COVID-19 Vaccine: A Way Out of Crisis

COVID-19 pandemic has taken toll on the entire globe at physical, emotional and administrative level; straining each and every aspect to its fullest. As on April 19/2021, COVID-19 has infected more than 140 million people around world with around 3 million deaths worldwide. Covid-19 vaccine has emerged as an important direction to walk the world out of this crisis. This chapter covers the basic aspects and principles of vaccination and Immunology and its application in COVID-19 pandemic. This chapter further covers the different type of vaccines being developed, their dosage schedule and route of administration, common adverse events and myths related to them.


Introduction
Immunization is a global health and development success story, saving millions of lives every year. Vaccines reduce risks of getting a disease by working with your body's natural defenses to build protection. When you get a vaccine, your immune system responds. Immunization is a key component of primary health care and an indisputable human right. It's also one of the best health investments money can buy. Vaccines are also critical to the prevention and control of infectious-disease outbreaks. They underpin global health security and will be a vital tool in the battle against antimicrobial resistance. The two terms vaccination and immunization has been used synonymously over the time but the two terms differ in their meaning [1].
"Vaccination" as per definition is defined as the process of administering the biochemical product referred to as vaccine in the human body whereas "Immunization" is defined as the process by which body develops immunity against the disease [2].
Vaccines train the immune system to develop antibodies and protect against the disease. As per World Health Organization (WHO) there are number of vaccines which had been developed against number of diseases namely Diphtheria, hepatitis B, measles, mumps, pertussis, polio and many more. On similar grounds, to tackle the menace of COVID-19, various vaccines have been developed [3].  Secondary Immune Response is the reaction of the immune system when it contacts an antigen for the second and subsequent times.

Appearance
Appears mainly in the lymph nodes and spleen.
Appears mainly in the bone marrow and then, in the spleen and lymph nodes.
3 Occurrence This occurs in response to the primary contact of the antigen.
This occurs in response to the second and subsequent exposure to the same antigen.

Antibody Peak
The antibody level reaches its peak in 7-10 days.
The antibody level reaches its peak in 3-5 days.

Affinity of Antibody
Low affinity to their antigens. High affinity to their antigens. 6 Responding Cells

Naive B cells and T cells Memory B cells 7 Antibodies
Both thymus-dependent and thymus-independent antibodies are involved in the primary immune response.
Only thymus-dependent antibodies are involved in the secondary immune response.
Types of Antibodies A large amount of IgM and a small amount of IgG are produced during the primary immune response.
A large amount of IgG, a small amount of IgM are produced during the secondary immune response. 10

Amount of Antibody
Few antibodies are produced in the primary immune response.
100-1000 times more antibodies are produced in the secondary immune response.

Strength of the Response
The primary immune response is usually weaker than secondary immune response.
The secondary immune response is stronger.

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Antibody level Antibody level declines to the point where it may be undetectable.
The antibody level tends to remain high for longer time.

Immunological response to vaccine
Immune response is divided into two types: 1. Primary immune response.
The differences between these two have been described in

Immunological responses to different type of COVID-19 vaccines
Inactivated vaccine: the genetic material is inactivated or destroyed in inactivated vaccine which after ingested by antigen presenting cell stimulate the helper T cells which in order stimulate B-cell to produce antibodies as described in Figure 1 [7].
Example: COVAXIN (Bharat Biotech).  Subunit Vaccine: In this vaccine only a part of the agent imitates like real infection and stimulate helper T cells which in turn stimulates B cells to produce antibodies as described in Figure 2.
Example: Novavax (protein subunit). Viral vector Vaccine: these vaccines use non-coronavirus vector modified to carry gene coding for the SARS-COV-2 antigen. This antigen gets expressed on the cells infected gets ingested by antigen presenting cell which then project the complex to helper T cells which then activates both the B-Cells and Cytotoxic T cell as described in

COVID-19 vaccines at glance
From February to June 2021, at least seven different vaccines across three platforms have been rolled out in countries [8][9][10]. Vulnerable populations in all countries are the highest priority for vaccination. At the same time, more than 200 additional vaccine candidates are in development, of which more than 60   are in clinical development. COVAX is part of the ACT Accelerator, which WHO launched with partners in 2020. Some of the vaccines which have been rolled out are described in Table 2.

Dosage and schedule for vaccination
Most COVID-19 vaccines are designed for a two-dose schedule. Two dose vaccination works by mimicking natural immunity. After a first vaccine dose, the immune system needs time to generate a response and to create memory cells that will recognize the pathogen if it is encountered again. The person is considered immune from COVID-19 disease 14 days after the second dose of vaccine in two dose vaccine schedule. All these vaccines are administered Intramuscularly in the deltoid muscle as described in Table 3.

Vaccine storage and cold chain maintenance
Delivering vaccines to all corners of the world is a complex undertaking. It takes a chain of precisely coordinated events in temperature-controlled environments to store, manage and transport these life-saving products. This is called a cold chain. Vaccines must be continuously stored in a limited temperature range -from the time they are manufactured until the moment of vaccination. This is because temperatures that are too high or too low can cause the vaccine to lose its potency

Adverse effects and contraindications of COVID-19 vaccine
Vaccination is the process of administering foreign agent in the body which is usually associated with various adverse effects which are mostly of mild intensity but may cause severe adverse events in some [11].
Some of the known adverse events following immunizations are fever, pain and swelling at injection site, fatigue, chills, and headache. Some of the vaccine recipients may experience some severe adverse events like Anaphylactic reaction but the incidence of this is rare.
Contraindication to COVID-19 vaccine include severe allergic/anaphylactic reaction to any ingredient of the vaccine or to the first dose of vaccine. COVID-19 vaccine is also contraindicated in pregnant women or those suspected to be pregnant due to paucity of data in this group.

Myths related to COVID-19 vaccine
Myths preventing people from taking the vaccine are many and will be mentioned in another chapter, however one of these is: COVID-19 vaccine was thought to cause infertility in women due to the resemblance of spike protein to the protein syncytin secreted by placenta. This was proved to be myth as the two protein have large difference in amino acid sequences hence ruling out the concern of infertility.
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