A review exploring molecular mimicry, potential autoimmune outcomes, and future directions in vaccine safety research
By Brian S.
Review of IgG cross-reactivity after COVID-19 vaccination: mechanisms, rare autoimmune risks, surveillance, and research needs.
Neutral stance note: This article raises scientific questions about a specific immunological mechanism — IgG cross-reactivity via molecular mimicry — without making claims beyond current evidence.
How IgG Cross-Reactivity Can Happen and Persist for Years
Molecular mimicry occurs when an immune response to a foreign antigen also targets self-proteins due to structural similarity. SARS-CoV-2 Spike protein shares certain peptide motifs with human proteins, which may result in cross-reactive IgG binding (Kanduc & Shoenfeld, 2020). This mechanism is also recognised in other viral and bacterial infections (Cusick et al., 2012).
Persistence of IgG is supported by evidence showing antibodies and memory B cells can last months to years post-vaccination (Goel et al., 2021). Even when titres drop, reactivation from other antigens or bystander effects may sustain cross-reactive antibodies.
Bystander activation and epitope spreading involve immune system stimulation leading to activation of autoreactive clones, broadening immune targets beyond the initial viral antigen (Vojdani et al., 2021).
Cross-reactive sources beyond the vaccine include microbiota and plant antigens with similar structural motifs to viral proteins (Li et al., 2023). These may interact with vaccine-induced immunity.
Potential Autoimmune Disorders Reported or Biologically Plausible
Reported post-vaccine conditions (rare, not necessarily causally proven) include:
- Myocarditis and pericarditis (Oster et al., 2022)
- Guillain-Barré syndrome (Patone et al., 2021)
- Immune thrombocytopenia (Lee et al., 2021)
- Autoimmune hepatitis (Bril et al., 2021)
- Small-vessel vasculitis, thyroiditis, systemic lupus erythematosus flares (Vojdani et al., 2021)
Who Might Be Most Susceptible
Risk factors include:
- Age/sex: Higher myocarditis rates in young males post-mRNA vaccine (Oster et al., 2022)
- Genetics: Certain HLA types associated with higher autoimmune risk (Cusick et al., 2012)
- Pre-existing autoimmunity or recent infection
- Hormonal influences: Sex hormones modulate immune responses differently in males and females.
Quantitative Perspective
With ~5.18 billion people fully vaccinated worldwide (WHO, 2024), estimates based on observed incidence suggest:
- 1 case/million → ~5,180 cases globally
- 10/million → ~51,800 cases
- 40/million (high subgroup rate) → ~207,200 cases
- 100/million (upper bound assumption) → ~518,000 cases
These figures are illustrative; most autoimmune events remain rare compared to the health impact of COVID-19 itself.
Why Many Cases May Be Missed
Reasons include nonspecific symptoms, long latency, complex serology, under-reporting, and strict causality standards in medical research (Black et al., 2009).
Benefits, Risks, and Future Outlook
While the mechanism of molecular mimicry is real, large-scale surveillance shows severe autoimmune events are rare. Continued monitoring, epitope mapping, and targeted risk mitigation could further improve safety.
References
Black, S., Eskola, J., Siegrist, C.A., Halsey, N., MacDonald, N., Law, B. and Miller, E., 2009. 'Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines'. The Lancet, 374(9707), pp.2115-2122.
Bril, F., Al Diffalha, S., Dean, M. and Fettig, D.M., 2021. 'Autoimmune hepatitis developing after coronavirus disease 2019 (COVID‐19) vaccine: Causality or casualty?'. Journal of Hepatology, 75(1), pp.222-224.
Cusick, M.F., Libbey, J.E. and Fujinami, R.S., 2012. 'Molecular mimicry as a mechanism of autoimmune disease'. Clinical Reviews in Allergy & Immunology, 42, pp.102–111.
Goel, R.R., Painter, M.M., Apostolidis, S.A., Mathew, D., Meng, W., Rosenfeld, A.M., Lundgreen, K.A., Reynaldi, A., Khoury, D.S., Pattekar, A. and Gouma, S., 2021. 'mRNA vaccines induce durable immune memory to SARS-CoV-2 and variants of concern'. Science, 374(6572), pp.abm0829.
Kanduc, D. and Shoenfeld, Y., 2020. 'Molecular mimicry between SARS‐CoV‐2 spike glycoprotein and mammalian proteomes: implications for the vaccine'. Journal of Autoimmunity, 111, p.102611.
Lee, E.J., Cines, D.B., Gernsheimer, T., Kessler, C., Michel, M., Tarantino, M.D., Semple, J.W., Arnold, D.M., Godeau, B., Lambert, M.P. and Bussel, J.B., 2021. 'Thrombocytopenia following Pfizer and Moderna SARS‐CoV‐2 vaccination'. American Journal of Hematology, 96(5), pp.534-537.
Li, X., Zhong, W., Wang, J., Wang, F., Xu, L. and Xu, X., 2023. 'Cross-reactivity of oral microbiota-induced antibodies with SARS-CoV-2 spike protein'. Frontiers in Immunology, 14, p.1122334.
Oster, M.E., Shay, D.K., Su, J.R., Gee, J., Creech, C.B., Broder, K.R., Edwards, K., Soslow, J.H., Dendy, J.M., Schlaudecker, E. and Lang, S.M., 2022. 'Myocarditis cases reported after mRNA-based COVID-19 vaccination in the US from December 2020 to August 2021'. JAMA, 327(4), pp.331-340.
Patone, M., Handunnetthi, L., Saatci, D., Pan, J., Katikireddi, S.V., Razvi, S., Hunt, D., Mei, X.W., Dixon, S., Zaccardi, F. and Shankar-Hari, M., 2021. 'Neurological complications after first dose of COVID-19 vaccines and SARS-CoV-2 infection'. Nature Medicine, 27(12), pp.2144-2153.
Vojdani, A., Kharrazian, D. and Vojdani, E., 2021. 'Reaction of human monoclonal antibodies to SARS-CoV-2 proteins with tissue antigens: implications for autoimmune diseases'. Frontiers in Immunology, 11, p.617089.
WHO, 2024. COVID-19 Dashboard. [online] Available at: https://covid19.who.int [Accessed 11 August 2025].
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