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PH9.4 | PH9.4 | Vaccine Pharmacology — SDL Guide — SDL Guide (Part 2)

Vaccine Pharmacology: Kinetics, Dynamics, Uses and Adverse Effects

The pharmacological properties of vaccines differ substantially between types, and understanding these differences is essential for correct clinical use.

Pharmacokinetics (immunological kinetics):
Following a primary dose, the antigen is processed by antigen-presenting cells (dendritic cells, macrophages) at the injection site. It is transported to regional lymph nodes, where naïve B- and T-cells are activated. The primary immune response takes 1–2 weeks to generate detectable antibodies. The antibody titre peaks at 4–6 weeks, then slowly declines. Memory cells persist. After a booster dose, the secondary response is rapid (2–3 days), high-magnitude, predominantly IgG, and long-lasting. Live attenuated vaccines generally produce longer-lasting immunity (often lifelong) from fewer doses because they replicate transiently, providing prolonged antigen exposure that drives stronger and more durable memory.

Adjuvants and dynamics:
Inactivated, subunit, and toxoid vaccines typically have lower immunogenicity than live vaccines. Adjuvants — aluminium salts (alum) being the most widely used — enhance immunity by activating innate immune pattern recognition receptors (Toll-like receptors), prolonging antigen retention at the injection site (depot effect), recruiting immune cells, and providing a 'danger signal' that promotes CD4+ T-helper cell activation. The AS04 adjuvant system (alum + MPL, a TLR4 agonist) used in Cervarix (bivalent HPV vaccine) produces stronger and more durable immunity than alum alone.

Adverse drug reactions by vaccine type:
- Local reactions (universal): Pain, redness, swelling at the injection site — due to innate immune activation; more marked with adjuvanted vaccines; generally mild and self-limiting.
- Systemic reactions: Fever, malaise, myalgia — especially after live attenuated and whole-cell pertussis vaccines; represent the innate inflammatory response to vaccine-induced 'infection'.
- Live vaccine-specific ADRs: Because live vaccines replicate, they can rarely cause vaccine-strain disease — OPV can cause vaccine-associated paralytic poliomyelitis (VAPP, ~1:750,000 doses); varicella vaccine can cause a mild varicella-like rash; yellow fever vaccine very rarely causes yellow fever-like viscerotropic disease in elderly first-time recipients. Disseminated vaccine-strain infection can occur in immunocompromised patients — a life-threatening complication that is the basis for absolute contraindication.
- Anaphylaxis: Rare but serious; occurs with any vaccine; protocol requires 15–30 minutes observation after administration.
- Vaccine-induced immune thrombocytopenia and thrombosis (VITT): Identified as a rare adverse event (approximately 1:100,000) after ChAdOx1 (AstraZeneca) and Ad26.COV2.S (Janssen) COVID-19 adenoviral vector vaccines; mechanism involves cross-reactive antibodies against platelet factor 4 (PF4) similar to heparin-induced thrombocytopenia; presents with thrombosis at unusual sites (cerebral venous sinus, splanchnic veins) combined with thrombocytopenia, 5–30 days after the first dose.
- mRNA vaccine ADRs: Injection site pain, fatigue, headache, myalgia (commonly); myocarditis/pericarditis (rare, predominantly in young males after second dose of mRNA COVID-19 vaccine — most cases mild and self-resolving).
- Guillain-Barré syndrome (GBS): Very rare association with some influenza vaccines (established historical association with the 1976 swine flu vaccine); also a potential association with some COVID-19 adenoviral vector vaccines — risk far outweighs benefit for most populations.

Clinical Use of Vaccines: Cold Chain, Schedule, Contraindications

Correct clinical use of vaccines requires knowledge of cold chain, schedule, contraindications, and the management of adverse events.

Cold chain management is the unbroken temperature-controlled supply chain from vaccine manufacturer to patient. Most vaccines require storage at +2°C to +8°C (refrigerator temperature). Key exceptions: OPV and varicella vaccine traditionally require -20°C (freezer storage), although newer vVPV formulations have improved thermostability. mRNA COVID-19 vaccines (BNT162b2) required -70°C storage initially, with shorter-term refrigerator storage after thawing. Freeze-sensitive vaccines (most aluminium-adjuvanted vaccines — DTwP, hepatitis B, IPV, DTaP) are DAMAGED BY FREEZING — a critical cold chain error. Healthcare providers must routinely check the Vaccine Vial Monitor (VVM) — a heat-sensitive label on the vial that changes colour irreversibly when cumulative heat exposure exceeds a threshold, signalling that the vaccine may be inactive.

India's Universal Immunisation Programme (UIP) and NMC-relevant schedule:
The core paediatric schedule includes: BCG at birth; OPV/IPV series; Hep B at birth and at 6, 10, 14 weeks; DTwP/DTaP; Hib; PCV; rotavirus; measles/MR/MMR; JE in endemic areas. For adolescents and adults: Tdap booster, Hep B catch-up, HPV vaccine for girls (9–14 years), influenza annually for high-risk groups, typhoid.

Contraindications — live vaccines (ABSOLUTE):
- All immunocompromised states: HIV with CD4 count <200/mm³, haematological malignancy, organ transplant recipients on immunosuppressants, high-dose corticosteroids (>20 mg/day prednisolone for >2 weeks), biologics and JAK inhibitors, primary immunodeficiencies
- Pregnancy (live viral vaccines — MMR, varicella, yellow fever; BCG is given to neonates but deferred in pregnancy)
- Known severe allergy (anaphylaxis) to a vaccine component (e.g. neomycin allergy and MMR)

False contraindications (common errors): Low-grade fever, minor illness, mild allergy, family history of adverse vaccine reactions, premature birth, breastfeeding — NONE of these are valid contraindications and patients should be vaccinated on schedule.

Vaccine hesitancy: The WHO identifies vaccine hesitancy as a top-10 threat to global health. Physicians are the most trusted source of vaccine information for most patients. Evidence-based counselling — addressing concerns about ingredients, autism (the MMR-autism link has been thoroughly disproved and the original study retracted), adjuvant safety, and mRNA technology — is a core clinical communication competency.

Self-Assessment

Test your understanding of vaccine pharmacology with the following questions.

1. Classify the following vaccines by type and state whether they can be given to an HIV-positive patient with CD4 count 150/mm³: (a) BCG, (b) hepatitis B vaccine, (c) IPV, (d) OPV, (e) varicella vaccine.

1. A healthcare worker receives only one dose of hepatitis B vaccine and then is lost to follow-up. She presents 5 years later asking about immunity. What is the likely serological status, and what is the recommended course of action?

1. Explain why the Hib plain polysaccharide vaccine was replaced by the conjugate vaccine for use in infant immunisation schedules. Which immunological mechanism does this exploit?

1. Name two adjuvants used in commercial vaccines and describe their mechanism of action.

1. A 25-year-old man receives the first dose of ChAdOx1 (AstraZeneca COVID-19) and 10 days later develops severe headache, leg pain, and is found to have cerebral venous sinus thrombosis with platelet count 40,000/mm³. Name the adverse event, its proposed mechanism, and the treatment approach (clue: standard heparin is contraindicated).