Researchers in Australia found that 49 patients who had previously been diagnosed with an allergy to the flu vaccine were actually misdiagnosed with anaphylaxis.

According to the authors of a research letter published today in the Medical Journal of Australia, some adverse reactions to influenza vaccines may be safe to de-label from “allergies” to “stress-related responses” because they do not fulfill the criteria for anaphylaxis.

“As it can be difficult to distinguish influenza vaccine-associated anaphylaxis — estimated by one American study to affect 1.35 people per one million doses — from the more numerous immunisation stress-related responses —affecting 4–7% of influenza vaccine recipients — the latter may be misdiagnosed as allergies,” say the authors, led by Dr Beau Carr, from Monash Health.

Carr and colleagues examined the clinical records of all people (18 years and older) with influenza vaccination allergies who visited the Monash Health adult vaccine allergy service between April 1, 2017 and August 31, 2021.

The researchers noted that seven of the 49 subjects had index reactions that “met the Brighton criteria for anaphylaxis; the most frequent symptoms were dermatologic (70%) or respiratory reactions (57%)”.

After split-dose (10 participants) or full-dose (39 participants) challenges, 20 individuals had symptoms consistent with immunization stress-related responses, but none fulfilled the Brighton criteria for anaphylaxis.

“Thirteen of the 20 were de-labelled because their symptoms were mild; the other seven were also de-labelled after challenge with a different influenza vaccine the following year.”

According to the data, influenza vaccine allergy may be overdiagnosed because illnesses that mimic vaccine anaphylaxis are more common than anaphylaxis itself.

Despite recommendations from the World Health Organization, Carr and colleagues found that it was challenging to distinguish “between anaphylaxis and an acute stress response in acute health care”.

The study authors recommend that “reactions be treated as allergic if clinically suspected, but also that the patient be promptly referred to an allergist for further assessment. It will probably be safe to de-label many patients because their reactions do not meet anaphylaxis criteria.”

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