What does it mean if breastmilk has anti-coronavirus antibodies? Not much!

Types of Antibodies. immunoglobulin structure

The lactivist community is obsessed with the idea that breastmilk might contain antibodies against the virus that causes COVID-19.

[Rebecca] Powell is an assistant professor of medicine and infectious diseases at Mount Sinai’s Icahn School of Medicine who studies the immune properties of human breast milk.

Her lab is hoping to pin down whether breast milk has antibodies specific to COVID-19, whether they might protect babies from COVID-19, and ultimately, whether they can be spun into a therapy against the illness for adults.

They’ve released their results as a pre-print, Evidence of a significant secretory-IgA-dominant SARS-CoV-2 immune response in human milk following recovery from COVID-19, a paper that has NOT been reviewed by other scientists:

There’s not much practical significance to a breastmilk antibody that reduces the risk of an already rare disease by only 8%.

In this preliminary report, 15 milk samples obtained from donors previously-infected with SARS-CoV-2 as well as 10 negative control samples obtained prior to December 2019 were tested for reactivity to the Receptor Binding Domain (RBD) of the SARS-CoV-2 Spike protein by ELISAs measuring IgA, IgG, IgM, and secretory Ab. Eighty percent of samples obtained post-COVID-19 exhibited IgA reactivity, and all these samples were also positive for secretory Ab reactivity, suggesting the IgA is predominantly sIgA. COVID-19 group mean OD values of undiluted milk were significantly greater for IgA (p<0.0001), secretory-type Abs (p<0.0001), and IgG (p=0.017), but not for IgM, compared to pre-pandemic group mean values. Overall, these data indicate that there is strong sIgA-dominant SARS-CoV-2 immune response in human milk after infection in the majority of individuals, and that a comprehensive study of this response is highly warranted.

Or as a pediatrician opined on Facebook:

Not only does the act of breastfeeding likely protect baby, but a potent antibody response within the breast milk could be even more effective than plasma or immunoglobulin infusions as a therapy for active COVID-19 infection!

Let’s assume for the moment that the results are true and breastmilk contains secretory IgA against COVID-19. What does it really mean? Not much!

We already know that breastmilk contains secretory IgA against respiratory and diarrheal illnesses. And we know that the secretory IgA reduces those illnesses by a — wait for it — only 8%! That’s not especially meaningful for the common cold; the antibodies DON’T prevent babies from getting the common cold (as any breastfeeding mother could tell you). They simply reduce the incidence by only 8%.

For an illness like COVID-19, where the incidence among infants is already low, the impact is likely to be negligible or even unmeasurable.


Because IgA is a subtype of antibody and not a particularly effective one. It is very different from what most people think about when they think about antibodies.

The most powerful antibodies against disease — the antibodies that you make if you are infected or vaccinated against the majority of childhood diseases — are IgM and IgG. IgM and IgG circulate in the bloodstream and seek out a specific virus or bacterium to tag it for destruction by white blood cells. These antibodies are so effective that they can be harvested in the plasma of people who have recovered from a disease in order to passively protect people who can’t make enough antibody on their own.

Secretory IgA, in contrast, acts on internal surfaces of the body like the respiratory and gastrointestinal tracts.

Secretory IgA (SIgA) plays an important role in the protection and homeostatic regulation of intestinal, respiratory, and urogenital mucosal epithelia separating the outside environment from the inside of the body. This primary function of SIgA is referred to as immune exclusion, a process that limits the access of numerous microorganisms and mucosal antigens to these thin and vulnerable mucosal barriers.

Secretory IgA reduces the risk that a virus or bacterium will gain entry to the body, but doesn’t act in the bloodstream where the organism wreaks its havoc. To use an analogy, if IgG and IgM are guns then secretory IgA is a fence. Fences are useful but they don’t do much to protect you once the invader has scaled the fence and dropped over the other side. Only a weapon could possibly protect you then.

That’s also why breastmilk is not protective against the majority of childhood diseases. While IgG can be transferred to a baby across the placenta, (hence vaccinations for mothers in the last trimester of pregnancy) it can’t be effectively transmitted in breastmilk because it will be digested in the baby’s stomach.

Moreover, to my knowledge, unlike IgM and IgG — which can be used to provide passive immunity in someone who can’t make or hasn’t yet made their own antibodies — IgA has NEVER been used effectively to provide passive immunity to anyone for anything. To put it another way, if secretory IgA were a useful therapy, we already would have used it to prevent the common cold and that hasn’t happened.

The bottom line is this: even if breastmilk contains antibodies to COVID-19, the practical significance is likely to be low and the possibility of using it as a therapy is vanishingly small. It’s just another example of breastfeeding researchers touting ever more arcane theoretical benefits of breastfeeding that make no difference in reality.