Evaluation of patients with suspected vaccine allergies in Singapore.

Background: Hypersensitivity reaction to vaccines has been reported to occur in 5 per 100,000 doses. Although hypersensitivity reactions can occur to either the active vaccine component or other components such as excipients, outcome data from skin testing and provocation remains limited.

Objective: To evaluate the role of skin testing and vaccine provocation in patients with an allergy label to vaccine.

Methods: This is a single centre, prospective study between March 2021 and November 2021 of adults with known allergy to non-COVID vaccine. All participants underwent skin prick testing (SPT) and intradermal testing (IDT) to vaccine and excipients. A subset of patients with negative skin testing underwent graded vaccine provocation.

Results: A total of 264 adults were evaluated. The most common index vaccine reactions were nonspecific rash (47.7%), angioedema (32.2%) and itch (25.0%). All patients had negative SPT to vaccines and excipients. Thirty patients (11.4%) had positive IDT to Hepatitis A, Hepatitis B, Human Papilloma Virus (HPV), Influenza, Measles-Mumps-Rubella (MMR), Pneumococcal, Rabies, Diphteria, Tetanus and Pertussis (DTaP). Out of 234 patients with negative IDT, 32 patients (12.1%) underwent vaccine provocation. Three patients (9.4%) developed reaction to influenza and MMR vaccine. One patient required systemic corticosteroids, one required antihistamine, and another patient did not require any treatment. None required admission or attendance at emergency department.

Conclusions: The majority of allergy labels to vaccine are inaccurate based on low skin test positivity and low reaction rates on vaccine provocation. Vaccine provocation is safe. Excipients are unlikely to be the main cause of hypersensitivity reactions in vaccines.