10.5 Vaccination, Immune Memory and Herd Immunity

Vaccination exposes the immune system to antigen without the uncontrolled disease risk of natural infection. A vaccine may contain attenuated or inactivated pathogen, purified antigen, an inactivated toxin, genetic instructions for an antigen, or a harmless vector delivering those instructions. The platforms differ, but each aims to create antigen presentation, lymphocyte selection and memory before a dangerous encounter.

An mRNA vaccine supplies a temporary RNA template encoding an antigen. Host ribosomes translate the mRNA, antigen is produced for a limited time and immune recognition follows. The mRNA does not need to enter the nucleus and is eventually degraded. A viral-vector vaccine uses a modified carrier virus to deliver antigen-coding DNA to cells. Neither platform requires the disease-causing pathogen to reproduce normally in the vaccinated person.

The first dose produces a primary response with a lag while selected B and T cells expand. A later booster reintroduces antigen to a larger memory-cell population, so antibody concentration rises sooner and often to a higher peak. Protection can decline if antibody and memory populations wane or if pathogen antigens change. Booster intervals therefore depend on immune persistence, disease risk and antigenic evolution rather than a single rule for all vaccines.

A vaccinated person may still be infected if pathogen particles enter and begin replicating. Protection means the primed response controls infection sooner, reducing the probability or severity of disease. Some vaccines also strongly reduce infection and onward transmission; others mainly prevent severe outcomes. These effects must be measured separately. The claim that any detected infection proves vaccine failure confuses sterilizing immunity with clinically valuable protection.

Vaccine effectiveness depends on the match between vaccine antigens and circulating variants, the recipient's age and immune condition, dose schedule and time since vaccination. Population studies compare outcomes among vaccinated and unvaccinated groups while accounting for exposure and confounding variables. No intervention needs to be perfect to produce a large reduction in expected harm.

Preclinical work begins in cells or tissues and then suitable animal models to investigate toxicity, dose and the kind of immune response produced. Model organisms share many pathways with humans but are not miniature humans; host specificity and immune differences limit inference. Evidence from an animal study supports progression, not automatic approval for general human use.

Phase I trials use a small number of volunteers and focus on safety and dose. Phase II expands participation and examines consistency, immune response and common side effects. Phase III includes many more participants and estimates efficacy while detecting less common adverse events. Regulators assess the combined evidence before licensing, and surveillance continues after deployment because extremely rare effects may only become visible across millions of doses.

Ethical testing requires informed consent, independent review, scientifically justified comparison groups and ongoing safety monitoring. Historical experiments that deliberately exposed children without modern consent would not meet present standards. Urgency can accelerate overlapping logistics and recruitment, but it does not remove the need to establish manufacturing quality, safety and efficacy.

When many people are immune, an infectious person is less likely to meet a susceptible person, so transmission chains end more often. This indirect protection is herd immunity. It is especially valuable for newborns, people with severe immune suppression and others who cannot receive or respond strongly to a vaccine. The protected person does not acquire memory cells from the surrounding community; risk falls because exposure becomes less likely.

For a simplified, well-mixed population with a vaccine that perfectly blocks transmission, the critical immune fraction is approximately 1 − 1/R₀, where R₀ is the average number of secondary cases produced by one case in a wholly susceptible population. Real thresholds are affected by incomplete vaccine effectiveness, uneven contact networks, waning immunity and behaviour. Highly transmissible pathogens require a larger immune fraction because more potential transmission links must be blocked.

Herd immunity applies only when protection interrupts person-to-person transmission. Tetanus bacteria persist in the environment and disease is not acquired from another person with tetanus, so vaccinated neighbours do not remove an individual's exposure route. By contrast, sustained high coverage helped eradicate smallpox because humans were the essential host, infection was identifiable and an effective vaccine interrupted transmission.

Eradication is more demanding than local control. Surveillance must find remaining cases, vaccination and containment must reach mobile or underserved populations, and countries must coordinate because reintroduction ignores borders. Smallpox combined favourable biology with international organization: no non-human reservoir maintained transmission, cases were recognizable, immunity was effective and coordinated campaigns could concentrate on outbreaks. A successful vaccine is necessary for many eradication programmes but is rarely sufficient by itself.