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
Other very rare vascular/hematologic signals (e.g., other clotting disorders)
What: Small safety signals for rare blood-clotting conditions and related hematologic issues in very large datasets.
Which vaccines: Some signals seen more with adenoviral-vector vaccines, other signals appear across platforms but with low absolute numbers.
How common: Extremely rare, detected only because surveillance datasets included tens of millions of people. PubMed+1
