Immunology deals with the study of the immune system in all its areas. Among them is vaccinology, the field of study and development of vaccines . Whether it is a vaccine designed to prevent a virus or bacterial infection, its objective is to prepare our immune system to defend itself against the pathogen, in the event that it comes into contact.
Not all vaccines are the same What is a sterilizing vaccine?
Today we have several types of vaccines that can be classified according to their design . On the other hand, they can also be evaluated according to the type of immune response that it provokes in our body. Thus, we find vaccines that help eliminate a current infection or that provide protection for the future in the short, medium or long term.
Ideally, all vaccines would confer long-term “immunity .” That is to say, that the immune system was trained to recognize and react quickly to a pathogen (or several) from the moment the vaccine is received for the rest of our lives. This would be an example of a long-term effective vaccine.
Even if a vaccine is effective in protecting us from a disease, the memory may only last a few months or years, requiring a booster dose to refresh our defenses . This depends on many factors, such as the type of vaccine or pathogen.
Although all vaccines have the same objective, all these factors generate different levels of effectiveness in protecting us. Among the vaccines that work in the long term because they generate a memory immune response, the best option is the one that generates a “sterilizing immunity” .
What is sterilizing immunity?
This term is known as a natural process by which, after a natural infection or vaccination, the immune system not only generates long-term protection from disease, but can even prevent infection . On some occasions, we can suffer from asymptomatic infections, because we do not present visible symptoms.
However, the infection may still be taking place at the cellular level , but because the immune system is primed, it does not spread throughout the body or cause an uncontrolled pro-inflammatory response.
A sterilizing immunity not only prevents symptoms and infection, but also prevents contagion because the pathogen is completely neutralized . In vaccine development, this is the Holy Grail, although it is not that easy to obtain.
Does sterilizing immunity really exist?
We are learning more and more about the interaction that takes place when a pathogen infects our cells. Viruses and bacteria have diverse infection mechanisms and through years of evolution, some of them have become stealth specialists .
These stealthy microorganisms have mechanisms by which they infect cells without activating a proinflammatory response on the part of the organism. Thus, they use our cellular machinery to reproduce in a controlled and silent way . Only when the infection is advanced, symptoms may appear, but the pathogen is ready to infect the next host.
The absence of symptoms makes it difficult to confirm if there is an infection at a specific time without prior suspicion. Due to this known mechanism, some experts in the field of immunology and microbiology question whether sterilizing immunity really exists .
The well-known example of the measles vaccine , which caused almost complete eradication in many populations, has been used as an example of a sterilizing vaccine. However, we now have much more sophisticated infection detection techniques than ever before. Although rare, some cases of measles can now be detected on a regular basis .
It is not unreasonable to assume that natural immunity can occur in a timely and natural manner several times throughout our lives. In a scenario where there has been a previous infection or vaccination, in the face of a low pathogen load, have the necessary tools to eliminate the threat without giving it the chance to infect . However, it may be more anecdotal than routine .
Will we have a sterilizing vaccine for COVID-19?
At the beginning of the development of vaccines, there was a Spanish vaccine developed at the CSIC (Centro Superior de Investigaciones Científicas) that promised to be one of the first sterilizing vaccines against COVID-19 . However, this vaccine has not completed clinical trials, so it is not available .
Setting as a goal for all vaccines the effect of generating long-term sterilizing immunity may be an impossible dream . In the case of the coronavirus, everything seems to indicate that the development of a sterilizing vaccine is very unlikely.
Although it cannot be predicted with certainty, the interaction with the virus, the mutation rate , the use of measures to prevent contagion, and current population dynamics are determining factors that make this task difficult .
Nor should we be discouraged by this prospect, since, in most cases, with the simple fact of preventing disease, serious symptoms or death , it is a success. Fortunately for us, those goals have already been achieved with current COVID-19 vaccines.
Pollard and Bijker. 2021. A guide to vaccinology: from basic principles to new developments . Nat Rev Immunol . doi: 10.1038/s41577-020-00479-7