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Wednesday, August 6, 2025

Bacterial vaccines


Bacterial vaccines represent one of the most critical public health interventions in modern medicine. These biologic preparations stimulate the immune system to recognize and respond to specific bacterial pathogens, thereby preventing severe, potentially fatal diseases. They may be composed of whole inactivated bacteria, live attenuated strains, bacterial subunits, conjugates, or toxoids.


1. Definition and Classification

A bacterial vaccine is a biologic agent derived from bacteria or their components (capsular polysaccharides, proteins, or toxins) that induces an immune response providing protection against future infections.

Main classifications:

  • Live attenuated bacterial vaccines

  • Inactivated (killed) whole-cell vaccines

  • Subunit vaccines

  • Toxoid vaccines (detoxified bacterial exotoxins)

  • Conjugate vaccines (polysaccharide-protein conjugates)

2. Mechanism of Action

Bacterial vaccines induce immunity by exposing the host immune system to bacterial antigens, either through:

  • Humoral immunity (primary response via B-cells and antibodies)

  • Cell-mediated immunity (especially with live vaccines)

  • Mucosal immunity (for orally or nasally administered vaccines)

The immune system recognizes the bacterial components as foreign and develops memory B-cells and T-cells, which allow for rapid and robust responses upon future exposure to the actual pathogen.

3. Major Types and Examples

A. Live Attenuated Bacterial Vaccines

  • Contain weakened bacteria capable of limited replication

  • Stimulate strong and long-lasting immunity

Examples:

  • BCG (Bacillus Calmette-Guérin) – for tuberculosis

  • Ty21a oral typhoid vaccine – for Salmonella typhi

B. Inactivated (Killed) Bacterial Vaccines

  • Entire bacterial cells killed by heat or chemicals

Examples:

  • Inactivated cholera vaccine (Dukoral®)

  • Whole-cell pertussis (component of DTP vaccine in some countries)

C. Subunit and Conjugate Bacterial Vaccines

  • Use purified bacterial components (proteins or polysaccharides)

  • Conjugates improve immunogenicity in young children

Examples:

  • Haemophilus influenzae type b (Hib) conjugate vaccine

  • Pneumococcal conjugate vaccine (PCV13, PCV15, PCV20)

  • Meningococcal conjugate vaccines (MenACWY, MenB)

D. Toxoid Vaccines

  • Contain inactivated bacterial exotoxins

Examples:

  • Diphtheria toxoid

  • Tetanus toxoid

4. Indications and Use

Routine Pediatric Vaccines:

  • DTaP (Diphtheria, Tetanus, acellular Pertussis)

  • Hib

  • PCV13 (Pneumococcal conjugate)

  • BCG (in endemic countries)

Adolescent and Adult Vaccines:

  • Td or Tdap booster (every 10 years)

  • Meningococcal conjugate (e.g., college entry)

  • Typhoid vaccine for travelers

Travel and Special Populations:

  • Cholera (e.g., Dukoral®)

  • Typhoid (Vi polysaccharide or Ty21a)

  • Anthrax (BioThrax®; for military/lab workers)

  • Plague (not routinely used; investigational stockpile in some countries)

Disease Eradication/Control Programs:

  • WHO Expanded Program on Immunization (EPI)

  • Maternal immunization with tetanus toxoid

  • Neonatal BCG for TB prevention

5. Immunogenic Components

  • Polysaccharide antigens: Often poorly immunogenic in infants unless conjugated

  • Protein antigens: Stronger T-cell-dependent immunity

  • Exotoxins: Rendered harmless (toxoid) but immunogenic

  • Adjuvants (e.g., aluminum salts): Enhance immune response

6. Administration Schedules

VaccineRoutePrimary Series (Infants)Booster Required
DTaPIntramuscular2, 4, 6, 15–18 monthsYes
HibIntramuscular2, 4, 6 months; booster at 12–15 mYes
PCV13/15/20Intramuscular2, 4, 6, 12–15 monthsYes
BCGIntradermalSingle dose at birthNo
Tdap/Td (adults)IntramuscularEvery 10 yearsYes



7. Adverse Reactions

Local:

  • Pain, redness, swelling at injection site

Systemic:

  • Fever, malaise

  • Irritability (in children)

  • Headache, fatigue

Rare but Serious:

  • Anaphylaxis (extremely rare; immediate hypersensitivity)

  • Seizures (febrile; transient)

  • Hypotonic-hyporesponsive episodes (DTaP)

  • Guillain-Barré syndrome (rare; associated with tetanus toxoid in rare reports)

8. Contraindications

  • Severe allergic reaction to vaccine or component (e.g., gelatin, neomycin, latex)

  • Live bacterial vaccines contraindicated in:

    • Immunocompromised individuals (e.g., HIV/AIDS, chemotherapy)

    • Pregnant women (live oral typhoid, BCG)

  • Moderate/severe illness with or without fever (temporary deferral)

  • History of encephalopathy not attributable to another cause within 7 days of pertussis vaccine

9. Precautions

  • Premature infants: May require monitoring after vaccination (apnea risk)

  • Hematologic malignancies or immunodeficiencies: Avoid live vaccines

  • Immunosuppressive therapy: Inactivated vaccines safe but may have reduced efficacy

10. Drug Interactions

  • Immunosuppressants (e.g., corticosteroids, chemotherapy): May blunt vaccine response, especially live vaccines

  • IVIG or blood products: Can interfere with live bacterial vaccines (e.g., BCG, Ty21a)

  • Antibiotics: May neutralize live oral vaccines like Ty21a

  • Concurrent vaccines: Multiple vaccines can be administered simultaneously if given at different injection sites

11. Storage and Handling

  • Must be stored per manufacturer instructions (usually 2–8°C)

  • Live vaccines are particularly temperature-sensitive (e.g., BCG loses potency rapidly if frozen or overheated)

  • Reconstituted vaccines (e.g., Hib, BCG) must be used within prescribed time frames

12. Regulatory Considerations and WHO Prequalification

FDA and EMA: Vaccines undergo rigorous pre-approval trials, including:

  • Phase I–III clinical trials

  • Post-marketing surveillance (Phase IV)

  • Annual updates in immunization schedules (CDC, ACIP, ECDC)

WHO Prequalification Program:

  • Certifies vaccines for international procurement (e.g., GAVI, UNICEF)

  • Ensures safety, efficacy, and cold chain compliance for global immunization

13. Public Health Impact

  • Major contributor to reduction in child mortality

  • Eradicated diseases (smallpox) and near-elimination of others (e.g., diphtheria, pertussis)

  • Herd immunity: Protects unvaccinated individuals via reduced transmission

  • Effective in outbreak containment and bioterrorism preparedness

14. Notable Licensed Bacterial Vaccines

Vaccine NamePathogen TargetedNotes
BCGMycobacterium tuberculosisLive attenuated; used in TB-endemic countries
DTaPC. diphtheriae, C. tetani, B. pertussisToxoids + acellular pertussis
HibHaemophilus influenzae type bConjugate vaccine; for infants and young children
PCV13/15/20Streptococcus pneumoniaeCovers 13–20 serotypes; conjugated
MenACWY, MenBNeisseria meningitidisConjugate and recombinant protein vaccines
Ty21a, Vi polysaccharideSalmonella typhiOral (live) and injectable (inactivated) forms
Cholera vaccine (Dukoral)Vibrio choleraeOral; inactivated + recombinant B subunit
Anthrax (BioThrax®)Bacillus anthracisAdsorbed protective antigen; used in biothreats



15. Special Populations and Vaccine Response

  • Infants and elderly: Require conjugated vaccines or adjuvanted formulations due to weaker immune response

  • Pregnant women: May receive tetanus toxoid and acellular pertussis to protect neonates (Tdap)

  • Immunocompromised: Avoid live vaccines; prioritize inactivated vaccines, though response may be blunted



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