A contraindication is a condition in a recipient that significantly increases the risk of a serious adverse reaction — vaccination must NOT be given when a true contraindication exists. A precaution is a condition that may increase the risk or reduce vaccine effectiveness, requiring clinical judgment on whether the benefit outweighs the risk. The most universally applicable contraindication is a history of severe allergic reaction (anaphylaxis) to a prior dose of the vaccine or to a vaccine component. Anaphylaxis is an IgE-mediated type I hypersensitivity reaction occurring within minutes to hours of exposure, involving mast cell and basophil degranulation with massive release of histamine, tryptase, prostaglandins, and leukotrienes, causing bronchospasm, laryngeal edema, vasodilation, and cardiovascular collapse. Live attenuated vaccines (MMR, varicella, LAIV, rotavirus, yellow fever, BCG) contain replication-competent organisms and are contraindicated in patients with severe immunodeficiency — including those with primary immunodeficiency, active leukemia/lymphoma, chemotherapy, radiation, high-dose systemic corticosteroids (≥20 mg/day prednisone equivalent for ≥14 days), solid organ transplant recipients on immunosuppressive therapy, and HIV patients with CD4 <200 cells/µL — because the attenuated organism can cause disseminated disease....