Learning Objective
Describe the major vaccine types, their mechanisms, advantages, disadvantages, and key clinical examples.
Vaccination induces active humoral and/or cellular immune responses to specific pathogens and creates immune memory.
Live Attenuated Vaccines
Description:
Microorganisms are weakened (attenuated) so they are nonpathogenic but can undergo limited replication in the host.
Pros:
- Strong cellular and humoral immunity
- Often lifelong protection
Cons:
- Rare reversion to virulent form
- Contraindicated in pregnancy and most immunodeficiencies
- Exception: MMR and varicella may be given to HIV patients with CD4 ≥ 200 cells/mm³ after specialist consultation
Examples:
- Adenovirus (non-attenuated, military recruits)
- Typhoid (Ty21a oral)
- Polio (Sabin)
- Varicella
- Smallpox
- BCG
- Yellow fever
- Influenza (intranasal)
- MMR
- Rotavirus
Killed (Inactivated) Vaccines
Description:
Pathogen is chemically or heat-inactivated; epitope structure preserved. Primarily induces a humoral response (no replication → minimal MHC I presentation).
Pros:
- Safer than live vaccines
Cons:
- Weaker immune response
- Mainly humoral
- Boosters required
Examples:
- Hepatitis A
- Typhoid (Vi polysaccharide IM)
- Rabies
- Influenza (intramuscular)
- Polio (SaLk = Killed)
Subunit, Recombinant, Polysaccharide, and Conjugate Vaccines
Description:
Use specific antigens or pathogen components to stimulate immunity. Polysaccharides alone → T-cell–independent (weaker, no memory; ex: PPSV23). Polysaccharide + protein conjugate → T-cell–dependent (stronger; PCV13/15/20).
Pros:
- Targets specific epitopes
- Fewer adverse reactions
- Safe in immunocompromised patients
Cons:
- More expensive
- Weaker immune response than live vaccines
Examples:
- HBV (HBsAg)
- HPV
- Acellular pertussis (aP)
- Neisseria meningitidis vaccines (various serogroups)
- Strep pneumoniae:
- PPSV23 = polysaccharide, T-independent
- PCV13/15/20 = conjugate, T-dependent
- Hib
- Herpes zoster (recombinant zoster vaccine)
Toxoid Vaccines
Description:
Denatured bacterial toxins with intact binding sites. Induce antibodies without causing disease.
Pros:
- Protect against toxin-mediated disease
Cons:
- Require boosters because antitoxin levels wane over time
Examples:
- Clostridium tetani (tetanus)
- Corynebacterium diphtheriae (diphtheria)
mRNA Vaccines
Description:
Lipid nanoparticle delivers mRNA encoding a foreign antigen (e.g., SARS-CoV-2 spike). Host cells synthesize protein → presented on MHC I and II → cellular + humoral immunity.
Pros:
- High efficacy
- Strong cellular and humoral responses
- Safe in pregnancy
Cons:
- Common transient reactions: fatigue, headache, myalgia
- Rare: myocarditis and pericarditis, especially in young males
Examples:
- SARS-CoV-2 mRNA vaccines
