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Cerebral venous sinus thrombosis (CVT) / Thrombosis with thrombocytopenia (VITT/TTS)
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What: Blood clots in brain veins sometimes accompanied by low platelet counts (can be life-threatening).
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Which vaccines: Adenoviral-vector vaccines (AstraZeneca, Johnson & Johnson) were most commonly implicated in VITT/TTS reports.
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How common: Very rare, but severe — prompted temporary pauses and age-specific recommendations in some countries when first identified. AHA Journals+1
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Acute Disseminated Encephalomyelitis (ADEM) and other rare neuro-inflammatory events
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What: Rare inflammation of brain and spinal cord; can cause neurologic deficits.
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Which vaccines: Reported as a signal in some large-scale safety surveillance datasets (rare).
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How common: Extremely rare — reported as safety signals requiring further study rather than confirmed causal relationships. PubMed
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Immune thrombocytopenia (ITP) / Low platelets (non-VITT)
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What: Low platelet counts leading to bleeding/bruising risk.
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Which vaccines: Reports have appeared across vaccine types; some cases were transient.
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How common: Rare; often manageable with standard therapies. ijmm.ir
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Anaphylaxis / Severe allergic reaction
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What: Rapid onset allergy (hives, breathing difficulties, hypotension).
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Which vaccines: Reported with mRNA and other vaccines but overall extremely rare.
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How common: Rare — vaccination sites monitor recipients for 15–30 minutes and are prepared to treat anaphylaxis immediately. PMC
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