## Passive Immunization in Rabies PEP **Key Point:** Human rabies immunoglobulin (HRIG) is the only effective passive immunizing agent for rabies post-exposure prophylaxis. It must be given alongside active vaccination (rabies vaccine) to ensure both immediate and long-term protection. ### Timing and Efficacy of HRIG - **Optimal window:** Within 7 days of exposure (most effective within 24–48 hours) - **Mechanism:** Neutralizing antibodies bind rabies virus glycoproteins and prevent attachment and entry into neurons - **Dose:** 20 IU/kg body weight - **Duration of protection:** ~21 days (half-life of immunoglobulin) ### Combined PEP Strategy 1. **Passive immunity (HRIG):** Immediate protection; administered once 2. **Active immunity (Rabies vaccine):** Delayed but long-lasting; given as a series - Cell culture vaccine (CCV) or purified chick embryo cell vaccine (PCECV) - Schedule: Days 0, 3, 7, 14, 28 (standard 5-dose regimen) or accelerated regimens **High-Yield:** HRIG + Rabies vaccine is the **gold standard** for complete rabies PEP. HRIG alone is insufficient; vaccine alone (without HRIG) is also suboptimal if given early. **Clinical Pearl:** In this patient (day 3 post-bite), HRIG can still be given effectively. The combination of HRIG (passive) + continued vaccine series (active) provides comprehensive protection. ### Comparison of Passive Immunizing Agents | Agent | Source | Efficacy | Serum Sickness Risk | Availability in India | |---|---|---|---|---| | HRIG | Human serum | Highest | Low | Limited | | ERIG | Equine serum | Good | Moderate | More available | | Monoclonal antibodies | Recombinant | High | Minimal | Emerging | **Mnemonic:** **HRIG = Human Rabies Immunoglobulin** — the **H** stands for human (safer than equine), providing **immediate** passive protection.
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