Vaccines

Introduction to Vaccines

  • Focus: The role of vaccines in the eradication of infectious diseases and the principles behind vaccine design.

Historical Context

  • Edward Jenner:

    • Regarded as the father of vaccination.

    • Noted for using vaccinia (cowpox) to induce immunity against smallpox.

    • Observed that milkmaids with cowpox lesions did not contract smallpox—a serious systemic disease.

    • Hypothesis: Exposure to cowpox led to protection against smallpox.

    • Method: Jenner proved immunity by challenging vaccinated individuals with the virulent smallpox pathogen, demonstrating they did not contract the disease.

    • Contribution: Demonstrated that the pus from cowpox lesions could be effectively transferred from person to person for vaccination, not just from cattle.

Terminology and Immunology

  • Challenge:

    • Defined as testing the efficacy of a vaccine by exposing individuals to the actual live virulent pathogen.

  • Antigens:

    • Shared surface antigens between cowpox and smallpox facilitate antibody cross-reactivity, leading to immunity.

    • Illustrated through green (common) and blue (non-shared) antigens, with some antigens producing antibodies that react with both viruses.

Purpose of Vaccination

  • Objective:

    • Induce long-lived memory T and B cells for protection against subsequent infections.

    • Generate immunity without inducing the disease.

  • Herd Immunity:

    • When a significant portion of the population is vaccinated, it protects unvaccinated individuals from exposure to contagious diseases.

Vaccination Dynamics

  • Scenarios illustrating disease contagion:

    • Unvaccinated Population:

    • High likelihood of disease spread among unimmunized individuals.

    • Partially Vaccinated Population:

    • Still significant contagion, but fewer infections due to some immunity.

    • Herd Immunity:

    • When a majority is vaccinated, infection spread is drastically reduced, protecting those who are unvaccinated.

Key Concepts of Vaccine Design

  • Long-lived Memory Response:

    • Essential in eliciting specific immune memory against pathogens.

  • Multivalent Vaccines:

    • Products containing multiple antigens to broaden the immune response.

  • Adjuvants:

    • Substances that enhance the immune response to vaccines, especially for weakly immunogenic substances.

Historical Impact of Vaccination

  • Infectious diseases caused significant mortality in the early 20th century:

    • 1900: 21,064 smallpox cases, 894 deaths.

    • 1920: 469,000 measles cases, 7,500 deaths.

    • Diphtheria: 150,000 cases, 13,000 deaths.

    • Pertussis: 100,000 cases, 5,100 deaths.

  • Post-vaccine era significantly reduced the incidence and mortality of these diseases.

    • Diphtheria and polio vaccines were successful in near eradication efforts.

    • Measles vaccine effectively decreases infection rates significantly.

Characteristics of Effective Vaccines

  • Must be:

    • Safe: No serious side effects or death.

    • Protective: Must protect from illness caused by virulent pathogen.

    • Long-lasting immunity: Provides prolonged protection.

  • Economic and Practical Considerations:

    • Low cost per dose, stability under varied conditions, ease of administration, minimal side effects, and effective elicitation of both neutralizing antibodies and T-cell responses.

Current Vaccination Practices in the US

  • Immunization Schedule:

    • Children receive multiple booster shots for various diseases.

    • Later vaccinations target conditions like human papillomavirus (HPV) and meningitis.

Vaccines Lacking Immunity

  • Tuberculosis:

    • BCG vaccine does not provide significant immunity; not approved for use in the US.

  • Malaria:

    • 1.1 million annual deaths; active research for vaccines.

  • Schistosomiasis, HIV/AIDS:

    • Ongoing research for effective vaccines against these diseases.

Specific Vaccine Examples

  • Measles Vaccine (MMR):

    • 93% efficacy after initial dose; 97% after booster.

    • Decline in measles cases since 2000, yet still poses significant risks, particularly in unvaccinated populations.

Types of Vaccine Platforms

  • Live Attenuated Vaccines:

    • Strong immune responses, often require just one dose.

    • Risks include potential reversion to virulence, side effects, etc.

  • Killed/Inactivated Vaccines:

    • Completely safe but often require boosters and less immunogenic.

  • Acellular/Subunit Vaccines:

    • Made from purified proteins or polysaccharides; require understanding of protective antigens and typically involve adjuvants.

Adjuvants in Vaccine Formulation

  • Definition:

    • Substances that enhance the immune response.

  • Mechanism:

    • Stimulate innate immunity, often via toll-like receptors, and prolong antigen exposure (antigen depot).

  • Example:

    • Alum: Induces dendritic cell maturation; currently the only adjuvant approved for human use in the US.

Advanced Vaccine Strategies

  • Recombinant Vectors:

    • Engineered microorganisms can carry genes from pathogens, facilitating immune responses against those pathogens.

  • Experimental Vaccines:

    • DNA vaccines, dendritic cell vaccines under investigation but faced challenges in efficacy.

Vaccine Development and Approval Process

  1. Preclinical Phase:

    • Testing in animals for immune response and safety.

  2. Phase I Trial:

    • Focus on safety; monitor side effects.

  3. Phase II Trial:

    • Assess immune response in larger cohort; no pathogen challenges.

  4. Phase III Trial:

    • Large-scale efficacy and safety trials across diverse populations.

T Helper Cell Types in Vaccine Design

  • Importance of aligning desired immune response types (e.g., Th1 for intracellular pathogens) with vaccine design choices.