What: Low platelet counts leading to bleeding/bruising risk.
Which vaccines: Reports have appeared across vaccine types; some cases were transient.
Anaphylaxis / Severe allergic reaction
What: Rapid onset allergy (hives, breathing difficulties, hypotension).
Which vaccines: Reported with mRNA and other vaccines but overall extremely rare.
How common: Rare — vaccination sites monitor recipients for 15–30 minutes and are prepared to treat anaphylaxis immediately. PMC
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Bell’s palsy (facial nerve palsy)
What: Sudden facial weakness on one side.
Which vaccines: Small signal noted in early mRNA vaccine trials and some surveillance, though causal link uncertain.
How common: Very rare; most cases recover. PMC
What: Inflammation of the spinal cord causing weakness, sensory changes.
Which vaccines: Reported as single cases/rare clusters in early surveillance; signal is low and investigations continue.
How common: Extremely rare. Taylor & Francis Online
Arrhythmias and palpitations (non-myocarditis cardiac events)
10. Arrhythmias and palpitations (non-myocarditis cardiac events)
What: Heart rhythm disturbances post-vaccination sometimes reported; many are benign and transient.
Which vaccines: Reported across types in surveillance data.
Multisystem inflammatory syndrome (MIS)-like presentations
What: Hyperinflammatory syndrome (more commonly reported after infection, rare after vaccination).
Which vaccines: Mostly linked to SARS-CoV-2 infection; very rare vaccine-associated reports exist and are investigated.
How common: Extremely rare. PMC
Appendicitis (signal in some early datasets)
What: Appendicitis was flagged in some early surveillance as a potential signal, though later evidence did not confirm a causal link.
Which vaccines: Mixed/uncertain.
How common: No consistent increased incidence established — area of ongoing study. Taylor & Francis Online
Autoimmune or auto-inflammatory flares (e.g., IBD, rheumatologic reactions)
What: Flare-ups of preexisting autoimmune conditions have been reported anecdotally; studies show most people with autoimmune disease tolerate vaccines well.
Which vaccines: Reported across types; causality is often unclear.
How common: Uncommon; benefits of vaccination typically outweigh small flare risk for most patients. Taylor & Francis Online
Rare dermatologic reactions (e.g., erythema multiforme, severe cutaneous reactions)
What: A variety of skin reactions ranging from mild rash to rare severe reactions.
Which vaccines: Reported across vaccine platforms; most are mild.
How common: Rare. PMC
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