Cerebral venous sinus thrombosis (CVT) / Thrombosis with thrombocytopenia (VITT/TTS)
What: Blood clots in brain veins sometimes accompanied by low platelet counts (can be life-threatening).
Which vaccines: Adenoviral-vector vaccines (AstraZeneca, Johnson & Johnson) were most commonly implicated in VITT/TTS reports.
How common: Very rare, but severe — prompted temporary pauses and age-specific recommendations in some countries when first identified. AHA Journals+1
Acute Disseminated Encephalomyelitis (ADEM) and other rare neuro-inflammatory events
What: Rare inflammation of brain and spinal cord; can cause neurologic deficits.
Which vaccines: Reported as a signal in some large-scale safety surveillance datasets (rare).
How common: Extremely rare — reported as safety signals requiring further study rather than confirmed causal relationships. PubMed
Immune thrombocytopenia (ITP) / Low platelets (non-VITT)
What: Low platelet counts leading to bleeding/bruising risk.
Which vaccines: Reports have appeared across vaccine types; some cases were transient.
How common: Rare; often manageable with standard therapies. ijmm.ir
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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