COVID-19 has been declared as pandemic by WHO at 11th March 2020. With more than 69.7 million cases worldwide and 1.58million approximate death worldwide death, it has been one of the tragic event in human history. With midst of the pandemic, everyone is hoping for vaccine against COVID-19. Ever wondered how a vaccine works against COVID-19.It all started In 1796, the scientist Edward Jenner injected material from a cowpox virusinto an eight-year-old boy providing him resistance against smallpox that was considered deadly at that time . The eight-year-old was inoculatedagainst the disease and this became the first ever vaccine.
But how did it work? To understand how vaccines function, we need to know its contents, types and working mechanism of vaccine
What does vaccine contain?
Modern day vaccines contain some or all of the following substances listed as:
1. Antigen
2. Preservatives
3. Surfactant
4. Stabilizers
5. Diluents
6. Adjuvant
7. Residuals
Antigen is the chief component of the vaccine. Antigen may be the weakened pathogen itself or its residue or parts that is responsible for triggering host immunity. Preservativeare the chemicals that preserves vaccines from being contaminated and losing its integrity once vial has been opened (2-phenoxyethane is most common preservatives). Surfactants are the chemicals that keep vaccine content blended restricting it to solidify or clump. Stabilizer are simple compounds that prevents further reaction within vaccine vial protecting chemical integrity of vaccines. Diluents are commonly sterile water that dilutes the vaccine to the correct concentration. Adjuvant are contained by some vaccines that acts as catalyst improving vaccine efficacy. Residuals are the leftover compounds that are used during vaccine development. Concentration of this residuals are very low.
Types of Vaccines
Various forms of vaccine are available now with variety of mode of working, effectiveness and
Live attenuated vaccines
First, we have live attenuated vaccines. These are made of the pathogen itselfbut a much weaker and controlled version. One or two doses of the vaccine can develop lifelong immunity in the host. The downside is that live attenuatedvaccines can be difficult to make, and because they're liveand quite powerful, people with weaker immune systemscan't have them. Some of the common live attenuated vaccines are:-
1. Rota vaccine
2. MMR(Measles. Mumps and Rabies) vaccine
3. Chicken Pox, Small Pox
Inactive vaccines
These are the vaccines in which the pathogens have been killed. The weakening and inactivationin both types of vaccine ensures that pathogens don't developinto the disease. But just like a disease,they trigger an immune response, teaching the body to recognize an attack by making a profileof pathogens in preparation.While inactive vaccinesdon't create long-lasting immunity and require booster doses to retain its effectiveness. Some of the inactive vaccines are:
1. Hepatitis vaccine
2. Rabies vaccine
3. Polio vaccine
Sub unit Vaccine
The subunit vaccine, is only made from one partof the pathogen such as specific proteins, lipids or any sugar components, called an antigen, the ingredient that actually triggersthe immune response. Some of the vaccine are: -
1. Hepatitis B vaccine
2. Pneumonococcal vaccine
Toxoid Vaccine
Toxoid vaccines make use of toxin (harmful product) to trigger immune response in host body. They create immunity to the parts of the germ that cause a disease instead of the germ itself. Immune response is targeted to the toxin instead of the whole germ hence requires booster dose for retention of vaccines. Some examples of Toxoid vaccine are :
1. Diphtheria
2. Tetanus
mRNA / DNA vaccine
RNA vaccines are a new generation of vaccines. Instead of the antigen itself, RNA vaccines contain a messenger RNA – mRNA – that encodes for the antigen. Once inside the body’s cells, the mRNA is translated into protein, the antigen, by the same process the cells use to make their own proteins. The antigen is then displayed on the cell surface where it is recognized by the immune system. From here, the sequence of events is similar to that of a conventional vaccine. Some RNA vaccines also contain additional mRNA coding for an enzyme, which, after being translated in host cells, can generate multiple copies of the antigen-encoding mRNA. This essentially amplifies the production of antigen from a small amount of vaccine, making the vaccine more effective. These are called self-amplifying RNA vaccines. RNA vaccines are easier and safer to produce than conventional vaccines. The latest COVID-19 vaccine development is focused in this method. Latest example of mRNA/ DNA vaccine includes
1. Pfizer
2. Covisheild
Working Mechanism
When foreign microbes invade us, the immune system triggers a series of responses in an attempt to identify and remove them from our bodies. The signs that this immune response is working are the coughing, sneezing, inflammation and fever we experience, which work to trap, deter and rid the body of threatening things, like bacteria. These innate immune responses also trigger our second line of defense, called adaptive immunity. Special cells called B cells and T cells are recruited to fight microbes, and also record information about them, creating a memory of what the invaders look like, and how best to fight them. This know-how becomes handy if the same pathogen invades the body again. But despite this smart response, there's still a risk involved. The body takes time to learn how to respond to pathogens and to build up these defenses. And even then, if a body is too weak or young to fight back when it's invaded, it might face very serious risk if the pathogen is particularly severe. But what if we could prepare the body's immune response, readying it before someone even got ill? This is where vaccines come in. Using the same principles that the body uses to defend itself; scientists use vaccines to trigger the body's adaptive immune system, without exposing humans to the full-strength disease. This has resulted in many vaccines, which each work uniquely, separated into many different types.
Does vaccine fail?
Yes, vaccine might fail to work for certain microorganisms such as virus as they go under constant mutation causing antigenic shifts or antigenic drifts. For instance, Influenza A virus are typed/named on the basis of HA and NA antigens that enables them to combine with the sugar(saccharides) of host cell. However, the mutation changes HA and NA antigens so that our immune system no longer recognizes them failing effectiveness of vaccines. So, it becomes hard for the development of vaccines for viral diseases such as Infuenza, HIV and other. The recent development of vaccine development i.e. mRNA enables us to address these mutation problems and modify vaccine quickly as RNA vaccines are easier and safer to produce than conventional vaccines.
Conclusion
Whateverthe mechanism of vaccine, the basic ideology behind is to trigger the host immune response through weakened antigen or its parts. Though vaccine is a milestone achievement in human history. Vaccine are not enough to fight against COVID-19 battle. There still remains a challenge for global distribution of the vaccine, which can be done via effective cold chain management, vaccine storage and transport handling.
(Public Health Students: Bachelor of Public Health, Pokhara University)
