As a neonatal nurse specialist, mothers often ask me about the antibodies found in breast milk and how they can work to protect their babies. To explain it to them, and to you, I have to delve into immunology, and those conversations are usually too technical for most readers and mothers I meet in the NICU. Unfortunately, the information that is readily available to mothers that isn’t highly technical is a large body of conflicting information. So I’m here to talk more about this very important topic in a way that makes it straightforward! My goal is to answer very common reader questions like these:

“Why does my infant need any vaccines at all since she’s getting all the antibodies she needs from my breast milk?” and,

“Why does my seven-month old son need to get a flu shot if I received it during my pregnancy?”

Both, excellent questions! Parents, take note – there are 2 ways babies acquire immunity: through passive transfer, and active transfer (also known as acquired immunity).

Passive Immunity During Pregnancy 

The first way for a baby to acquire immunity is before and just after birth, and it is called passive immunity. When a mother is pregnant, antibodies are produced by the mother’s immune system and then transported across the placenta to the fetus’s blood supply.  These types of antibodies are called immunoglobulin g. You’ll see these abbreviated as “IgG,” and they are the only antibody type that passes through the placenta to the growing fetus. They are called passive maternal IgGs, because of what they are and how they’re transferred to the baby.

IgGs are the most common type of antibody in our bodies and protect us – as well as unborn babies – from viral and bacterial illness (you can learn more about antibody types by clicking here). Human babies are born with all of the passive maternal IgG antibodies they will ever have, regardless of their feeding method.

Maternal IgG antibodies are temporary, however, and gradually disappear within about 6-8 months after birth. Fortunately, after birth the baby begins to make their own IgG antibodies in response to viruses and bacteria in their environment. By six months, just when the mother’s IgG antibodies are no longer present, a health baby produces their own IgG antibodies at normal levels, due to their immune system maturing.

Passive Immunity Through Breast Milk


Breast milk provides IgA antibodies for newborns (courtesy: Amanda Simons)

The second way a baby can acquire passive immunity is through breast milk. Colostrum is the first milk a woman produces when she begins to breastfeed, and it contains large numbers of antibodies called secretory immunoglobulin. You’ll see this abbreviated as IgA (first we discussed IgG, and now, IgA).  Human babies are not able to absorb these passive maternal antibodies from milk or colostrum directly into their bloodstream. Instead, the IgA antibodies in milk and colostrum protect against infection by working inside the baby’s gastrointestinal tract, a clear protective role of human milk, which you can read more about here. These IgA antibodies penetrate and protect the mucous membranes in the baby’s mouth, airway, throat, and intestines.This passive breastfeeding IgA immunity continues until a baby is weaned.

This protection is invaluable to a newborn, and is absolutely necessary in developing countries where access to clean water is problematic. However, it only protects against infection through their digestive tract; it does not provide bloodstream antibodies to protect against various vaccine-avoidable illnesses, which is something that only the IgGs provide (see above).

So to answer our previous question,”why does my infant need any vaccines at all since she’s getting all the antibodies she needs from my breast milk”? The answer is, the antibodies we get from vaccinating ourselves against deadly illnesses aren’t the type that are transmitted through breast milk.

This means to best protect infants from illnesses, mothers should strongly consider following vaccination recommendations during and after pregnancy, to provide critical IgG antibodies to children.

Strengthen Passive Immunity By Getting Vaccinated During Pregnancy

Getting vaccinated in the 2nd and 3rd trimester improves an unborn babys immunity

Getting vaccinated in the 2nd and 3rd trimester improves an unborn baby’s immunity

Low levels of IgG antibodies are transferred from mother to baby starting in the early second trimester of pregnancy (between 13-27 weeks), and reach their peak transfer in the third trimester. Preemies, for example, are more vulnerable to infection and illness, because they were not able to gain the maximum level of maternal passive immunity available because they were born earlier.

Mothers can be vaccinated during pregnancy (or given booster shots) in order to produce IgG antibodies that transfer to their baby through the placenta, and protect the baby after birth.

Key vaccinations that a mother should consider getting during her pregnancy include the following:

Tdap Vaccine – Covering Tetanus, Diphtheria, and Pertussis (Whooping Cough):  The Tdap vaccine is given in the third trimester, between 27 and 36 weeks, both to prevent mothers from contracting pertussis and to produce IgG antibodies which help protect babies after delivery. You can listen to an example of what an infant sounds like with whooping cough here, to get an idea of why prevention is so critical.

Click on the photo for the CDC's Immunization & Pregnancy Guide

Click on the photo for the CDC’s Immunization & Pregnancy Guide

Influenza Vaccine (“The Flu Shot”): The influenza vaccine is also recommended during pregnancy.  Unlike Tdap, the flu shot can be given at any time during pregnancy, but especially when it first becomes available, as it is released seasonally in late summer and changes each year. The flu shot is an important one to get because pregnant women are more likely to get severely ill if they contract the flu while pregnant, and also because pregnancy complications, such as preterm birth and low birthweight are more common in women who contract influenza while pregnant. The IgG protection passed from mother to fetus also benefits newborns whose mothers received their flu shot while pregnant; some studies suggest a 50% lower risk of hospitalization for influenza in babies whose mothers were vaccinated during pregnancy compared to babies of unvaccinated mothers.

Any Remaining Childhood Vaccines: For mothers who have not had all of their routine vaccinations, it is important to speak to your physician directly about what shots can and cannot be done during your pregnancy, and complete them as appropriate. Prior vaccinations still within their period of effectiveness will provide unborn children the passive IgG maternal immunity, so a catch up plan either prior to or during pregnancy may be needed for missing or expired vaccinations.

Active (Acquired) Immunity

Finally, a baby’s immune system is at its weakest right after birth. Since passive immunity from both forms of maternal immunity is temporary, and breastfeeding antibodies can only protect within the GI tract while breastfeeding is occuring, these measures are not enough to fully protect a child from infectious disease beyond the first 5-6 months of their life. At 6 months, a baby’s IgG antibodies – acquired passively from their mother – are gone. Their immune systems have begun to mature and if they are healthy, they are producing their own IgG antibodies from the germs they are encountering in their world. This is what is known as active or acquired immunitythe type of immunity that your body develops only after you’ve been exposed to a virus or germ.


Active immunity is the only way to protect children from infectious disease past 6 months of age (Courtesy: CDC)

To continue the process of protection, they need to actively acquire immunity, and fortunately, vaccination is a safe and effective way to achieve it.

When I’m asked questions such as,”why does my seven-month old son need to get a flu shot if I received it during my pregnancy”? My answer is, because your flu shot can no longer protect him. At seven months old, this child no longer has passive immunity from his mother.

Vaccines are tested again and again to be sure that they are safe for children and for nursing mothers. If you are concerned about whether or not a particular vaccine is safe to receive while breastfeeding, check the CDC’s list of vaccines that are safe for nursing women and babies. And finally, during the early months prior to receiving vaccinations, babies are counting on their parents, family, friends, caregivers, doctors, nurses, lactation consultants and anyone them to protect them from diseases. Making sure that everyone is up-to-date with their recommended vaccines is the best way for a community to support a newborn’s health.

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Categories: Ages + Stages, Food, Nutrition, + Infant Feeding, Infectious Disease + Vaccines, Newborns + Infants