Infectious Diseases, Immune System and Vaccines

Since ancient times the causes of severe infectious diseases in humans have been puzzling to people. Specifically, the recurrence of outbreaks, the severity of outcomes after infection ranging from permanent sequelae to premature death as well as the large impact in numbers in human populations have created the need to search for ways to learn about and prevent them. By continuous observations and through trial and error, civilizations were able to identify and distinguish the common features of certain infectious diseases and so to differentiate one from each other. Once fairly identified, people have been searching for effective and disease-specific ways to prevent them. It is in this search for prevention how vaccines appeared as tools against the nasty outcomes of infection. In this essay I will attempt a brief review of concepts related to how the immune system and vaccines work, and some key events in vaccine development.

Infectious diseases with severe outcomes in humans are several and new diseases continuously appearing. Yet, using the particular causes or the origins of infectious diseases, they can broadly be classified into four groups: bacterial, viral, parasitic and fungi. Among these causes, there are not yet vaccines against any parasite or fungus causing severe disease in humans. Among all those diseases caused by bacteria or viruses only a few of them have a vaccine to prevent humans from their burden. Even less, the available vaccines are only against a few strains, serotypes or serogroups of a given bacterium or virus. Therefore, the number of pathogens (bacteria and viruses) that could cause severe infectious diseases in humans is several times larger than the number of vaccines we have against them.

In general, bacterial and viral diseases are often grouped as pathogens causing diseases

Humans have been exposed to environments full of dangerous pathogens. These continuous exposure to pathogens caused outbreaks and diseases. The threat of infection requires response from human bodies and so the human body often reacts and fights the attacks of pathogens using an innate nonspecific immunity level, which is part of the immunity system. This is often called “primary response” which occurs following the first exposure to a foreign agent. If the response is quick and effective, the infection will be contained or eliminated. Sometimes, however, the response fails so infection would cause a disease. This could happen when the human body’s immunity is low and/or when the pathogen has a virulence to overcome the response, rapidly multiplying itself inside the human body and causing great damage to cells and tissues.

When humans were or are able to survive and recover from certain infectious diseases, their bodies generate some specific level of immunity to reinfections. It is because the human bodies have an immune system which is essential for our survival and where some specific memory about previous infections is stored and keeps the defense abreast. Specifically, that specific memory set in the immune system works as a first line of defense when potential reinfections with the same pathogens happen. Sometimes, such memory could wane in time and so make humans again susceptible to infection and disease; other times, exposure to the same pathogen would boost a decaying specific immunity and keep the protection against the attack of that pathogen high. Another important reason for humans becoming susceptible again is when the specific pathogen mutates and evolves, becoming unidentifiable and so making useless the previous specific memory of protection in our immune system.

Among all types of pathogens, the new or foreign ones of great concern are those that cause severe disease and often overwhelm the immune system by making their response slow and ineffective. Often are those that cause a heavy disease burden in the human population with plenty of sequelae or death. One way to prepare the immune system to such rapid and virulent attacks and to build a defense against them is with vaccines. Although there are different types of vaccines even against the same pathogen, in general all of them work like sparring for box or combat fighters; namely, as a sparring does, a vaccine safely and effectively stimulates the immune system, trains its rapid response and prepares it to fight. Effective vaccines are often able to mimic some or all specific characteristics – also known as antigens– of the pathogen. To keep the training of the immune system safe, these antigens in the vaccine are often weakened or killed versions of the real pathogen. In average, during the first two weeks after vaccination, the vaccine is able to prime the immune system to respond fast and with force, should an infection with the particular pathogen in full strength happen in the future. Some refer to this process as programming of the immune system and to the rapid and specific response as “secondary response” or “vaccine-induced response.” An effective secondary response is often faster and with force in the production of pathogen-specific antibodies and in creating more cellular memory in the immune system for future attacks. Yet, vaccine’s protection also could wane with time or the specific pathogen could mutate and evolve. Similar to natural immunity that could also wane, vaccine immunity could also wane or become useless against new versions of the pathogen making human bodies susceptible again to infection.

From the first vaccine developed in 1796 to the latest Covid19 vaccine, they are considered one of the greatest achievements of modern medicine. Throughout the last three centuries, several developments ranging from the advances in laboratory techniques, the discoveries in germ theory, the introduction of attenuated toxins, the cultivation of viruses, the evolution of cell culture to the introduction of recombinant DNA and the whole genome sequencing techniques have allowed the development of different vaccines and enjoy the progress and benefits we see today from them. In general, diseases that have caused the highest burden in morbidity and mortality in societies have received more attention and resources from those interested in developing vaccines.

Although vaccines have been an effective tool against infectious diseases, the development of each vaccine for a specific pathogen has often been a slow and research intensive process. Still, there were several attempts against some pathogens that did not end up with a safe and effective vaccine. Vaccine development follows a rigorous scientific process focused primarily on safety and then on its effectiveness. All these processes should have a sound foundation on basic biology research, scientific discovery of vaccine candidates as viable proof of concept. This is the point when the application of the investigation of a new vaccine should be submitted. After approval, a three-phase clinical studies and trial begins.

Roughly summarized, Phase I is focused on vaccine safety conducting trials in small groups of both animals and humans; phase II is focused on vaccine effectiveness trials in an increased number of humans. Phase III is one of the most important ones. In this phase, thousands of individuals are recruited and the sample distribution should be representative of the population where the vaccine will be targeted. Individuals are then randomly assigned to often two groups – this is referred to as random control trials or RCT. RCTs often have a control group not exposed to the vaccine (control) and a trial group that will be exposed to the vaccine (trial group). Except for the exposure to the candidate vaccine, it is expected that both groups have very similar demographics and clinical characteristics to avoid confounding factors. Then, both groups are followed and clinical and lab data collected. By comparing the rates of disease cases in the exposed group with those of the control group is how the vaccine safety and effectiveness are determined. If successfully safe and effective, the vaccine data is then submitted for analysis by independent institutions and then the vaccine is licensed and recommended.

In the recent past, the research and development of new vaccines used to take about 15 to 20 years. However, thanks to more than three decades of advances in DNA sequencing and the stable production of proteins in human cells exposed to messenger RNA (mRNA), the research and development time has been reduced substantially and made possible the development of Covid19 vaccines in less than a year. Had the Covid19 pandemic occurred 30 years ago, we would have been in worse shape to control or prevent their consequences.

Overall, human’s immune systems could rely on more effective and safe vaccines to prime their response against pathogens’ attacks. Yet, the burden of infectious diseases could only be partially reduced by vaccines. As mentioned above, among all those diseases caused by bacteria or viruses only a few of them have a vaccine to prevent humans from their burden and for those with a vaccine, the protection is often partial and may wane in time.

*Note: All the ideas developed in this essay were based on the references listed below.

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