Vaccines are among the most pharmacologically complex therapeutic agents — combining immunology, formulation science, and population-level pharmacoepidemiology. This guide covers vaccine platforms, mechanism of action, adjuvant science, immunosuppressive drug interactions, and herd immunity thresholds.
The timing of vaccination relative to immunosuppressive therapy is critical to achieving adequate seroprotection. Key principles:
Methotrexate
Hold for 2 weeks post-vaccine (if disease allows) — improves influenza and COVID-19 seroconversion (EULAR recommendation).
Rituximab
Vaccinate ≥4 weeks before infusion or wait ≥6 months post-infusion. B-cell depletion abolishes humoral vaccine response for 6–12 months.
JAK inhibitors
Reduce humoral responses to COVID-19 mRNA and influenza vaccines. Vaccinate before starting therapy where possible.
Mycophenolate / azathioprine
Modest reduction in vaccine response. Inactivated vaccines generally safe to administer. Avoid live vaccines.
Systemic corticosteroids
Avoid live vaccines at ≥20 mg/day prednisone for ≥2 weeks. Inactivated vaccines can be given but timing with lower-dose periods improves response.
TNF inhibitors
Live vaccines contraindicated. Inactivated vaccines should be administered before starting TNF therapy where possible. Pneumococcal and annual influenza are priority vaccines.
Herd immunity threshold = 1 − 1/R₀. The basic reproduction number (R₀) determines the proportion of the population that must be immune to halt sustained transmission.
Disease
R₀ range
Herd immunity threshold
Vaccine
Measles
12–18
~94%
MMR (2 doses)
Polio
5–7
~85%
IPV / OPV
Pertussis
12–17
~92–94%
DTaP / Tdap
Smallpox
5–7
~83%
Vaccinia (eradicated)
COVID-19 (original)
2–3
~50–67%
mRNA / viral vector
COVID-19 (Omicron)
8–15
~88–93%
Updated bivalent mRNA
Frequently asked questions
How do vaccines work pharmacologically?
Vaccines prime adaptive immunity by presenting antigen safely. Humoral response: B cells → plasma cells → antigen-specific IgG/IgA/IgM antibodies + memory B cells. Cellular response: CD4+ T helper cells support B cells; CD8+ cytotoxic T cells destroy infected cells. On re-exposure, memory cells mount a rapid secondary response. Adjuvants (aluminium salts, AS04, MF59) amplify innate immune activation to improve immunogenicity. See WHO immunisation recommendations ↗.
What are the differences between mRNA, viral vector, subunit, and live-attenuated vaccines?
Live-attenuated (MMR, varicella, BCG): strong durable immunity; cold chain required; avoid in immunocompromised. Inactivated/subunit (hepatitis B, HPV, pneumococcal): safe in immunocompromised; may need boosters; adjuvants often needed. Viral vector (adenovirus-based): DNA delivered via attenuated virus; risk of pre-existing vector immunity reducing efficacy. mRNA (lipid nanoparticle-encapsulated): no DNA integration; strong TH1 response; adaptable to new variants rapidly. For immunocompromised patients, see the drug allergy and hypersensitivity guide for allergy screening. CDC Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book) ↗.
How do immunosuppressive drugs affect vaccine responses?
Immunosuppression impairs vaccine response. Key interactions: Methotrexate: reduces seasonal influenza and COVID-19 vaccine responses — temporary hold (2 weeks) improves seroconversion (EULAR guidance ↗); Rituximab (anti-CD20): depletes B cells — vaccine response minimal for 6–12 months post-infusion; time vaccines before rituximab or wait 6+ months; JAK inhibitors: reduce humoral responses; High-dose corticosteroids (≥20 mg/day ≥2 weeks): avoid live vaccines. Live vaccines (MMR, varicella) contraindicated in significant immunosuppression — see pharmacogenomics for immune-related pharmacological variation.
What is herd immunity and how does vaccination achieve it?
Herd immunity threshold = 1 − 1/R₀. Examples: measles (R₀ 12–18) → ~94% coverage needed (explains why measles returns when MMR coverage drops below ~95%); polio (R₀ ~5–7) → ~85%; COVID-19 original strain (R₀ ~2.5) → ~60% (new variants with R₀ >10 significantly raised this). Population immunity gaps due to vaccine hesitancy or access disparities create pockets of susceptibility. WHO herd immunity explainer ↗. Related: antimicrobial stewardship addresses analogous population-level drug exposure management.
What are adjuvants and why are they added to vaccines?
Adjuvants activate innate pattern recognition receptors (TLRs, NLRs) to amplify adaptive immune responses to antigens. Without them, purified antigens produce weak, short-lived immunity. Common adjuvants: Alum (aluminium hydroxide/phosphate) — depot effect; enhances antibody responses; oldest and most widely used. AS04 (alum + MPL, TLR4 agonist) — Cervarix HPV vaccine ↗. MF59 (squalene-based oil-in-water emulsion) — Fluad (influenza, elderly) ↗. AS01B (MPL + QS-21 saponin) — Shingrix (shingles), Mosquirix (malaria). Understanding adjuvants is central to vaccine pharmacology; see immunogenicity vs bioavailability parallels.