The severity of a first anaphylactic reaction doesn’t reliably predict how severe the next one will be. The next reaction could be more severe, milder or about the same as before, research shows.
A research team reviewed 12 years of medical data on children who were treated at two Quebec hospitals. They identified 142 patients who had been treated for two separate food-triggered anaphylactic reactions. They then categorized anaphylactic reactions as mild, moderate of severe based on symptoms.
About 57 percent of second anaphylactic reactions to the same allergen had a similar severity to the first reaction. About 23 percent of subsequent reactions were more severe than the first, while just under 20 percent were less severe.
Anaphylaxis severity “was unpredictable over time,” says Joseph Najem, first author of the study and a medical student at McGill University in Montreal. “An individual could have a mild reaction to a food the first time, but have a moderate or severe reaction the second time.”
The reverse also occurred: some children with severe or moderate reactions the first time experienced milder symptoms the next.
Accidental exposures to peanut, tree nut, milk, eggs and fish were the most common allergens that sent kids to the emergency departments at Montreal Children’s Hospital and the Hôpital Sacré-Coeur.
Overall, about 80 percent of reactions among children coming to the two emergency departments were classified as moderate or severe, Najem says. The average age at the first reaction was age 4.5 years, and 61 percent were boys.
On average, 17 months passed between the children’s first and second anaphylactic reactions.
The study team included 13 Canadian hospitals and institutes. Researchers did not know the dose, or amount of allergen, consumed in each case. Nor did they know how doses differed between each patient’s two reactions.
Anaphylaxis Symptoms Tend to Repeat
One consistent pattern did emerge. Symptom type tended to repeat. If a child experienced skin, respiratory, gastrointestinal or cardiovascular symptoms during their first reaction, they were likely to have similar symptoms in the next reaction.
For instance, about 73 percent of children with skin symptoms such as hives and itch the first time had them again. About 84.5 percent of those with respiratory symptoms during their first reaction also had them during the second reaction.
Najem presented the study at the 2026 American Academy of Allergy, Asthma and Immunology annual meeting.
In this study, anaphylaxis was defined as involving at least two of skin, gut, respiratory or cardiovascular symptoms after allergen exposure. Or it could be faintness on its own, due to low blood pressure.
The unpredictability of anaphylaxis severity mirrors a well-known older study. In a report of 48 cases of fatal anaphylaxis between 1999 and 2006 in the United Kingdom, over half of food‐related fatalities occurred in people whose previous reactions had been mild.
“It definitely suggests the importance of having epinephrine on hand. A mild reaction might not happen the second time,” Najem says. “You can’t assume.”
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