Clinical meaning
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. Inactivated vaccines (Tdap, hepatitis B, IPV, HPV, pneumococcal, influenza injection) are generally safe in immunocompromised patients but may produce a diminished antibody response. Pregnancy is a contraindication to all live vaccines due to theoretical risk of fetal infection; however, inactivated vaccines including Tdap (recommended at 27-36 weeks each pregnancy) and inactivated influenza are safe and recommended. The NP must distinguish between true contraindications (which prohibit vaccination) and common misconceptions that are NOT contraindications — including mild acute illness with or without fever, current antimicrobial therapy, disease exposure or convalescence, premature birth, personal or family history of allergies (non-vaccine), and breastfeeding.