Table of Contents
Basic Idea of Active Immunization
Active immunization is a medical procedure that intentionally stimulates the body’s own specific immune response so that it builds a long-lasting “immunological memory” against a particular pathogen or toxin. After active immunization, the body itself produces antibodies and memory cells; protection develops with a delay but often lasts for years or decades.
Active immunization is usually carried out with vaccines. These contain antigens (or genetic information for antigens) that resemble the real pathogen enough to trigger a protective immune response, but without causing the full disease in healthy individuals.
In contrast to passive immunization (ready-made antibodies, fast but short-lived protection), active immunization is slow to develop but generally long-term and sometimes lifelong.
Components and Types of Vaccines
All vaccines are based on presenting antigens to the immune system in a form that is safe but immunogenic (able to stimulate a strong immune response). Different strategies are used:
1. Live Attenuated Vaccines
These vaccines contain living pathogens (viruses or bacteria) that have been weakened (attenuated) so they can still replicate to a limited extent but no longer cause severe disease in people with normal immune systems.
Typical examples (conceptual, not a full list) include vaccines against some viral childhood diseases.
Features:
- Mimic a natural infection relatively well.
- Strong and often long-lasting immune response (both antibody-mediated and cell-mediated immunity).
- Often only 1–2 doses needed for long-term protection.
- Can sometimes cause mild, short-lived symptoms similar to a very mild form of the disease.
- Must not be given to people with severe immune deficiencies or in some cases during pregnancy, because even weakened pathogens can be dangerous for them.
- Often sensitive to heat and require a cold chain during storage and transport.
2. Inactivated (Killed) Vaccines
These vaccines contain pathogens that have been killed (e.g., by heat or chemicals), or parts of them, so they cannot replicate.
Features:
- Safe for immunocompromised people because the pathogen cannot multiply.
- Mainly stimulate antibody-mediated (humoral) immunity.
- Usually require repeated doses and booster shots to maintain protection.
- Less risk of reverting to a disease-causing form (because the pathogen is not alive).
Inactivated vaccines include:
- Whole-pathogen vaccines: entire killed bacteria or viruses.
- Subunit vaccines: only selected components (e.g., surface proteins, polysaccharides).
3. Toxoid Vaccines
Some bacteria cause disease primarily by producing toxins. Toxoid vaccines do not contain the bacteria themselves but chemically inactivated toxins (toxins are converted to harmless toxoids that still retain their antigenic structure).
Features:
- The immune system learns to recognize and neutralize the toxin.
- Protection focuses on the effects of the toxin rather than preventing colonization entirely.
- Usually require booster vaccinations at regular intervals to maintain high antibody levels.
4. Subunit, Conjugate, and Recombinant Vaccines
These vaccines use only specific purified components of the pathogen rather than the whole organism.
Subunit Vaccines
- Contain only selected antigens, such as a particular protein from the surface of a virus or bacterium.
- Reduced side effects because unnecessary components of the pathogen are removed.
- Often require adjuvants (see below) and multiple doses.
Conjugate Vaccines
Some bacteria are surrounded by capsules made of polysaccharides that are poorly recognized by the immature immune system of young children. In conjugate vaccines, these polysaccharides are chemically linked to a carrier protein.
Advantages:
- Stronger and more reliable immune response.
- Effective even in infants and small children.
- Better immunological memory.
Recombinant Vaccines
Antigens are produced by genetically engineered microorganisms (e.g., yeast or bacteria). The organism’s genome is modified to produce a pathogen’s antigen, which is then purified and used as a vaccine.
Advantages:
- No need to handle large quantities of the actual pathogen.
- Very pure and well-defined composition.
- Flexible design and relatively rapid adaptation to new strains or variants.
5. Nucleic Acid Vaccines (DNA and mRNA Vaccines)
These vaccines do not contain the antigen itself but give cells the genetic information to produce the antigen temporarily.
DNA Vaccines
- Plasmid DNA encoding one or more antigens is injected.
- Some of the recipient’s cells take up the DNA and produce the antigen.
- Still under development in many contexts; used in some veterinary vaccines.
mRNA Vaccines
- Contain messenger RNA encapsulated in lipid nanoparticles.
- Cells take up the mRNA, produce the antigen for a limited time, and present it to the immune system.
General features of nucleic acid vaccines:
- No live pathogen; no risk of infection from the vaccine.
- Rapidly adaptable to new variants by changing the genetic sequence.
- Often elicit both antibody and T cell responses.
- Require careful formulation and storage (e.g., sensitivity of mRNA to degradation).
Role of Adjuvants and Additives
Many vaccines, especially inactivated, subunit, and toxoid vaccines, need additional components to elicit a strong and long-lasting immune response.
Adjuvants
Adjuvants are substances added to vaccines to enhance the immune response to the antigen.
- Common adjuvants include aluminum salts (alum) and newer formulations using emulsions or other molecules.
- They often act by causing a mild, local inflammatory reaction that attracts immune cells and activates them.
- Without adjuvants, some purified antigens would be too weakly immunogenic.
Other Additives
Vaccines may also contain:
- Stabilizers (e.g., sugars, proteins) to maintain the antigen’s structure.
- Preservatives in multi-dose vials to prevent contamination.
- Residual traces of substances used in production or inactivation (e.g., formaldehyde), usually in extremely low amounts well below toxic thresholds.
These components are specific to vaccine technology and formulation and are strictly regulated.
How Active Immunization Works in the Body
The general principles of the immune response and immunological memory are introduced elsewhere; here the focus is on how a vaccine initiates these processes.
First Contact: Primary Immune Response
After vaccination:
- Antigen uptake
Antigen-presenting cells (APCs) such as dendritic cells take up the vaccine antigens at the injection site. - Migration and presentation
APCs migrate to nearby lymph nodes and present processed antigen fragments on their surface in combination with MHC molecules. - Activation of lymphocytes
- Naive T and B cells specific for this antigen encounter the APCs.
- T helper cells become activated and help B cells.
- B cells differentiate into plasma cells and begin producing specific antibodies.
- Formation of memory cells
Some activated B and T cells become long-lived memory cells that persist after the initial response has waned.
This primary response takes days to weeks to reach full strength, which is why protection soon after the first vaccination is incomplete.
Booster Vaccinations and Secondary Immune Response
Many vaccines are given in several doses (basic immunization and later boosters). Each additional contact with the antigen stimulates a secondary immune response:
- Faster onset.
- Much higher antibody levels.
- Higher affinity antibodies (better binding quality due to affinity maturation).
- Larger pool of memory cells.
This improved response after booster shots is the immunological basis for schedules that include multiple doses and periodic refreshers.
Vaccination Schedules and Strategies
Public health authorities establish vaccination recommendations and schedules based on:
- Typical age at which a disease is most dangerous or common.
- How long vaccine-induced protection usually lasts.
- Specific risks for some groups (e.g., pregnant people, elderly, health-care workers, travelers).
Basic Immunization
- Initial series of vaccinations that builds primary protection.
- Given in early childhood for many infectious diseases, and in adulthood for certain others.
- Often consists of 2–3 doses within a defined time frame.
Booster Vaccinations
- Given after years when antibody levels and rapid responsiveness may decline.
- Aim to maintain or restore protection throughout life.
- Intervals vary depending on the disease and vaccine (can range from a few years to decades).
Herd Immunity (Population-Level Effect)
When a sufficiently large fraction of a population is actively immunized against a disease spread from person to person:
- The pathogen has fewer opportunities to find susceptible hosts.
- Transmission chains break more easily.
- Even people who cannot be vaccinated (e.g., some immunocompromised individuals) gain indirect protection because they are less likely to encounter the pathogen.
The threshold for herd immunity depends on how contagious the disease is.
Indications and Contraindications
Not every vaccine is suitable for every person at every time. Decisions about active immunization consider:
Typical Indications
- Routine childhood and adolescent vaccination schedules.
- Vaccination before travel to regions with particular disease risks.
- Vaccination for occupational exposure (e.g., laboratory staff, health-care workers).
- Catch-up vaccination in adults who missed earlier immunizations.
- Vaccination of at-risk groups (e.g., people with certain chronic illnesses).
Contraindications and Precautions
Examples include:
- Severe acute illness with fever: vaccination may be postponed.
- Known severe allergy to a vaccine component: requires alternative preparations or special precaution.
- Severe immune deficiency: live attenuated vaccines are usually contraindicated; inactivated vaccines may still be possible but may be less effective.
- Pregnancy: many inactivated vaccines are considered safe when indicated; live attenuated vaccines are usually avoided.
Decisions are based on individual risk–benefit assessment.
Side Effects and Risks of Active Immunization
Vaccines are designed to be as safe as possible, but they deliberately stimulate the immune system, so reactions are expected.
Common, Mild Reactions
These are signs that the immune system is working and usually resolve within a few days without treatment or with simple measures.
- Local reactions at the injection site: redness, warmth, swelling, pain.
- Mild general symptoms: fatigue, low-grade fever, headache, muscle aches.
- In children: transient irritability or reduced appetite.
Rare but More Serious Reactions
- High fever, extensive swelling, or strong injection site reactions.
- Allergic reactions, including very rare severe reactions (e.g., anaphylaxis) soon after vaccination.
- Very rare specific complications associated with certain vaccines; these are monitored closely.
Because vaccines are given to healthy people, safety requirements are particularly strict. The overall risk from vaccines is generally far lower than the risk from the diseases they prevent.
Development, Testing, and Surveillance of Vaccines
Active immunization depends on a multi-step process to ensure effectiveness and safety.
Development Phases
- Preclinical research
Identification of antigens, choice of vaccine type, laboratory and animal studies for immunogenicity and toxicity. - Clinical trials in humans
Conducted in phases: - Phase I: small groups, mainly safety and dosing.
- Phase II: larger groups, immunogenicity (antibody levels, cellular responses).
- Phase III: large groups, effectiveness in preventing disease and detailed safety analysis.
Approval and Quality Control
- Regulatory authorities evaluate results and may approve vaccines only if benefits clearly outweigh risks.
- Each batch is tested for identity, purity, and potency before it can be used.
Post-Approval Surveillance
- Ongoing monitoring of vaccine effectiveness and safety.
- Reporting systems for suspected vaccine side effects.
- Adjustments to recommendations and formulations if necessary (e.g., in response to new pathogen variants).
Advantages and Limitations of Active Immunization
Advantages
- Often long-lasting, sometimes lifelong protection.
- Prepares the immune system before exposure to the real pathogen.
- Can significantly reduce or even eliminate diseases at the population level.
- Cost-effective prevention compared to treating diseases and complications.
- Reduces misuse of antibiotics by preventing bacterial infections and complications.
Limitations
- Protection is not immediate; time is needed to develop immunity after vaccination.
- Some individuals respond weakly or not at all (non-responders).
- Booster doses may be necessary.
- Not all diseases currently have effective vaccines (e.g., certain chronic infections, rapidly changing pathogens).
- Requires infrastructure and organization (cold chain, trained personnel, record keeping).
Summary
Active immunization uses vaccines to stimulate the body’s own immune system to form specific antibodies and memory cells against pathogens or their toxins. Different vaccine types—live attenuated, inactivated, subunit, toxoid, conjugate, recombinant, and nucleic acid vaccines—present antigens in various ways but share the goal of safe, effective induction of long-term protection. With appropriate scheduling, monitoring, and safety measures, active immunization is one of the most powerful tools for preventing infectious diseases at both individual and population levels.