1. Clinical Overview
Purified Vi Polysaccharide Typhoid Vaccine is a sterile solution containing the purified capsular polysaccharide antigen (Vi antigen) extracted from Salmonella Typhi strain Ty2. It is a non-living, subunit vaccine that induces a T-cell independent immune response, primarily producing IgG antibodies against the Vi polysaccharide. It provides protection against typhoid fever caused by Salmonella Typhi. In the Indian context, it is a critical tool for prophylaxis in endemic regions, for travelers, and for high-risk groups like food handlers and laboratory workers.
| Onset | Duration | Bioavailability |
|---|---|---|
| Protective antibody levels are typically achieved within 7-14 days post-vaccination. | Protection lasts for approximately 3 years. Revaccination is recommended every 3 years for continued risk. | Not applicable (Parenteral administration). Immunogenicity, measured as seroconversion (≥1 μg/mL anti-Vi IgG), is >90% in healthy adults. |
2. Mechanism of Action
The vaccine contains the purified Vi (virulence) capsular polysaccharide antigen from Salmonella Typhi. Upon intramuscular injection, the antigen is taken up by antigen-presenting cells and presented to B-lymphocytes in a T-cell independent manner (Type 2 T-independent antigen). This direct stimulation of B-cells leads to the production of specific IgG antibodies, primarily of the IgG2 subclass, without engaging significant T-helper cell memory. These circulating antibodies opsonize the bacteria, facilitating complement-mediated lysis and phagocytosis, thereby preventing invasive disease.
3. Indications & Uses
- Active immunization against typhoid fever caused by Salmonella Typhi in individuals 2 years of age and older.
- Prophylaxis for travelers from non-endemic to typhoid-endemic regions, including most parts of India.
- Vaccination of high-risk groups: laboratory personnel handling Salmonella Typhi, healthcare workers in endemic zones, and food handlers.
4. Dosage & Administration
Adult Dosage: Single dose of 0.5 mL (25 mcg of Vi polysaccharide) intramuscularly.
Administration: For intramuscular use only. Shake the vial or pre-filled syringe well before use. The preferred site is the deltoid muscle. Do not administer intravenously, intradermally, or subcutaneously. Use a separate, sterile syringe and needle for each patient. Observe for syncope for 15 minutes post-vaccination.
5. Side Effects
Common side effects may include:
- Pain, tenderness, redness, or induration at the injection site (≥10%).
- Low-grade fever (<38°C).
- Headache.
- Malaise, myalgia.
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Immunosuppressants (e.g., corticosteroids, chemotherapy, biologics) | Diminished immunogenic response; may not confer adequate protection. | Major |
| Other Vaccines (e.g., Yellow Fever, Hepatitis A) | Can be administered simultaneously at different injection sites. No clinically significant interaction. | Minor |
| Anticoagulants (e.g., Warfarin) | Increased risk of injection site hematoma. Administer with caution using fine needle and apply firm pressure. | Moderate |
7. Patient Counselling
- DO complete the vaccination at least 2 weeks before potential exposure to typhoid.
- DO inform the doctor about any allergies, current illnesses, or medications, especially immunosuppressants.
- DO apply a cold compress if there is pain or swelling at the injection site.
- DO NOT take the vaccine if you have a high fever or severe active infection.
- DO NOT assume this vaccine protects against paratyphoid fever.
8. Toxicology & Storage
Overdose: Unlikely due to single-dose administration in a healthcare setting. Theoretical overdose could exacerbate local and systemic reactions (e.g., high fever, severe local inflammation).
Storage: Store between +2°C to +8°C (in a refrigerator). Do not freeze. Protect from light. Do not use if frozen or if the solution is discolored or contains particulate matter. The vaccine is sensitive to heat. Use immediately after opening the vial. Transport in a cold chain.