Principles of vaccines and immunisation

Active and passive immunity, the main vaccine types, the cold chain and how Colombia's Expanded Programme on Immunisation (PAI) delivers vaccines.

Updated Nov 2, 2026

A vaccine teaches the immune system to recognise a microbe or toxin before a real encounter, leaving memory cells that respond quickly later. Immunoglobulins and antitoxins instead give ready-made antibodies for immediate, short-lived protection. Vaccine programmes protect not only the vaccinated person but also people who cannot be vaccinated, by limiting circulation of the microbe.

Key points

Active immunity is produced by the person's own immune system (after infection or vaccination) and usually lasts years; passive immunity is transferred antibody and lasts weeks to months.
Live attenuated vaccines (measles–mumps–rubella, yellow fever, BCG, oral rotavirus) give strong, long immunity but are avoided in pregnancy and in most immunosuppressed people.
Inactivated vaccines (whole, subunit, toxoid, polysaccharide, conjugate, recombinant) cannot replicate, usually need several doses and boosters.
Plain polysaccharide vaccines work poorly in young children; joining the sugar to a protein (conjugate vaccines) recruits T-cell help and memory.
In Colombia the PAI of the Ministry of Health runs the national schedule and the cold chain, which keeps vaccines between 2 °C and 8 °C at vaccination sites.

Active and passive immunity

Passive protection comes from maternal IgG that crosses the placenta, from breast milk, and from products such as human normal immunoglobulin, hyperimmune globulins (tetanus, rabies, hepatitis B) and animal antitoxins and antivenoms. It acts at once but fades as the antibody is broken down. Rabies post-exposure care combines both: immunoglobulin for immediate cover and vaccine for lasting immunity.

Vaccine types

Live attenuated: weakened microbe that replicates; often one or two doses give long immunity
Inactivated whole organism: killed virus or bacterium
Subunit and recombinant: purified or engineered proteins, such as hepatitis B and HPV vaccines
Toxoid: inactivated toxin (tetanus, diphtheria); protects against the toxin, not colonisation
Polysaccharide and conjugate: capsule sugars, alone or linked to a carrier protein (Hib, pneumococcal, meningococcal)
Nucleic acid and viral-vector vaccines: the body's cells make the antigen

Safety and timing

Most reactions are local pain, fever or malaise. Severe allergy to a previous dose or a component is a contraindication; live vaccines are generally avoided in pregnancy and in most forms of immunosuppression because the vaccine strain can cause disease. Immunoglobulin products can blunt the response to some live-virus vaccines, so the timing between them matters. In Colombia, events supposedly attributable to vaccination or immunisation (ESAVI, also called EAPV) are tracked by vaccine-safety surveillance; an event that follows a vaccine is not necessarily caused by it.

The cold chain and the PAI

Vaccines lose potency with heat and some are destroyed by freezing. The cold chain is the logistic process that keeps them at the right temperature from the manufacturer to the moment of injection: in Colombia, from a national store through departmental and municipal centres to local vaccination sites, with WHO-prequalified refrigerators and cold boxes, and solar refrigerators where there is no electricity. The PAI sets the national schedule for children, pregnant women and older adults; the current ages and doses are on the Ministry of Health's vaccination pages.

Herd (community) immunity

When enough people are immune, a microbe cannot find new hosts easily and stops circulating. This protects newborns, pregnant women and people who cannot receive live vaccines. Falling coverage allows outbreaks to return.

Test yourself

Think of the answer, then tap the question to see it.

Why are live attenuated vaccines generally avoided in severely immunosuppressed patients?

The weakened microbe still replicates and can cause severe or disseminated disease when immunity is poor.

Why are conjugate vaccines used in infants instead of plain polysaccharide vaccines?

Plain polysaccharides trigger a T-cell-independent response that is weak and short in young children; linking them to a protein recruits T-cell help and memory.

In rabies post-exposure care, why are immunoglobulin and vaccine both given?

Immunoglobulin gives immediate passive protection while the vaccine induces the person's own lasting antibodies.

What is the cold chain?

The logistic process that keeps vaccines at the required temperature from production to administration; in Colombia's PAI vaccination sites keep them between 2 °C and 8 °C.

Add to my cards (4)

Example drugs

Connections

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Chapter 1: Principles of Vaccination | Pink Book | CDC (Centers for Disease Control and Prevention (CDC), 2021); ¿Cómo funcionan las vacunas? (Ministerio de Salud y Protección Social de Colombia (PAI), 2026); La cadena de frío del Programa Ampliado de Inmunizaciones - PAI (Ministerio de Salud y Protección Social de Colombia (PAI), 2026); Vigilancia en vacunación EAPV/ESAVI (Ministerio de Salud y Protección Social de Colombia (PAI), 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.