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Viral Vaccines - Types, Mechanism of Action, and Side Effects.

Published on Aug 30, 2022 and last reviewed on Mar 07, 2023   -  5 min read


Viral vaccines are used to boost our immune system and prevent viral infections. This article explains how viral vaccines work and provide protection.


A vaccine is introduced into the body to produce immunity to a disease. The vaccine is an agent that is similar to a microorganism-causing disease and is often made from weakened or killed forms of the microbe. Vaccines help our body to fight against invading microorganisms such as bacteria and viruses. Viral vaccines contain either inactivated viruses or attenuated (alive but not capable of causing disease) viruses, protein fragments of a virus, and genetic material of the virus (DNA or RNA), which help to produce an immune response and protect the body from further infection with the virus.

What Are Viruses?

Viruses are infectious microscopic agents that contain either DNA or RNA as genetic material. They can replicate only within a host organism (unable to survive without a living host). Viruses cause infection in living organisms, like plants, animals, and bacteria. When infected, a host cell is often forced to rapidly-produce thousands of copies of the original virus. Viruses can cause a large number of diseases in humans, most of which are fatal and debilitating.

What Are Viral Vaccines?

Viral vaccines are used for the prevention of viral infections. Viral vaccines contain either attenuated (alive but cannot cause disease) viruses, inactivated or dead viruses, viral genetic material, or proteins. Because few drugs are useful against viral infections, prevention of infection by the use of vaccines is very important.

What Are the Components of a Viral Vaccine?

  • Active Ingredient - Small quantities of the harmless form of the virus that we are immunized against.

  • Adjuvants - Helps in creating a better immune response to the vaccine. Very small quantities are used; hence no side effects (example-aluminum).

  • Preservatives and Stabilizers - Maintain vaccine quality, safe storage, and contamination (example-sorbitol)

  • Water - Main ingredient.

What Are the Different Types of Viral Vaccines?

1) Live Attenuated Viral Vaccines - The virus is functional or alive but weakened, and it can replicate in the body and generate an immune response, not causing the disease. Examples:

These vaccines may cause severe disease as a result of uncontrolled replication of the vaccine virus in a person who has a weak immune system (immunocompromised), for example, HIV infection and leukemia.

2) Inactivated or Dead Viral Vaccines - Viruses in this vaccine are inactivated during the process of making the vaccine. They are not able to cause the disease, but the presence of adjuvants or immune enhancers helps to generate an immune response. Examples:

  • Hepatitis A vaccine.

  • Rabies vaccine.

  • Injected poliovirus vaccine (Salk vaccine).

  • Most influenza vaccines.

  • Some COVID-19 vaccines: Covaxin, CoronaVac, SinopharmBIBP.

3) Subunit Viral Vaccines - They are composed of protein or glycoprotein components of a virus that are capable of inducing a protective immune response.

  • Protein vaccines: Include fragments extracted from a virus. Example: virus-like particles in hepatitis B and human papillomavirus (HPV) vaccines.

  • Pure polysaccharide vaccines.

  • Nucleic acid-based vaccines: They use genetic material from a disease-causing virus to stimulate an immune response against it. Depending on the vaccine, the genetic material could be DNA or RNA. Examples: Zika virus, HIV, COVID-19.

4) Viral Vector Vaccines - They use a modified version of a different virus to deliver protection. Viruses like measles, adenovirus, influenza, and vesicular stomatitis virus (VSV) are used as vectors. In COVID-19 vaccines, adenovirus is used as a viral vector.

How Do Viral Vaccines Work?

Viral vaccines work by stimulating a response from the immune system to a virus. This response creates a memory in the immune system and allows the body to remember a specific virus so that it can protect the body against this virus and prevent the disease that it causes during further exposure.

  • Live Attenuated Viral Vaccines and Dead, Inactivated Viral Vaccines - Viral vaccines contain a weakened or killed or inactivated form of the virus that is not capable of causing disease. This is called an antigen. When a person gets a vaccine, their immune system recognizes the antigen as foreign, and antibodies are produced against the disease-causing viruses. It also activates the immune cells called T-cells and B-cells in the blood and bone marrow. Later during contact with the same virus, these antibodies protect the person from the disease.

  • Protein-Based Viral Vaccines - These vaccines contain protein from a virus’s surface. The proteins made in the laboratory (contained in the vaccine) stimulate the immune system and do not cause infection. Protein-based vaccines containadjuvants to strengthen the immune response.

  • mRNA Vaccines - These vaccines contain instructions for the human cells to makeantigen proteins.When a person receives an mRNA vaccine, human cells read these instructions and produce antigen protein. The immune cells recognize the antigen protein and produce antibodies against them. Example: Modena COVID-19 vaccine, Pfizer-BioNTech (mRNA vaccine).

  • Viral Vector Vaccines: Genetic material from a virus is placed in a modified version of a different virus (viral vector). The viral vector enters the human body and produces an immune response, and results in the production of antibodies. Example: AstraZeneca and the University of Oxford vaccine, The Janssen/Johnson & Johnson COVID-19 vaccine.

How Are Vaccines Important in Preventing Viral Diseases?

Getting vaccinated will save our lives. Strong protection against serious medical conditions, hospitalization, and death can be reduced with the use of vaccines. Evidence suggests that being vaccinated will reduce the chances of passing the virus on to others.

What Are the Benefits of Getting Vaccinated?

Vaccines help to provide protection against diseases that used to be more common, like meningitis, diphtheria, measles, mumps, tetanus, pertussis (whooping cough), and polio. These infections can cause life-threatening or serious illnesses and result in life-long health conditions. The use of vaccines and preventive measures reduced the infectivity of these diseases, and many of these diseases are rare nowadays.

What Are the Side Effects of Vaccines?

Some people develop side effects after vaccination, which are normal signs. It indicates that their body is building protection against the disease. These side effects may result in reduced ability to do day-to-day activities, but they should go away in a few days. They are:

  • Pain, redness, and swelling on the arm where you got the shot.

  • Tiredness.

  • Chills.

  • Headache.

  • Fever.

  • Nausea.

  • Muscle pain.

Adverse effects that could cause long-term health problems are extremely rare following any vaccination. Adverse effects usually happen within six weeks to seven weeks of receiving the vaccination.


Viral infections have a major impact on overall health. Hence the development of preventive measures against viral infections is really important. Vaccination is the most cost-effective medical intervention for preventing mortality and morbidity against infectious viral diseases. Many effective and safe vaccines are currently available against several viral diseases of significant medical importance. Prevention of viral infections can be achieved by the use of vaccines that induce an immune response and further result in the production of antibodies that help the body cells to fight against viral infection.

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Last reviewed at:
07 Mar 2023  -  5 min read




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