Original article: L’intricato mondo dei vaccini: tutto quello che occorre sapere, by Giovanna Spinosa
Vaccines generally constitute the set of antigens of a disease that are injected into a subject to induce an immune response that will protect him from the onset of the disease itself. We are talking about a natural mechanism activated by the human body when it detects the presence of a pathogen. It responds to invasion by producing antibodies and competent cells in adequate quantities, specifically targeting the infection or disease.
This all happens in two stages:
- During the first encounter with the infectious agent (the pathogen), the defense mechanism has not yet been activated and set in motion and the disease thus has time to develop;
- With the second encounter, however, the pathogen will be immediately recognized and eliminated before it has time to cause the disease.
In fact, if an antibody response is stimulated in an individual against a germ present in the area to which it may be exposed, it is possible to carry out a work of prevention, reducing or eliminating the cases of disease for that particular pathogen. This type of method is part of the specific prophylaxis.
The general characteristics of vaccines are:
- Efficacy, as they must be able to induce an immune response;
- Ease of production;
- Simplicity in administration;
- Zero or minimal side effects, but still manageable. They must be commensurate with the risk;
- Economic acceptability.
General classifications of vaccines
Vaccines are divided into six classes. In particular, the following are distinguished:
- Live and virulent vaccines, in the past they were used in tribal customs and are no longer used today;
- Inactivated vaccines, formed by complete microorganisms, that are inactivated by chemical or physical methods that do not produce changes in the proteins in order not to alter the immune response. These vaccines are stable and safe, but usually induce a lower immune response than attenuated ones. Also, there are often booster doses for them. An example is the vaccine for hepatitis A;
- Attenuated vaccines, consist in the use of germs with the same antigenic characteristics as those that cause disease, but do not cause damage because they have lost their virulence. Examples include the Sabin polio vaccine (OPV), yellow fever, measles, mumps, rubella, chicken pox, rotavirus and smallpox vaccines;
- Detoxified microbial processes, represented by detoxified toxins through peculiar biochemical processes, as some diseases are not linked to the germ, but to a toxin produced by the latter. The most classic example is the tetanus vaccine;
- Antigenic fractions from germs, whereby the germ responsible for the disease is cultivated, inactivated, fractionated and, therefore, the most characteristic antigens are isolated in such a way as to have a more targeted action. Such vaccines have high industrial and research costs. An example is the Haemophilus influenzae vaccine;
- Recombinant DNA vaccines, produced from the DNA portions of microorganisms that code for a specific antigen through a biotechnological process. Examples are the hepatitis B vaccine and the meningococcal B vaccine.
Preparation of vaccines
Preparing a vaccine involves:
- The choice of the strain of the pathogen: you have to choose the most suitable one. For example, based on the area in which it must be administered, the most common in the population will be selected;
- The choice of the strain culture: it is necessary to consider the soil that allows maximum growth. In particular, liquid medium is used for bacteria, while cell suspensions or even duck embryos are used for viruses;
- Attenuation / inactivation / detoxification depending on the class of vaccine to be prepared. Specifically, first of all, a purification process must be carried out, since in the soil there could be other potentially toxic substances or that can cause allergic phenomena;
- Lyophilization: necessary for conservation, because the vaccine in the absence of water is less subject to contamination;
- Controls that follow very strict protocols, as it is necessary to be sure that there are no other components in the vaccine produced. They can be divided into three classes: harmlessness, sterility and efficacy control.
Eventually the vaccine is ready for distribution and administration. So the vaccine preparation chain is very long and it takes time, at least one year, before it can be made available to the population in total safety.
How are vaccines introduced?
To assess the need or not to introduce a certain vaccine, the impact of the disease and its spread nationwide is first of all checked. The efficacy and safety of the vaccine in question is then considered, also comparing it in terms of quality and cost to alternative interventions, including perhaps existing but older vaccines.
On the basis of other planning elements, such as the methods of distribution, its pharmaceutical presentation, the availability of doses and the organizational capacity of the local health units that must distribute it, it is decided whether or not to produce the new vaccination.
Vaccination coverage and herd immunity
By vaccination coverage we mean the percentage of vaccinated individuals, compared to the total: if there is good vaccination coverage, the disease spreads very slowly and it is possible to manage it.
This is the effectiveness of the so-called herd immunity, also called group or herd immunity; it consists in the ability of a group to resist the attack of an infection to which a large portion of members are immune, thus protecting everyone, even those who for some reason are not vaccinated.
The term “herd immunity” was coined in 1923, referring to the fact that when a sufficient number of American children are infected and have developed immunity in a population of American children, the rate of infections also falls among those still susceptible to being infected.
Specifically, it is the fraction of the population that must be immune to a certain virus, so that the average number of individuals infected by each infected subject is less than 1.
In particular, the percentage of immune individuals who prevent an infectious disease from spreading in an epidemic way is called minimum herd immunity threshold (HIT). In fact, by increasing the number of immune individuals, the likelihood of contact between infected and susceptible individuals is reduced. If the percentage of immune in the population is higher than HIT, the epidemic disease can be contained or eliminated.
We refer to a dynamic phenomenon, for example, vaccinations with waning immunity over time require booster events.
Therefore, vaccines are currently the safest method for not getting sick and not transmitting a disease to others. They are a winning weapon against pathologies wrongly believed to have disappeared, such as diphtheria and polio, still present in some parts of the world.
Credits of images
- Figure 1: https://www.fondazioneveronesi.it/magazine/tools-della-salute/glossario/vaccinazione;
- Image 2: https://www.fondoasim.it/sviluppo-vaccino-tempo/;
- Figure 3: https://www.adnkronos.com/vaccini-ecco-il-calendario-quali-fare-anno-per-anno_6uGid6niqEbZXI3kZ2dicY?refresh_ce;
- Image 4: https://www.sifweb.org/sif-magazine/articolo/siamo-vicini-all-immunita-di-gregge-per-la-covid-19-2020-07-30.