Adding a probiotic supplement to formula milk slashed rates of respiratory infections in babies who weren’t breastfed. Infections are more common in formula-fed babies, but the supplement – which contained both sugars from breast milk and the bacteria needed to ferment them – produced fatty molecules that seemed to strengthen the infants’ immune systems. This helped them eliminate viruses that cause symptoms like sneezing and a runny nose.

“It’s phenomenally exciting,” says Daniel Davis at Imperial College London, who wasn’t involved in the research. A high burden of early-life infections has been linked to more moderate to severe infections in later life and an increased risk of allergies. “Modifying formula milk to help set up good immune health for babies is a really powerful concept,” says Davis.

However, such interventions could have unintended consequences, including actually increasing the risk of allergies. “If you increase the power of someone’s immune response against infections, that might make them more likely to be susceptible to allergies or autoimmunity,” says Davis, who learned about the supplement at the Immunology of the Exposome conference at the Royal Society in London on 12 October. “These interventions are likely to have both pros and cons; we don’t know what those all are yet.”

Prior trials suggest that supplements containing sugars from breast milk, called human milk oligosaccharides (HMOs), cut rates of respiratory infections in such infants. These sugars are fermented by gut bacteria, including Bifidobacterium longum infantis, to produce short-chain fatty acids, molecules that seem to enhance immune responses against pathogens like viruses and bacteria.

But some babies, especially those in Western nations, lack B. in their guts, which may be related to their mother’s diet. Previously, Petter Brodin at Imperial College London and his colleagues found that supplements that boost levels of this bacterium seemed to improve immune health in breast-fed babies.

Now, Brodin’s team has developed a supplement containing both HMOs and B. longum infantis and tested it on formula-fed babies, who are more likely to lack B. longum infantis.

With funding from Nestlé, which develops formula milk, the researchers recruited 223 infants – aged between 2 and 5 weeks – who were exclusively formula fed in Manila, the Philippines. About half of the infants were randomly assigned to drink formula milk made from a powder containing the probiotic supplement, while the remaining infants consumed the same milk without the supplement.

The parents – who were blinded as to which group their baby was in – logged whenever they showed symptoms of respiratory infections, such as sneezing, coughing or a runny nose, until they were 6 months old.

During this time, 23 per cent of the infants in the supplement group were recorded as developing an infection, compared with 40 per cent in the formula-only group. The former infants also recovered a few days faster from the infections, on average. “The respiratory infections were fewer and their duration was shorter,” says Brodin.

Further analysis of stool samples revealed that the supplement seems to boost levels of B. longum infantis in the gut, while reducing inflammation and supporting immune responses against pathogens.

The researchers need to carry out large trials that track the supplement’s effects in different countries over time, says Davis. Brodin says the team is planning to explore how it affects the risk of allergies and asthma.