Infectious diseases are caused by pathogens such as viruses, bacteria, fungi or parasites. Vaccines exist mainly against diseases caused by bacteria or viruses. Viruses cause, for example, polio, measles, rubella or hepatitis. Bacteria cause diphtheria and tetanus. Many people in Germany are vaccinated against these diseases. Vaccinations are also part of basic health care worldwide.
Researchers are trying to develop vaccines against dangerous pathogens. Whether this is successful depends in no small part on the properties of the pathogen. Parasites, such as the causative agent of malaria (Plasmodium), trick the human immune system. The single-celled organism has a highly complex structure and can adapt to the human host again and again as soon as it comes under pressure from drugs or vaccine candidates. For example, to date there is only one approved vaccine against malaria (Mosquirix), whose protective effect is limited. The HI virus, which can trigger the immunodeficiency disease AIDS, is also very adaptable and variable. These characteristics complicate the development of a vaccine. However, suitable vaccines have been developed against diseases such as poliomyelitis (polio), measles and tetanus. In the case of tuberculosis, on the other hand, intensive work is being done on improved vaccines, since the available vaccine does not show a good effect against pulmonary tuberculosis.
Prof. Dr. Stefan H. E. Kaufmann on vaccine development
"In chronic disease settings, the vaccine must be better than natural immunity."
Prof. Dr. Helga Rubsamen-Schaeff on vaccine development
"The reason why there is no vaccine for HIV is that the virus is highly variable."
Prof. Dr. Stefan H. E. Kaufmann on research into a vaccine against tuberculosis
"What we need here is a new vaccine."
How does a vaccination work in the body?
Vaccines protect against infectious diseases by preparing the immune system for a pathogen. The vaccine fakes an actual infection by imitating the virus or bacterium. Depending on the type and quality of the vaccine, the immune system reacts similarly to an actual infection: it begins to produce antibodies and immune cells. If one comes in contact with the pathogen after vaccination, the immune system recognizes it. The body’s own defense mechanisms are now activated quickly and specifically.
However, there are differences in how vaccinations work. Most vaccines approved to date produce high antibody titers, i.e., high levels of antibodies that neutralize pathogens. Viruses, for example, can be intercepted by antibodies and thus no longer penetrate the cells of the body. Infection is avoided and the likelihood of infecting other people is reduced. However, with certain vaccines, the viruses manage to enter the body’s cells and multiply despite vaccination. Then, although symptoms are often observed, they are lighter due to the trained immune system. As a result, the disease takes a comparatively mild course. In these cases, vaccinated persons can continue to carry the virus. How infectious vaccinated persons actually are depends, among other things, on the vaccine and the properties of the virus.
Prof. Dr. Stefan H. E. Kaufmann on strategies of individual pathogens and the effect of vaccines
"The easiest way is to vaccinate against the toxin of a pathogen."
Prof. Dr. Stefan H. E. Kaufmann on other vaccine strategies
Making the pathogen palatable for scavenger cells or killing body cells.
There are basically three types of vaccines: inactivated vaccines, live vaccines and nucleic acid-based vaccines. In dead vaccines, killed pathogens or fragments of them are administered. These are sufficient to activate the immune system. Such vaccines must be refreshed because the body does not form an immunological memory. In the case of live vaccines, the body is administered attenuated pathogens, for example viruses, which can no longer cause the disease. The immune system nevertheless recognizes them as invaders and develops defense strategies. The body also forms memory cells that recognize the pathogen in the event of a new invasion and already know how to combat it. A vaccination with live vaccines such as the measles vaccination therefore usually has a lifelong effect.
Genetic information as the basis for novel vaccines
For several years, another type of vaccination has been thoroughly researched: nucleic acid-based vaccines. The novelty of these vaccines compared to vaccination with conventional dead or live vaccines is that no pathogens or pathogen components are vaccinated that directly trigger an immune reaction. Instead, they rely on genetic information from the pathogens. This new vaccine category includes mRNA and DNA vaccines, as well as vector vaccines.
In the case of mRNA and DNA vaccines, the direct "blueprint" for components of the pathogen (messenger ribonucleic acid, mRNA) or the carrier of genetic information underlying this blueprint (deoxyribonucleic acid, DNA) is introduced into the cell. According to this "blueprint", the cell produces a protein of the pathogen, which then triggers the immune response. The immune system produces soluble defense substances, the antibodies, and forms specific T cells, each of which is directed against the pathogen protein. If a vaccinated person later comes into contact with the pathogen, the immune system recognizes it by its characteristics and can fight it off. Several mRNA vaccines were first licensed against COVID-19 in 2020. Incorporation of mRNA into the human genome is impossible, since the conversion of RNA to DNA is not possible in human cells.
In vector vaccines, so-called gene shuttles (vectors) are used as "means of transport. These genetically modified viruses introduce the gene, i.e. the DNA for the respective pathogen component, into cells. According to current knowledge, the viral vectors used for vaccination are harmless to humans and cannot cause any disease. Vector vaccines are used against dengue fever and Ebola fever.