The subject of vaccination has always been controversial. Despite stringent measures implemented in the design and testing of vaccines today, many people remain suspicious. While some believe that vaccines induce rather than prevent disease, others adhere to fictional theories alleging that vaccinations are overt schemes developed to alter people’s DNA or implant surveillance microchips! The bottom line is much of the opposition to vaccines stems from a lack of knowledge, which spurs unvalidated theories. In this short piece, I will use my practical experience in the field of vaccine development to explain the general principles underpinning how vaccines stimulate the immune system against a given pathogen.
Throughout our lifetimes, we are constantly exposed to a myriad of pathogens such as bacteria and viruses. In many cases, our immune system protects us from succumbing to a perpetual state of infection. The mechanisms underlying how the immune system recognizes and eliminates pathogens are very complex and have fascinated scientists for hundreds of years. Years of research have shown that the immune system’s ability to recognize different types of proteins(peptides) plays a critical role in the recognition and elimination of various types of pathogens— a virus-like SARS-Cov-2 being no exception.
There are proteins that are naturally found in the body and are essential for its survival. From here onwards, we will call this set of proteins ‘self-proteins’. Much like us, many pathogens express their own proteins that are pivotal for their replication. Their proteins differ from ours in terms of composition, so we will call these proteins ‘non-self’ proteins. Amazingly, it turns out that our immune system (T cells, B cells, antibodies, etc) can distinguish and respond differently when it encounters ‘self’ and ‘non-self’ proteins.
Before the body has been invaded by a new pathogen, the proteins that immune cells typically encounter as they scout the body are ‘self-proteins’. Since these are associated with the normal state of the body, they do not trigger events that activate the immune system dramatically. Picture it as friends passing each other and gesturing a few warm exchanges—no provocation occurs between immune cells and other cells that display ‘self’ proteins.
However, when infection occurs, the paradigm shifts. The pathogens, coated with ‘non-self’ proteins, start accumulating in the body. If the pathogen is a virus, it enters normal human cells and converts them into virus-producing factories. As these infected cells produce viruses, they also start displaying ‘non-self’ proteins derived from the viruses that they are churning out. When the immune system encounters both the pathogen and the infected cells, there are two main ways it can respond. It can deploy antibodies that bind strongly to the ‘non-self’ protein coats found on the pathogen or the infected cell, thereby flagging them for destruction by other specialized immune cells. T cells can also directly recognize the ‘non-self’ peptides displayed on infected cells. These T cells then go on to destroy the infected cells, thereby eliminating the pathogen’s source. Ultimately, the body is rid of the infection and the host/patient recovers.
As the immune system returns to baseline, it retains a ‘memory’ of the pathogen such that if an exposure occurs again in the future, it can be activated promptly and eliminate the pathogen. Thus, the second exposure would not be as aggressive as the first and in many cases, the pathogen fails to cause observable symptoms.
SARS-Cov-2, is a novel virus and at the time of infection in the lungs, the body does not have an existing memory of it. Many individuals with strong immune systems develop mild symptoms as their immune systems fight off the invading pathogen. In a few days, they recover. Unfortunately, the immune systems in high-risk individuals, particularly those with existing conditions, may not effectively respond to the infection. In addition, some of the responses that occur may hyperactivate the immune system, causing high levels of inflammation in the lungs. As a result of this inflammation, the patient undergoes respiratory distress and would require a ventilator for assisted breathing.
Vaccines are designed to prime the immune system so that it responds to the infection as if it has encountered it before. In a research laboratory, scientists study the protein composition of the pathogen. They identify proteins that are abundantly expressed, vital for the pathogen to replicate and at the same time, provoke the immune system strongly. DNA, or messenger RNAs encoding for these protein fragments are then packaged in a vaccine construct and delivered to patients after long rounds of safety and immunogenicity clinical trials. When the vaccine enters the body and the ‘non-self’ proteins derived from the pathogen start accumulating, the immune system becomes activated. Recipients of the vaccines may experience common side effects like soreness at the injection site, a mild headache and fatigue can for a day or two after vaccination. This reflects that the immune system is responding and building immunity against the ‘non-self’ proteins. Historically, severe side-effects after vaccination have been recorded within the first 60 days after the administration of the final dose. Hence the possibilities of long-term effects years after vaccination, at least for most of the population, are very low.
The immune activation caused by the vaccine is short-lived because of the absence of an active infection. As the initial activation wanes, the immune system keeps a record of the ‘non-self’ proteins it encountered. This record can be in the form of antibody-producing B cells and T cells that are effective at eliminating any cell or pathogen that displays the same configuration of the ‘non-self’ proteins that were delivered through the vaccine.
When a vaccinated individual encounters the pathogen in the future, the immune system takes advantage of the memory it acquired through vaccination to quickly eliminate the pathogen before it can cause further harm. In other words, the immune system behaves as if it has seen the pathogen before and it has been preparing to defeat it for a long time. This right here, underscores the power of vaccines in priming the immune system, readying it for any future infection by the same pathogen.
In summary, the role of vaccines is to harness the power inherent in the existing immune system. Unlike some chemical drugs, vaccines on their own cannot eliminate an infection. However, it is the interaction between the vaccine and the immune that results in potent immune responses, which protect the individual from succumbing to the disease that is caused by the given pathogen.
By Itai Muzhingi
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