Most people who receive the smallpox vaccine experience only mild side effects, such as soreness, redness, swelling, or itching at the vaccination site, along with fatigue, fever, headache, or swollen lymph nodes. Rare but serious complications include severe skin reactions, inflammation of the heart (myocarditis or pericarditis), brain inflammation (encephalitis), and progressive vaccinia (painless vaccination lesion with worsening ulceration and necrosis), particularly in people with weakened immune systems or certain skin conditions.
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