Newborns treated with antibiotics respond less well to vaccines, study shows

The Guardian

02-04-2025

Babies who are treated with antibiotics as newborns have reduced immune responses to vaccines in later infancy, likely due to changes in the gut microbiome, new research suggests.
The Australian study tracked 191 healthy babies from birth, finding that those who received antibiotics in the first few weeks of life had significantly lower levels of antibodies against multiple vaccines at seven and 15 months.
Of the babies, 111 were exposed to antibiotics in the newborn period – either through direct treatment (32 newborns), or indirectly through mothers who took antibiotics during labour (49) or the first six weeks postpartum (30).
The study, published in the journal Nature, assessed the infants' microbiomes around the time of their first routine vaccinations at six weeks. It found that the babies directly treated with antibiotics at birth had less of a group of beneficial gut bacteria known as Bifidobacterium.
The reduction in Bifidobacterium was linked to lower antibody levels against multiple components of the pneumococcal vaccine, as well as Haemophilus influenzae type b, at both seven and 15 months.
Prof David Lynn, a program director at the South Australian Health and Medical Research Institute, who co-led the research, said Bifidobacterium was commonly found in the healthy infant gut in the first weeks of life.
'They are particularly well adapted to metabolising human milk oligosaccharides [a type of carbohydrate] in breastmilk, and so you'll see high rates of those bacteria in infants that are breastfed,' he said.
Lynn, also a professor of systems immunology at Flinders University, said the bacteria gave the immune system 'an extra kick', priming it for an optimal antibody response to vaccines.
'What we think is probably most important is the composition of the microbiome around the time of vaccination,' he said. 'Antibiotics can disrupt the normal colonisation by bifidobacteria and allow other types of bacteria to colonise the intestine instead.'
The research also studied immunity in germ-free mice, finding that antibody responses to the pneumococcal vaccine strongly depended on the presence of Bifidobacterium and improved when the mice were given a probiotic containing the bacteria.
The study, involving researchers from 12 institutions across Australia, did not find reduced vaccine responses in babies whose mothers received antibiotics during labour, suggesting direct antibiotic treatment in newborns had a more persistent impact on the gut microbiome.
Lynn said the study results should not 'unduly worry' parents of newborns treated with antibiotics. 'There's usually very good reason for giving the neonates those antibiotics, given that infections and sepsis in that critical early life period can be very serious.'
The antibiotic-treated newborns still mounted 'pretty decent responses to all of the vaccines,' he said. 'Around that seven-month time point, most of the infants are above what's called the seroprotective threshold, so they will be expected to be protected against infection.'
'What does seem to happen is that, over time, those responses wane a bit quicker in the infants that directly have antibiotics.'
The study had a 'relatively modest' sample size, the authors noted, and did not include babies who had been born via Caesarean section.
In the coming months, the researchers will commence a clinical trial to test whether giving antibiotic-treated newborns a probiotic containing Bifidobacterium would improve antibody responses to routine vaccinations.
The probiotic is safe and already widely used in hospitals to protect preterm infants against a condition called necrotising enterocolitis.