Active immunization involves the use of inactivated or attenuated pathogens, or selected components of these pathogens, to induce an immune response in the vaccinated individual. The purpose of vaccination is to establish protective immunity that can prevent disease when the individual is later exposed to the actual pathogen.

Several types of vaccines are available, and in some cases they may be used in combination to achieve the desired protective effect. Vaccines are not administered arbitrarily; they are used according to dosages and protocols established through extensive research and development by pharmaceutical companies. Their application takes into account both indications and contraindications. These prescribing guidelines are evaluated and approved by the relevant regulatory authorities (European, national, or international) based on experimental and analytical evidence, in accordance with the principles of evidence-based medicine.

Since each vaccine type differs in its mode of action, the most important question is always: what is the vaccine designed to protect against? Let us take a closer look at the different categories.

Inactivated Vaccines

Inactivated vaccines contain killed pathogens. As a result, they are considered safe, since the pathogen cannot replicate in the vaccinated animal or human. Immunity is induced by the antigens present at the site of administration. For certain highly dangerous diseases, inactivated vaccines are the only acceptable option due to biosafety concerns. A well-known example is the rabies vaccine.

The magnitude and duration of the immune response depend largely on the pathogen itself, with immunity typically developing within 10–14 days after vaccination. These vaccines are particularly effective for boosting pre-existing immunity. The immune response can be enhanced through the use of adjuvants.

Attenuated Vaccines

Attenuated vaccines contain live but weakened pathogens. Because the pathogen can still replicate within the host, these vaccines are not entirely risk-free and may occasionally cause mild symptoms following administration. The degree of attenuation varies between vaccines and is an important consideration.

Their major advantage is the strong and long-lasting immune response they induce, often beginning as early as 5–7 days after vaccination. Some vaccine strains are naturally attenuated and possess reduced pathogenicity. Another significant benefit is their suitability for mass vaccination programs, for example through aerosol or spray administration. Since vaccine strains may spread among animals, a more homogeneous level of immunity can be achieved within a population.

Subunit Vaccines

Subunit vaccines contain purified antigens, typically surface proteins responsible for inducing protective immunity. Because they contain only selected components of the pathogen, they generally cause fewer adverse effects. However, they are often more expensive to produce, and the immunity they induce may be weaker than that generated by vaccines containing the entire pathogen.

Marker Vaccines

The principle of marker vaccines is based on the use of vaccine strains that differ slightly from the wild-type, virulent pathogen, such as deletion mutants lacking specific proteins. As a result, the immune response induced by vaccination can be distinguished from that caused by natural infection using specific diagnostic methods. This allows vaccinated animals to be differentiated from infected animals, an approach commonly referred to as DIVA (Differentiating Infected from Vaccinated Animals).

Biosynthetic (Recombinant) Vaccines

These vaccines are produced by inserting a gene encoding an important surface protein of the pathogen into a simple, easily cultured microorganism, such as a bacterium. The host microorganism then produces the foreign protein, which can be purified and used as a vaccine antigen.

This technology is similar to the production of recombinant insulin and has become a widely used method in modern vaccine development.

Vector Vaccines

Vector vaccines use a harmless virus as a carrier, or vector, for genetic material derived from another pathogen. A gene encoding one of the pathogen's proteins is inserted into the vector virus, creating a recombinant virus that can safely stimulate an immune response.

Following vaccination, the host develops immunity against the protein originating from the target pathogen. Multiple genes from different pathogens can be incorporated into a single vector, potentially providing protection against several diseases simultaneously.

Nucleic Acid (DNA and RNA) Vaccines

Among the newest generation of vaccines, nucleic acid-based vaccines have a particularly innovative mechanism of action. These vaccines deliver genetic information encoding a specific pathogen protein into the host's cells. The vaccinated individual's own cells then produce the antigen, which stimulates the immune system to develop protective immunity.

In this sense, nucleic acid vaccines can be considered a form of subunit vaccine, as only a single protein of the pathogen is produced. Consequently, their safety profile is generally comparable to that of traditional subunit vaccines.

Beyond infectious diseases, this technology also holds promise for cancer therapy as well as the treatment of certain autoimmune and allergic disorders.

Conclusion

Even this brief overview demonstrates how far vaccine science has advanced in the fight against infectious diseases. In reality, the field of vaccinology is far more complex and diverse than can be fully captured in a short summary, and numerous factors influence vaccine development and application.

These factors include the age and immunological status of the individual being vaccinated, the presence or absence of maternal antibodies, vaccine stability and storage conditions, environmental influences, and many other considerations. The level of protection achieved also depends heavily on the characteristics of the pathogen, regional epidemiological conditions, and international regulatory requirements.

For these reasons, it is difficult to provide universal rules that apply to all vaccines. A comprehensive understanding of vaccinology requires extensive knowledge of microbiology, immunology, epidemiology, and related disciplines. Yet it is precisely this complexity that makes vaccine research such a fascinating and continually evolving field.