Yellow fever vaccine
Yellow Fever Vaccine
Generic Name
Yellow Fever Vaccine
Mechanism
- Live‑attenuated YFV‑17D enters host cells via endocytosis, primarily infecting dendritic cells and macrophages at the injection site.
- Intracellular viral replication produces non‑structural and structural antigens that stimulate both innate (type‑I interferon, NK cell activation) and adaptive immunity.
- T‑cell mediated responses (CD4⁺ Th1 and CD8⁺ cytotoxic) produce cytokines that clear infected cells.
- B‑cell activation generates neutralizing IgM that converts to durable IgG, providing lifelong protection after a single dose.
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Pharmacokinetics
- Absorption: Rapid local uptake; peak viral replication at 4–7 days, reflecting viremia.
- Distribution: Systemic dissemination via bloodstream; virus detected transiently in lymphoid tissues.
- Metabolism & Elimination: Viral RNA is degraded by host nucleases; no appreciable drug concentration remains beyond the age of the immune response.
- Duration: Protective neutralizing antibodies persist for life; no need for routine revaccination.
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Indications
- Prevention of yellow fever in individuals traveling to, residing in, or returning from endemic regions (Central & South America, sub‑Saharan Africa).
- Prophylaxis in outbreak settings, health‑care workers, and laboratory personnel with exposure risk.
- Required for entry into many countries with yellow‑fever outbreak risk.
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Contraindications
| Category | Remarks |
| Contraindicated |
• Severe allergic reaction (anaphylaxis) to vaccine components (gelatin, yeast products). • Known immunodeficiency disorders (e.g., AIDS with CD4 < 200 cells/µL), active chemotherapy, or immunosuppressive therapy. • Severe concurrent illness that might obscure vaccine reaction. • Infants < 6 months of age (routine use from 6 months; data limited below). |
| Caution |
• Pregnancy & breastfeeding: not recommended; risk–benefit must be evaluated. • Children < 9 months: not recommended unless indicated by local public‑health protocols. |
| Warnings |
• Rare viscerotropic (2–3 × 10⁻⁵) and neurotropic (≈1 × 10⁻⁴) disease occurrence. • Monitor for severe symptoms within 10–90 days post‑vaccination. |
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Dosing
| Population | Dose | Route | Needle Size | Site | Special Instructions |
| Adults & children ≥ 6 mo | 50 µL (0.5 mL, 1 × 10⁶ PFU) | Intramuscular (IM) | 25 G | Deltoid or anterolateral thigh | Ensure proper refrigeration (2–8 °C); thaw trays at 2‑8 °C before use. Avoid freeze‑thaw cycles. |
| Adolescents & adults | Same | IM | Same | Same | No booster needed; a new dose may be administered after 10–14 days of immunosuppressive therapy. |
| Immunocompromised (optional) | 50 µL | Same | Same | Same | Consider 2nd dose 3–6 months later only if at high risk; monitor for adverse events. |
Storage: 2–8 °C; do not refreeze after thawing. Maintain cold chain from vaccination to administration.
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Adverse Effects
| Adverse Effect | Frequency (per 100 k) | Comments |
| Local reactions (pain, erythema, induration) | 30–40 % | Resolve within 7 days. |
| Systemic symptoms (fever ≤ 38 °C, headache, myalgia, rash) | 10–15 % | Occur 4–8 days post‑vaccination. |
| Serious adverse reactions | ||
| • Viscerotropic disease | 1–2 per 100 k | Fever, hepatosplenomegaly, multi‑organ failure; high fatality (~30–40 %). |
| • Neurotropic disease | 1 per 100 k | Encephalitis, myelitis; mortality up to 40 %. |
| • Severe allergic reaction (anaphylaxis) | < 1 per 10 k | Immediate epinephrine needed. |
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Monitoring
- Immediate post‑vaccination: Observe for 30 minutes for anaphylaxis.
- Day 10–90: Monitor for unexplained fever, malaise, or rash; consider laboratory work‑up (CBC, LFTs) if viscerotropic signs appear.
- High‑risk patients (immunosuppressed): Reassess 3–6 months after vaccination; repeat dose if active disease risk persists.
- Documentation: Keep vaccination card; verify validity per destination country requirements.
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Clinical Pearls
1. Lifelong Immunity – A single IM dose provides permanent protection; no routine booster is required, simplifying travel preparation.
2. Rapid-Onset Protection – Protective neutralizing antibodies develop in 10–14 days; administer at least 10 days prior to travel, but vaccine is “effective essentially immediately.”
3. Temperature Tolerance – YFV‑17D can be stored frozen without loss of potency (up to 40 days), enabling safe distribution to remote sites.
4. Pregnancy Caution – Although data are sparse, live‑attenuated vaccines are contraindicated in pregnancy; alternative prevention strategies (sunscreen, mosquito nets) are mandatory.
5. Identify Allergens Early – Gelatin and yeast by‑products are common allergens; a detailed allergy history can avert anaphylaxis.
6. Report Serious Reactions – In case of viscerotropic or neurotropic disease, immediate reporting to public‑health authorities aids outbreak surveillance.
7. Travel‑Related Scheduling – For travelers, a 4–5 day window between vaccination and arrival in endemic areas is recommended; still, the vaccine confers protection “essentially immediately.”
8. Collectively Vaccinated Populations – In mass‑vaccination campaigns (e.g., Angola 2021), high coverage reduces herd‑immunity thresholds (~80 %) and curbs epidemic spread.
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• Key Takeaway
The Yellow Fever Vaccine (YFV‑17D) remains the gold standard for preventing a potentially fatal arboviral infection. Its single‑dose, live‑attenuated design offers durable immunity, but careful screening for contraindications and monitoring for rare but severe adverse events are essential for safe clinical practice.