Vaccinations have provided immunity against infectious diseases since the 18th century. It all began with a doctor named Edward Jenner. At the end of the 18th century, this English doctor discovered that infection with the relatively harmless cowpox provided immunity against the extremely deadly smallpox virus. The word ‘vaccination’ comes from the Latin word for cowpox: vaccinia. However, Jenner could not explain how the immunity that he had discovered actually worked. It took a few more years before it became clear that our immune system has a very good memory. Our immune system remembers which pathogens it has previously fought. As a result, it can react much faster to a subsequent infection and you no longer get sick. This principle is what makes vaccinations effective.
Our immune system’s memory works on the basis of the molecules that pathogens have on their surface, known as antigens. As soon as the pathogens enter our body, our immune system recognises these molecules as foreign and initiates a defensive response. A virusmakes good use of its molecules. After all, a virus is just a protein package with some genetic material in it. As a result, it needs a host cell to replicate. In some cases, such as the coronavirus or the flu virus, the host is human.
The virus lands on the surface of a cell, to which it gains access with the help of the molecules on its surface. A very specific key-lock principle applies, which is why a human virus generally has no effect on other organisms (and vice versa) unless the virus mutates. Once it is inside the cell, the virus starts to build in its genetic material. It is like the virus is holding the cell hostage and encouraging it to produce multiple new viruses. In response, the human cell displays the virus antigens on its surface. This allows the infected cell to be recognised by your immune system. Specialised white blood cells called cytotoxic T lymphocytes go on the attack. They bind to the surface of the infected cell and kill it, giving the virus no chance to spread.
One of the immune system’s first steps is to bind antibodies to the foreign antigens. Antibodies are specialised protein molecules that are found on the surface of certain white blood cells, among other places. The binding of antibodies to an antigen tells the immune system that the intruder must be eliminated. This process also stimulates the white blood cells to divide. Some daughter cells will produce more antibodies, while others will become memory cells. These memory cells are indispensable for immunity. Without these memory cells, it takes about ten days for the body to produce antibodies against an unknown intruder. During this time, the micro-organism can multiply and cause disease. If your immune system already knows the intruder, however, the antibodies start to form after just three days. Much faster!
The purpose of a vaccine is to elicit this type of immunity through the use of dead or weakened pathogens, or fragments of them. The important thing is for the vaccine to contain the antigens so that the immune system can produce the antibodies accordingly. They will not make you sick, but they do provide immunity against the specific pathogen. However, even a safe and effective vaccine is not enough to completely eliminate an infectious disease. Vaccination is not only intended to protect an individual, but also the entire population. The more people have been vaccinated, the lower the number of possible hosts for the pathogen. At a certain point, not enough victims are left to maintain the pathogen. When this point is reached, the disease becomes rare – a stage that is known as herd immunity.
At the moment, various laboratories worldwide are developing vaccines against the SARS-CoV-2 virus. Different approaches are being used. It is hard to say how long this will take, but experts anticipate at least one and a half to two years. It is therefore important for all of us to be careful until then and to keep following the safety regulations issued by the National Institute for Public Health and the Environment (RIVM).