Vaccines introduce materials mimicking pathogens (so-called antigens) into the body that make the immune system to produce antibodies and develop other types of defense mechanisms (e.g. direct defense mechanisms by immune cells) against real pathogens.
This process is called active immunity. Live, attenuated and non-live vaccines are used to develop active immune response. In certain cases, it is possible to introduce preformed antibodies into the body, thereby providing a transient defense – this form of immunization is called passive immunisation.
Live, attenuated vaccines
Live, attenuated vaccines contain a version of the living pathogen that has been weakened. BCG vaccine for tuberculosis (TB), measles (ten-day measles, rubeola), rubella (german-measles, three-day measles), mumps, varicella (chickenpox) vaccines and vaccines against yellow fever belong to this category.
- after introducing to the body, they reproduce, and create their effect by mimicking the natural pathways of infection;
- rarely, they might cause vaccine reactions;
- they provide long-term protection.
However, vaccination of immunodeficient or immunocompromised patients with live, attenuated vaccines can lead to severe disease, therefore it is advised to consult a specialist before vaccinating them with live, attenuated vaccines.
In Hungary, the following live, attenuated vaccines are authorized:
- BCG-vaccines (tuberculosis), MMR-vaccines (measles, mumps and rubella), varicella-vaccine (chickenpox), Rotarix/Rotateq (rotavirus) vaccines;
- yellow-fever (Stamaril) vaccines (available only at special vaccination points).
Non-live vaccines contain an active agent that does not reproduce in the body. The adequate immune protection is developed after more than one dose of vaccines (basal immunization/ primary series of vaccination and booster vaccines).
Depending on the active agent, we can differentiate the following categories:
- vaccines containing the whole killed or inactivated pathogen (pertussis, cholera, hepatitis A, IPV – poliovirus, rabies, tick-born encephalitis virus, flu virus);
- vaccines containing pathogen particles (acellular pertussis vaccines);
- vaccines containing pathogen particles produced by gene technology (hepatitis B-virus, human papillomavirus – HPV);
- vaccines containing particles of the bacteria’s capsule (pneumococcus, typhus);
- conjugate vaccines containing parts of the bacteria’s capsule conjugated with a protein that enhances the vaccine’s effect (Pneumococcus, Meningococcus);
- toxoid vaccines containing weakened, inactivated toxins (diphtheria, tetanus, cholera);
- mRNA-vaccines containing some genetic material coding for proteins of pathogens (certain types of COVID-19 vaccines);
- viral vector vaccines that introduce particulates of the pathogen’s genetic material into the body with the help of another, so-called virus vector (e.g., certain types of COVID-19 vaccines).
Maternal or passive antibodies do not influence the effect of vaccines, therefore (in case of hepatitis A and B, or tetanus infection or rabies) active and passive immunization can be used at the same time.
In immunodeficient patients the immune response may not develop by itself, thereby passive immunization is necessary for their protection.
Different routes of vaccination
Vaccines can be introduced to the body in the following ways:
- injection into the muscles (so-called intramuscular injections);
- injection into the skin (so-called intracutaneous injections);
- orally, into the gastrointestinal tract;
- into the respiratory mucosa via nasal inhalation.
Vaccines must be introduced to the body according to the recommendations and instructions of the pharmaceutical company manufacturing them.
The vaccines’ mechanism of action
During infections, phagocytes (a certain type of cells of the immune system) ingest germs, kill and dismember them. They also “introduce” certain parts of the pathogens (which we call antigens) to other cells of the immune system. This is the phase when the immune system “gets to know” the characteristics of the new microbe, and it usually takes approximately 7 days when a certain type of infection occurs for the first time in the body. During this stage, the early symptoms of the infection might develop.
After that, certain immune cells start to multiply and produce proteins (antibodies) against the pathogen. Antibodies can destroy germs with the help of immune cells. The typical symptoms of the disease develop in this stage: fever, pain, inflammations, rashes, etc. These symptoms are caused by the toxins of the destroyed microbes, materials released from the injured cells and the destroying effect of reproducing pathogens and the proteins produced by the defense mechanisms of the immune system.
When long-lasting immunity forms, the next infection by the same pathogen will trigger this second phase, the response of the immune system within 7 days, without developing symptoms or the disease itself.
Vaccines are the teachers of the immune system
By vaccination, pathogens which may infect us later in our lives are introduced to our immune system in controlled circumstances. Vaccination teaches our immune system how to produce antibodies and memory cells against the given pathogen so that in case of a recurrent infection, the immune system could react immediately and recruit the immune cells. As an effect of the vaccines, with a recurrent infection, the immune system disarms germs before the symptoms of the disease develop.
For how long are you protected by the vaccine?
Several vaccines produce lifelong immunity against the disease, although the length of the immunity depends on the type of the vaccine.
The length of the immunity depends on:
- the pathogen;
- the type of the vaccine;
- the health condition of the individual.
Some vaccines provide permanent protection, while others must be repeated at certain time intervals in order to achieve adequate protection.
Difference between medications and vaccines
An important difference is that while medications are mainly used for treating already developed diseases, with the help of vaccines we “teach” the immune system to specifically protect the body against pathogens we had not met before. Therefore, in contrast with medications, vaccines do not mean the actual protection, but a strong immunity developed with the help of the vaccine that provides a good chance to prevent severe disease. This is the reason, why vaccines do not produce immediate protection, and that developing a strong immune response needs time, and occasionally repeated vaccinations or exposure to pathogens.
The expression ‘herd immunity’ means that if a large portion of a community is vaccinated against a pathogen, it provides protection for the non-vaccinated individuals as well, as they cannot get the infection from anyone. This phenomenon has a great role in preventing pandemics. However, if the proportion of the non-vaccinated individuals falls below a critical threshold, the herd immunity effect ceases to exist.
With measles, this threshold is 95%, with mumps it is 86% as it depends on how contagious the pathogen is in the community. One great danger of vaccine-criticism is that with an incorrect decision smaller or greater communities’ health could be put at risk.