, Budylowski P, Poutanen S, Yau Y, Jenkins C, Anwer S, Christie-Holmes N, Kiss A, Mazzulli T, Johnstone J, McGeer A, Whittle W, Parvez B, Gray-Owen SD, Stone D, O’Connor DL. SARS-CoV-2 antibodies and their neutralizing capacity against live virus in human milk after COVID-19 infection and vaccination: prospective cohort studies. The American Journal of Clinical Nutrition. 2023 Dec 14. doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study, published in The American Journal of Clinical Nutrition, reported that SARS-CoV-2 infection and vaccination affected the antibodies found in human milk. This held despite the type of variant causing the infection and the vaccine dose interval. However, the presence of antibodies in maternal milk did not consistently predict live virus neutralization in the baby. Although human milk is unequivocally the best way to nourish infants, on its own it may not alone protect infants from SARS-CoV-2 infection. Guidance on protection to infants through human milk feeding after maternal infection/vaccination may require more nuanced messaging. The study was led by Dr. Deborah O’Connor (University of Toronto) in collaboration with Dr. Allison McGeer (Sinai Health System).

Milk was collected from participants with SARS-CoV-2 infection during pregnancy or lactation (confirmed by qPCR or presumptive) (Cohort-1), as well as from milk bank donors (Cohort-2), from March 2020 to July 2021 at three sequential four-week intervals. Milk was also collected from lactating participants who were vaccinated against COVID-19 at varying dose intervals (Cohort-3) from January to October 2021. Cohort-1 and Cohort-3 were recruited from Sinai Health (patients) and through social media. Cohort-2 included Ontario Milk Bank donors. Milk was examined for SARS-CoV-2 antibodies and live virus neutralization.

Key findings:

  • In Cohort-1 (n=57)
    • Of females with COVID-19, 53% of all individuals and 67.5% of confirmed positive symptomatic individuals had anti-SARS-CoV-2 IgA antibodies in at least one milk sample.
    • Including samples across all time points, 40% of IgA positive samples were neutralizing compared with 25% of IgA negative samples (odds ratio [OR]: 2.18).
    • Both IgA positivity and neutralization decreased over time from symptom onset with neutralization decreasing from 38% to 20% (0–100 days compared with 201+ days: OR: 4.30).
  • In Cohort-2 (n=318)
    • No sample tested positive for SARS-CoV-2 RNA.
    • Excluding milk samples from milk bank donors following vaccination and/or a COVID-19 diagnosis or febrile illness of the participant or household contact, 4.3% had IgA antibodies and 23% of IgA-positive samples were neutralizing.
  • In Cohort-3 (n=78)
    • There was no statistically significant interaction between collection time and either vaccine type or time interval between doses. However, vaccination was associated with a small increase in milk neutralization capacity at week two after dose two compared with pre-vaccination levels.
    • Vaccination with Moderna resulted in higher IgA values than Pfizer at all time points after vaccination (P < 0.04) except day 7 after dose 1.
    • Shorter vaccine intervals (three to <six weeks) with Moderna resulted in higher IgG levels than longer intervals (six to <16 weeks) with Pfizer.
    • Neutralizing capacity overall was higher for Moderna compared with Pfizer (P < 0.001) and for individuals who received their first dose during lactation compared with pregnancy (P = 0.03).
    • Neutralizing capacity increased post-vaccination but was not associated with antibody positivity. There was no significant association between IgA or IgG positivity and neutralizing capacity (neutralizing compared with not neutralizing) following vaccination.

Human milk IgA can help neutralize the SARS-CoV-2 virus. However, antibodies may also be present in the milk of uninfected and unvaccinated lactating individuals, a finding that could be due to cross-reactivity or false-positivity. Factors such as vaccine type (Moderna versus Pfizer) and interval (short versus long) can influence antibody levels in milk, but their presence doesn’t guarantee virus neutralization. Therefore, guidance on infant protection through human milk feeding during COVID-19 and possibly future pandemics, requires nuanced messaging.