A study to be presented at the 2026 American Academy of Allergy, Asthma & Immunology (AAAAI) Annual Meeting found that younger siblings of children who regularly consume peanuts were more likely to show higher rates of peanut sensitization and allergy risk. The findings, drawn from an analysis of the landmark Learning Early About Peanut Allergy (LEAP) trial, suggest that higher levels of peanut protein in the home environment may increase the likelihood of sensitization if an infant is exposed to the allergen but does not consume it directly.
The research was led by Dr Gideon Lack of King’s College London, who described the phenomenon as an “unintentional randomized trial.” The study followed 298 younger siblings of LEAP participants and found that peanut sensitization and allergy risk were independently associated with increased family peanut consumption. In other words, younger siblings of children in the peanut-consuming group showed higher rates of sensitization compared to siblings of children who avoided peanuts.
Among younger siblings of older siblings who avoided peanuts, the rate of peanut sensitization was 20%. That figure rose to 30.4% among siblings of peanut consumers. Researchers note that this difference was statistically significant. The findings support the “dual allergen exposure hypothesis,” which proposes that environmental exposure to peanut protein—such as through skin contact or inhalation—may promote sensitization, whereas early oral consumption promotes immune tolerance.
Importantly, early dietary introduction appeared to mitigate this risk. For younger siblings of peanut consumers, those who were introduced to peanuts within the first year of life had a sensitization rate of 18.3%. In contrast, the rate increased to 44.4% among those who did not consume peanuts during that time. As Dr Lack emphasized during the presentation, the effect was more apparent among younger siblings who did not eat peanuts in the first year of life.
A similar pattern was observed for confirmed peanut allergy. Among younger siblings of peanut consumers who did not eat peanut products early, 17.9% developed peanut allergy, compared to just 3.7% of those who were introduced early. These findings suggest that increased environmental peanut exposure in the household may elevate risk for infants who are not yet consuming peanut themselves, but that early, intentional introduction substantially reduces that risk.
According to the researchers, the investigation was prompted by reports from parents during the original LEAP trial who noted allergic reactions in younger children. This led the team to examine whether higher household peanut exposure could be associated with sensitization and allergy in younger siblings.
Overall, the study reinforces current medical guidance supporting early, intentional introduction of peanut in infancy, particularly in households where peanut is already being consumed. The findings underscore that environmental exposure alone may increase risk, but oral consumption during infancy remains a key strategy for promoting long-term tolerance.
