It's vaccination day and you're focused on the needle, the cry, the comfort after. You're relieved to know that your little darling will now have some protection and build immunity against certain diseases. You did the hard part, right? (They did the crying part, but still.)
Except the real story of whether that vaccine actually works started weeks ago, in a place you can't see, doing invisible work you would never imagine. It started in their gut.
A 2025 landmark study published in Nature found that the bacteria living in your baby's intestines play a significant role in how well their body responds to vaccines. Their gut bacteria are shaping the immune response from the inside.
The key player is called Bifidobacterium, a group of friendly bacteria that live in your baby's gut. They help break down food and produce compounds that train the immune system. Think of them as a coach: they show the immune cells what to pay attention to, so when a vaccine arrives, the body knows what to do with it. (If you've read our article on Baby's First Ecosystem, you've already met these bacteria. This is what they've been building towards.)
Based on Ryan et al. (2025): the microbiota-immune-vaccine connection in infants
Researchers followed 191 healthy babies from birth to 15 months, measuring their gut bacteria and their antibody levels after vaccinations. The pattern was clear: babies who had received antibiotics in their first weeks of life had significantly lower antibody levels at 7 months and again at 15 months. The antibiotics had reduced their Bifidobacterium levels. With fewer of those bacteria present at the time of vaccination, the immune response was weaker.
In controlled lab studies, the connection held up further. When researchers supplemented Bifidobacterium in subjects with depleted gut bacteria, vaccine responses improved.
Breast milk contains compounds called Human Milk Oligosaccharides (HMOs), specialised carbohydrates that exist to feed Bifidobacterium. Your baby can't digest them. They're food for the bacteria. (If you've read Your Milk Has a Clock, you already know breast milk is doing more than feeding your baby. This is another layer of that same story.)
Across populations where breastfeeding rates remain high, Bifidobacterium dominance in the infant gut is the norm. In populations where breastfeeding stops earlier, researchers see lower levels. This difference shows up by as early as the first month of life.
Based on research on HMOs and Bifidobacterium colonization: the global breastfeeding difference
For formula-fed babies, some modern formulas now include added HMOs or prebiotic compounds designed to support Bifidobacterium growth. The research in this area is still evolving, but it's worth knowing that this is something formula manufacturers are actively working on.
If you're breastfeeding, you're already doing the thing. Your milk is actively building your baby's immune system through the bacteria it feeds. There's nothing you need to add or change. The HMOs are working.
If antibiotics were necessary early on, know that this can be recovered. One course of neonatal antibiotics doesn't doom your baby's immune system, but it does mean Bifidobacterium has to recolonize. If you're breastfeeding afterward, the HMOs in your milk actively support that recolonization. Continued breastfeeding supports that recovery over time.
If you're formula-feeding or have questions about your baby's gut health, talk to your paediatrician. Every baby's situation is different, and your doctor can help you understand what's relevant for yours.
It's amazing how sophisticated our bodies are. Every little thing designed to help our little ones thrive. So next vaccination day, you'll know in your gut that the real work started long before the needle. It started in theirs.