Clinical bottom line
Allergic reactions to COVID-19 protein subunit vaccines, such as ZF2001, are rare but clinically important to recognize and manage. Current evidence suggests these vaccines have a favorable safety profile with fewer immediate hypersensitivity events compared to mRNA vaccines. However, clinicians should remain vigilant for immediate allergic reactions, including anaphylaxis, particularly in patients with a history of vaccine or excipient allergies. Diagnosis relies on clinical assessment supported by skin testing and serum markers when available. Management follows established protocols for vaccine-related allergic reactions, emphasizing prompt recognition and treatment. Ongoing research is needed to clarify immunologic mechanisms and optimize risk stratification.
What the evidence shows
Protein subunit vaccines against SARS-CoV-2, such as ZF2001, utilize recombinant spike protein fragments combined with adjuvants to elicit immunity (Chen et al., 2023). Compared to mRNA vaccines, these formulations appear to have a lower incidence of immediate allergic reactions, including anaphylaxis, although direct head-to-head comparative data remain limited (Chen et al., 2023; Lorenz et al., 2025). The allergenic potential is thought to be related primarily to vaccine excipients rather than the protein antigen itself (Moghimi et al., 2021).
Reports of allergic reactions to protein subunit vaccines have emerged but remain rare relative to the large number of doses administered globally (Reactions Weekly, 2023). Clinical presentations range from mild urticaria and angioedema to severe anaphylaxis. Wang et al. (2023) characterized immune profiles in patients with immediate allergic reactions to COVID-19 vaccines, noting distinct patterns of immune activation and autoimmunity that may inform diagnostic approaches.
Diagnostic recommendations emphasize detailed clinical history, skin prick and intradermal testing with vaccine components when feasible, and measurement of serum tryptase or other mast cell activation markers during acute reactions (Maki et al., 2021). These approaches help differentiate true IgE-mediated allergy from non-allergic hypersensitivity or anxiety-related responses.
Management aligns with general principles for vaccine-induced allergic reactions: immediate administration of intramuscular epinephrine for anaphylaxis, supportive care, and observation. For patients with confirmed or suspected allergy to protein subunit vaccines, referral to an allergist for further evaluation and consideration of alternative vaccine platforms is advised (Maki et al., 2021; Lorenz et al., 2025).
Caveats and uncertainty
Data on allergic reactions to protein subunit COVID-19 vaccines remain limited compared to mRNA and viral vector vaccines, reflecting their more recent introduction and lower global usage. Most available evidence derives from case reports, pharmacovigilance databases, and small observational cohorts, which may underreport or misclassify events (Reactions Weekly, 2023).
The immunologic mechanisms underlying allergic reactions to protein subunit vaccines are incompletely understood. While excipients such as polysorbates are suspected triggers, definitive causal links and biomarkers are lacking (Moghimi et al., 2021). The role of autoimmunity and non-IgE-mediated pathways requires further elucidation (Wang et al., 2023).
Diagnostic testing protocols are not standardized for protein subunit vaccines, and skin testing reagents are not widely available or validated. False positives and negatives complicate interpretation, limiting clinical utility (Maki et al., 2021).
Finally, the risk-benefit balance of vaccination in patients with prior allergic reactions to COVID-19 vaccines or components must be individualized, considering local epidemiology and patient preferences.
How this may change practice
Clinicians should be aware that protein subunit COVID-19 vaccines represent an alternative platform with potentially lower allergenic risk compared to mRNA vaccines. This knowledge may inform shared decision-making for patients with prior vaccine allergies or concerns about mRNA vaccine components.
Allergy evaluation protocols should incorporate emerging data on protein subunit vaccines, including consideration of skin testing and immune profiling where available. Prompt recognition and management of allergic reactions remain essential to ensure patient safety and maintain confidence in vaccination programs.
Referral pathways to allergy specialists for complex cases or suspected vaccine allergy should be established. Clinicians should stay updated on evolving evidence and guidelines as more data on protein subunit vaccine safety and allergy emerge.