W9 - Vaccinations

What was variolation in 17th-century India and China, and what were its primary risks?

Scabs from smallpox individuals were inhaled or injected under the skin; while it protected many, smallpox killed 10–20% of the population at the time, and variolation caused active smallpox disease in 1–2% of recipients.

What five specific epidemiological and biological factors made Smallpox Eradication (1977) uniquely successful?

1. Humans were the only host reservoir, with only a single viral serovar.

2. Infection always caused visible symptoms (rashes/pustules), making cases easy to trace and isolate.

3. Incubation time to symptoms/infectivity was short, leaving less time to transmit.

4. Survivors gained robust immunity, and the vaccine was highly effective.

What are the key differences between Passive Immunity and Active Immunity regarding memory and duration?

• Passive Immunity: Provides transient protection, helps alleviate existing conditions, and does not generate immune memory (lasts only as long as antibodies persist).

• Active Immunity: Generates long-term protective immune memory by activating host T and B cells via natural exposure or vaccination.

What are standard agents and major cons associated with Passive Immunity?

• Agents: Antivenom (spider/snake/botulism), pooled human immune gamma globulin (measles, rabies, tetanus), and horse-derived diphtheria antitoxin.

• Cons: Short-lived, no long-term memory, expensive human pooled antibodies, and risk of serum sickness from animal serums.

What are the defining characteristics of an Ideal Vaccine?

Must not cause disease, provide population-level protection, induce lifelong memory T and B cells, require only a single dose with zero side effects, be chemically stable without needing a cold chain, and be easy to administer (e.g., oral/vaccine banana) and cheap to produce.

What are the advantages and disadvantages of Live Attenuated Vaccines?

• Advantages: Excellent immunogens, induce long-lived/appropriate immunity, often require a single dose, and do not need adjuvants.

• Disadvantages: Less stable; carry a risk of reverting to a virulent form, potential contamination, and danger in immunocompromised hosts.

Give examples of Subunit & Toxoid vaccines based on recombinant proteins, toxoids, and conjugates.

• Recombinant Protein: Hepatitis B expressed in yeast.

• Toxoid: Bordetella pertussis (whooping cough) toxin-based vaccine.

• Conjugate: Meningococcal polysaccharide vaccine linked to a protein carrier.

What are the key safety and structural properties of Genetic (mRNA / DNA) Vaccines?

They contain no live, attenuated, inactivated, or recombinant virus (nor final protein). mRNA is short-lived, acts transiently as a protein template, and never enters the nucleus or integrates into the host genome.

What is a Recombinant Viral Vector Vaccine?

A vaccine utilizing a heterologous virus expression system (such as a modified Adenovirus) engineered to express a foreign pathogen antigen, like the SARS-CoV-2 spike protein.

What are the primary functions of Adjuvants (like ALUM / Aluminium salts) in vaccine formulations?

They boost immune responses by retaining the antigen near the injection site, facilitating phagocytosis, and lowering the total antigen dose required.

What regulatory standard applies to formaldehyde used as an inactivating agent in Australian vaccine standards?

It is strictly constrained to <0.02\% w/v free-formaldehyde.

What did extensive meta-analyses (such as Taylor et al., 2014, covering over 1.2 million children) definitively prove regarding the Wakefield autism myth?

They proved no association whatsoever between vaccines (including MMR, thimerosal, or mercury) and autism or autism spectrum disorders, confirming the Wakefield paper was a fraudulent retraction.

How do Inactivated (Killed) Vaccines differ from Live Attenuated vaccines regarding production and immunity induced?

• Production: Produced via chemical or gamma-ray inactivation of virulent pathogens (using agents like heat, phenol, or formaldehyde).

• Immunity Induced: Mainly induces humoral immunity (antibody production) rather than robust cell-mediated responses, and typically requires multiple boosters.

What are the relative stability and reversion risk profiles of DNA vs. mRNA vaccines?

• DNA Vaccines: Highly stable and cannot revert to virulence.

• mRNA Vaccines: Require ultra-low temperature cold storage stability, and cannot revert to virulence.

What was the purpose of Thiomersal in vaccine manufacturing, and what is its current status in modern pediatric schedules?

Thiomersal is a mercury-containing compound historically used as a preservative to prevent bacterial and fungal growth; modern children's schedules utilize thiomersal-free or trace formulations.

Why are trace amounts of antibiotics present in certain vaccine preparations?

They are trace carryovers resulting from tissue culture maintenance used strictly to preserve sterility during manufacturing.

What statistical impact have vaccines had globally since the launch of the Expanded Program on Immunization in 1974?

Over 154 million lives have been saved globally, with 40% of the drop in infant mortality directly attributable to vaccines.

What are the defining structural, manufacturing, and immunological characteristics of Live-Attenuated Vaccines versus Inactivated (Killed) Vaccines?

• Live-Attenuated Vaccines: Contain weakened, replication-competent pathogens (e.g., MMR, oral polio, BCG). They induce robust cell-mediated (CD8+ and CD4+) and humoral immunity closely mimicking natural infection, but carry a rare risk of reversion to virulence in immunocompromised hosts.

• Inactivated (Killed) Vaccines: Contain pathogens treated with heat, chemicals, or radiation so they cannot replicate (e.g., injectable polio, influenza, hepatitis A). They are safer for immunocompromised individuals but generally require multiple booster doses and adjuvants because they primarily stimulate humoral antibody responses rather than strong cytotoxic T-cell immunity.

How do Subunit, Recombinant, and Conjugate Vaccines function, and why are conjugate vaccines necessary for infants against encapsulated bacteria?

• Mechanism: Contain purified antigenic fragments (e.g., surface proteins or capsular polysaccharides) rather than whole organisms.

• Why Conjugates are Necessary: Polysaccharide antigens alone are T-independent antigens that elicit poor memory and weak antibody responses in infants under 2 years old. Conjugating the polysaccharide to an immunogenic carrier protein (like tetanus toxoid) allows engagement of CD4+ helper T cells, enabling class-switching, affinity maturation, and immunological memory.

What is the primary role of an adjuvant in vaccine formulations, and how does Alum (Aluminum salts) function?

• Role: Additives mixed into vaccines to enhance and modulate the magnitude of the immunogenic response to an antigen.

• Alum Mechanism: Acts as a depot to slowly release antigen over time, increases antigen uptake by antigen-presenting cells, and activates the NLRP3 inflammasome, driving a strong Th2-skewed humoral response.

What is the Herd Immunity Threshold (H_c), and how is it calculated based on the basic reproduction number (R₀) of an infectious agent?

• Definition: The minimum proportion of a population that must be immune (via vaccination or prior infection) to halt the sustained spread of an infectious disease within that population.

• Formula:

  $$H_c = 1 - \frac{1}{R_0}$$

  (where R₀ is the average number of secondary infections generated by a single infected individual in a completely susceptible population).

What are the key operational and immunological differences between Active Immunization and Passive Immunization?

• Active Immunization: Administration of a vaccine to stimulate the host’s own immune system to mount an adaptive response, resulting in long-term immunological memory and endogenous antibody/T-cell production.

• Passive Immunization: Direct transfer of pre-formed exogenous antibodies (e.g., human immunoglobulins, antivenoms, or monoclonal antibodies) into a recipient, providing immediate but temporary protection without generating host immunological memory.

What is the clinical purpose of administering Post-Exposure Prophylaxis (PEP) using specific immunoglobulins (such as Hepatitis B immune globulin or Rabies immune globulin)?

To instantly neutralize high-risk pathogen exposures before the host's own naive immune system has time to mount a primary adaptive response, preventing the establishment of acute, potentially fatal infection.

How do Adverse Events Following Immunization (AEFI) differ when classified as vaccine product-related reactions versus programmatic errors?

• Vaccine Product-Related Reactions: Physiological adverse effects inherent to the vaccine components (e.g., mild fever, injection-site soreness, or rare systemic hypersensitivity).

• Programmatic Errors: Preventable incidents resulting from mistakes in vaccine handling, cold-chain storage failure, incorrect reconstitution, or improper administration technique (e.g., sterile abscess from unsterile injection or wrong dosage).

Why is the maintenance of the Cold Chain (typically +2C to +8C for most refrigerated vaccines) critical during vaccine distribution and storage?

Exposing biological vaccines (especially live-attenuated viral strains and protein-based subunits) to temperatures outside the recommended cold-chain range can denature the tertiary structure of antigens, degrade viral viability, or ruin adjuvants, permanently destroying vaccine efficacy.

What is the biochemical mechanism of action of mRNA vaccines (e.g., Pfizer-BioNTech, Moderna) upon administration into host tissue?

Synthetic mRNA encoding a specific viral antigen (encapsulated in lipid nanoparticles for cellular uptake) is translated by host cellular ribosomes in the cytoplasm to produce the target antigen. The synthesized protein is then degraded and presented via MHC Class I and Class II pathways, stimulating both humoral antibody production and cellular ($\text{CD8}^+$ cytotoxic T-cell) immunity.

Why are Lipid Nanoparticles (LNPs) required for the delivery of mRNA vaccines, given that naked mRNA is unstable in extracellular fluids?

Naked mRNA is rapidly degraded by ubiquitous extracellular ribonucleases (RNases) and carries a negative charge that impedes crossing the negatively charged hydrophobic plasma membrane. LNPs protect the mRNA from enzymatic degradation, neutralize its charge, and facilitate endosomal escape into the host cell cytoplasm.

How do Viral Vector Vaccines (e.g., non-replicating adenovirus vector vaccines) deliver genetic material to host cells?

They use a modified, replication-deficient virus (such as an adenovirus) engineered to carry the DNA sequence encoding the target antigen into host cells. Once inside the nucleus, the DNA is transcribed into mRNA, which is subsequently translated into the target antigen by host ribosomes.

What is Anti-Vector Immunity, and how can it impact repeated administrations of viral vector-based vaccines?

The immune system mounts neutralizing antibodies and T-cell responses against the capsid proteins of the viral vector itself (rather than just the target antigen) upon initial exposure. During booster doses, these pre-existing anti-vector antibodies can rapidly neutralize and clear the vector before it successfully enters host cells, reducing the vaccine's efficacy.

What is the primary immunoglobulin isotype involved in mucosal immunity, and how does it reach mucosal surfaces (e.g., gut or respiratory lumen)?

• Isotype: Secretory IgA (sIgA), typically existing as a dimer.

• Transport Mechanism: Produced by plasma cells in lamina propria, IgA binds to the Polymeric Immunoglobulin Receptor (pIgR) on the basolateral surface of mucosal epithelial cells, is transcytosed across the cell, and is released into the mucosal secretions with a cleaved fragment of pIgR (the secretory component) which protects it from proteolytic degradation.

What is the immunological rationale for using a Prime-Boost strategy (heterologous or homologous) in vaccination?

The "prime" dose introduces the antigen to naive T and B cells, establishing primary clonal expansion and low-level immunological memory. The subsequent "boost" dose re-exposes the immune system to the antigen, stimulating the rapid proliferation of memory cells, affinity maturation, somatic hypermutation, and the production of high-affinity neutralizing antibodies at titers significantly higher than the primary response.

What is the difference between Primary Vaccine Failure and Secondary Vaccine Failure?

• Primary Vaccine Failure: Occurs when an individual fails to mount an initial protective immune response following vaccination (due to factors such as improper storage/cold-chain failure, incorrect administration, immunosuppression, or interference from maternal antibodies).

• Secondary Vaccine Failure: Occurs when an individual initially mounts an adequate protective immune response, but immunity wanes over time due to natural antibody decay and memory cell attrition, leading to reinfection upon exposure.

How does a pathogen's Basic Reproduction Number (R0) directly influence the critical vaccination coverage needed to achieve herd immunity?

R0 represents the average number of secondary cases generated by a single index case in a completely susceptible population. Higher R0 values (e.g., Measles with R0 ~12-18) require a significantly higher proportion of the population to be immunized to interrupt transmission chains compared to pathogens with lower R0 values (e.g., Influenza).

What is the distinction between Passive Surveillance Systems (such as VAERS - Vaccine Adverse Event Reporting System) and Active Surveillance Systems (such as vaccine safety datalinks) in pharmacovigilance?

• Passive Surveillance: Relies on voluntary reporting of adverse events by healthcare providers, vaccine manufacturers, or patients after vaccination. While effective at generating early signals for rare events, it is subject to underreporting and lack of an unexposed control group.

• Active Surveillance: Involves pre-planned, systematic collection of health data across large populations linked through electronic health records, allowing researchers to actively monitor, compare, and quantify the true incidence of adverse events against baseline rates in unvaccinated populations.