–Types of Viral Vaccines

Various approaches for Vaccine Development. Credit:https://www.genscript.com

There are several types of viral vaccines, each developed using different methods:

1. Live Attenuated Viral Vaccines: Live attenuated viral vaccines (LAVs) are among the most effective immunization strategies against viral diseases. They contain viruses that have been weakened (attenuated) so that they can no longer cause disease but can still stimulate a strong immune response. The development of these vaccines involves several key stages:i. Selection of Virus Strain: The process begins by identifying a wild-type virus that is known to cause the disease. This virus is then used as the starting point for attenuation.

Attenuation Process: The goal of attenuation is to weaken the virus, making it unable to cause              disease but still capable of replication at reduced levels to stimulate the immune system. This is                   typically done using the following methods:

• Serial Passage: The virus is repeatedly cultured in non-native environments, such as in cells or animals that are not its normal hosts (e.g., human viruses passaged in animal cells). Over time, the virus adapts to these new conditions, losing its virulence for humans.

• Temperature Sensitivity: Some viruses are adapted to specific temperatures. By growing the virus at suboptimal temperatures (e.g., lower than body temperature), the virus may lose its ability to replicate efficiently in human tissues.
• Genetic Manipulation: With advances in molecular biology, specific genetic modifications can be made to reduce the virus’s ability to cause disease. Deleting or altering certain genes reduces virulence without affecting the virus’s ability to trigger an immune response.

• Examples of Live Attenuated Viral Vaccines
  • Measles, Mumps, and Rubella (MMR): Developed by attenuating the respective viruses through serial passage in cell cultures.
  • Oral Polio Vaccine (OPV): Developed through attenuation by serial passage of poliovirus in monkey kidney cells.
  • Yellow Fever Vaccine (17D strain): Attenuated by serial passage in chick embryos.
  • Varicella-Zoster Vaccine: Created by attenuation of the virus through cell culture adaptation.

  Advantages and Challenges

  • Advantages: LAVs stimulate strong, long-lasting immunity, often with just one or two doses. They elicit both humoral (antibody) and cellular immune responses.
  • Challenges: There is a risk of reversion to virulence, particularly in immunocompromised individuals. Additionally, LAVs require careful storage (cold chain) to maintain stability.

2. Whole-inactivated (Killed) Viral Vaccines

• Whole-inactivated viral (WIV) vaccines are one of the earliest types of vaccines developed and have played a critical role in preventing viral diseases. These vaccines use viruses that have been “inactivated” (killed) so they cannot replicate or cause disease, but still stimulate an immune response.
• The concept of inactivated vaccines dates back to the late 19th and early 20th centuries. One of the earliest examples is the rabies vaccine developed by Louis Pasteur in 1885, which laid the groundwork for the use of inactivated pathogens in immunization. The technique was later refined, and the first formal WIV vaccine was created in the early 20th century, most notably for diseases such as polio (Salk vaccine), influenza, and typhoid.
• Viral Selection: The first step in developing a WIV vaccine is selecting the virus strain. The chosen strain must effectively represent the circulating virus and induce a broad immune response. This step may involve isolating a wild-type virus or creating an attenuated strain in a laboratory.Virus Inactivation: The key step in WIV development is rendering the virus non-infectious while preserving its antigenic structure. This is typically achieved by:
  • Chemical Methods: Chemicals such as formaldehyde or β-propiolactone are commonly used to inactivate the virus. These agents modify viral proteins and nucleic acids, preventing replication but maintaining the virus’s surface structures that are recognized by the immune system.
  • Physical Methods: Heat or radiation can also be used to inactivate viruses, although these methods are less commonly employed due to the risk of damaging essential antigenic components.
  • 

  Purification and Formulation: After inactivation, the virus is purified to remove any toxic substances or unwanted cellular debris. The purified virus is then formulated with stabilizers and adjuvants (substances that enhance the immune response). Adjuvants, such as aluminum salts, are often used to improve the vaccine’s immunogenicity.

  Advantages

  • Broad immune response, as the vaccine contains the entire virus, including multiple antigens.
  • Established production methods and safety profiles for many WIV vaccines.

  Limitations

  • May require booster doses to maintain immunity, as WIV vaccines tend to produce a weaker immune response compared to live vaccines.
  • Potential for side effects due to the presence of residual inactivating agents or other viral components.
  • Risk of incomplete inactivation, though stringent quality control measures minimize this risk.

  Examples of WIV Vaccines

  • Polio (Salk vaccine): Developed in 1955, this inactivated polio vaccine (IPV) is still in use today.
  • Influenza: Annual inactivated influenza vaccines are widely used and updated based on circulating strains.
  • Hepatitis A: An inactivated hepatitis A vaccine has been available since the 1990s.
  • Rabies: Inactivated rabies vaccines remain a key preventive measure for both humans and animals.
  • COVID 19: Sinopharm vaccine for Sinovac-CoronaVac vaccine

3. Recombinant Viral Vaccines

• Recombinant vaccines are produced using recombinant DNA technology. These vaccines use a harmless virus or yeast to deliver viral genetic material into the body, allowing cells to produce viral proteins that stimulate immunity.

Development Process

• Gene Cloning: Viral genes that encode immunogenic proteins are cloned into a vector, such as yeast, bacteria, or mammalian cells.
• Protein Expression: These vectors are introduced into cells that then produce the viral protein.

Diagram representing the Recombinant Hepatitis B Vaccine Production

• Purification and Formulation: The viral protein is purified and formulated into a vaccine.
• Example: The Human Papillomavirus (HPV) vaccine, which uses recombinant DNA technology to produce virus-like particles (VLPs).

a) Viral Vector Vaccines

• Viral vector vaccines use a modified, harmless virus (the vector) to deliver genetic material from a pathogen into human cells, triggering an immune response. The vector virus does not cause the disease itself, but it instructs the body to produce viral proteins that simulate infection, helping the immune system recognize and fight the pathogen in the future.

Development Process

• Selection of Vector: A virus that is non-pathogenic or attenuated (weakened) is chosen as the delivery system. Common vectors include adenoviruses and vesicular stomatitis virus (VSV). These viruses are engineered to be replication-defective, meaning they can enter cells but cannot replicate.
• Insertion of Pathogen Gene: The gene coding for a key protein of the target virus (such as the spike protein of SARS-CoV-2) is inserted into the genome of the vector. This recombinant virus will now carry the genetic instructions for producing the viral protein.

Diagram representing the formation of vector vaccine

• Cellular Immune Response: Once the vaccine is administered, the vector virus enters host  cells and delivers the genetic material. The cells then produce the viral protein (antigen) encoded by he pathogen gene. This stimulates the body to mount both humoral (antibody) and cellular (T-cell mediated) immune responses.

Diagram representing the delivery method of vector vaccine

• Immune Memory: The immune system learns to recognize and attack the protein, and if the actual virus is encountered later, the body is prepared to mount a rapid immune response.

Advantages

• Strong Immune Response: Viral vector vaccines stimulate a broad immune response, including both antibodies and T cells, which can offer durable protection.
• Single Dose Efficacy: Many viral vector vaccines provide strong protection with just a single dose due to their potent ability to activate immune cells.
• No Need for Adjuvants: Unlike subunit vaccines, viral vector vaccines often don’t require additional adjuvants to stimulate a robust immune response.

Examples of Viral Vector Vaccines

• AstraZeneca/Oxford University COVID-19 Vaccine (ChAdOx1-S): Uses a chimpanzee adenovirus to deliver the spike protein gene of SARS-CoV-2.
• Johnson & Johnson COVID-19 Vaccine (Ad26.COV2.S): Utilizes a human adenovirus (Ad26) to deliver the SARS-CoV-2 spike protein gene.
• Sputnik V COVID-19 Vaccine: A two-vector vaccine, utilizing two different adenovirus vectors (Ad26 and Ad5) in a prime-boost strategy.

b)DNA Viral Vaccines

• The development of DNA viral vaccines use plasmid DNA encoding viral antigens to stimulate an immune response.
• The idea of DNA vaccines first emerged in the early 1990s when researchers discovered that injecting genetic material (plasmid DNA) into muscle cells could lead to the expression of foreign proteins, which then triggered an immune response. The concept was revolutionary because it bypassed the need for traditional methods of using live-attenuated or inactivated viruses.
• 

Development of Delivery Methods

• Needle-free injectors (gene guns) to deliver DNA-coated particles directly into skin cells.
• Electroporation, which applies short electrical pulses to enhance cellular uptake of DNA.
• The COVID-19 pandemic accelerated vaccine development, with DNA vaccine platforms being explored for SARS-CoV-2.
• While mRNA vaccines took the lead in mass deployment, several DNA vaccines, particularly ZyCoV-D, a COVID-19 DNA vaccine developed by Zydus Cadila, were granted emergency use authorization.
• This marked the first time a DNA vaccine was approved for human use, proving the feasibility of this technology.

Key Advantages

• Safety: DNA vaccines do not use live viruses, minimizing the risk of infection.
• Stability: They are more stable at room temperature, making them easier to transport and store.
• Versatility: DNA sequences can be quickly adapted to target emerging viral strains.

  c) Protein subunit Viral vaccines

• Protein subunit viral vaccines are a type of vaccine that use specific pieces of a virus, typically its proteins, to stimulate an immune response without introducing the entire virus. This approach provides a safe and targeted method of immunization, reducing the risks associated with live-attenuated or inactivated viral vaccines.

Development Process

1. Antigen Identification: The first step in developing a protein subunit vaccine is identifying the viral proteins that are most likely to generate a protective immune response. This is usually a surface protein that plays a critical role in the virus’s ability to infect cells (e.g., the spike protein in SARS-CoV-2).
2. Recombinant Protein Production: The gene encoding the viral protein is inserted into a production system, typically using yeast, bacteria, or mammalian cells, to produce large quantities of the protein.
3. Purification and Formulation: Once the protein is produced, it is purified and combined with an adjuvant to boost immune response before being formulated into a vaccine that can be delivered to patients.

CHO cell = Chinese Hamster Ovary cell; gD = glycoprotein D

Mechanism of Action

1. Introduction to the Body: Once injected, the viral protein or subunit enters the body and is recognized as foreign by the immune system.
2. Immune System Activation: Dendritic cells (antigen-presenting cells) process the protein and present it to T-cells, which help activate B-cells. The B-cells then produce antibodies specifically targeting the viral protein.
3. Memory Formation: The immune system creates memory cells that “remember” the viral protein, ensuring a faster and stronger response if the actual virus infects the body later.

Examples of Protein Subunit Viral Vaccines

1. Hepatitis B Vaccine: One of the earliest and most successful protein subunit vaccines, the hepatitis B vaccine uses a purified piece of the virus’s surface antigen (HBsAg) to induce protective immunity.
2. Human Papillomavirus (HPV) Vaccine: The HPV vaccine, such as Gardasil, is based on virus-like particles (VLPs) containing the L1 protein of HPV. These particles resemble the virus but contain no genetic material, making them safe and highly effective.
3. Novavax COVID-19 Vaccine: This is a protein subunit vaccine for SARS-CoV-2 that uses the spike protein of the virus, combined with an adjuvant to boost the immune response. It has shown promising results in clinical trials for generating a strong immune response without using live or mRNA-based virus platforms.

Advantages of Protein Subunit Viral Vaccines

1. Safety: As they only use a part of the virus, protein subunit vaccines cannot cause the disease or replicate in the body, which makes them extremely safe, especially for immunocompromised individuals.
2. Reduced Risk of Side Effects: Since these vaccines contain only essential viral proteins, they tend to produce fewer side effects compared to whole-virus vaccines.
3. Targeted Immune Response: They can be designed to focus on specific viral components that are known to elicit a protective immune response, increasing their efficacy.
4. Stability: Protein subunit vaccines are typically more stable than mRNA or live-virus vaccines, which may require strict cold-chain storage.

Challenges and Limitations

1. Weaker Immune Response: Protein subunit vaccines sometimes generate a weaker immune response compared to live-attenuated or mRNA vaccines. This is why they often require adjuvants and booster doses.
2. Production Complexity: Manufacturing protein subunit vaccines involves the use of recombinant DNA technology to produce viral proteins, which can be more complex and time-consuming compared to some other vaccine platforms.
3. Dependence on Adjuvants: To elicit a strong immune response, protein subunit vaccines often rely on adjuvants. While safe, this adds an extra component that must be carefully monitored for safety and efficacy.

4. Virus-Like Particle (VLP) vaccines

• Virus-Like Particle (VLP) vaccines are a type of vaccine that uses structures resembling viruses but without any viral genetic material, making them non-infectious. VLPs are formed by the self-assembly of viral proteins and can mimic the virus’s structure closely enough to elicit a strong immune response. Here’s an overview of VLP vaccines, their mechanism of action, benefits, challenges, and examples.

Key Features of Virus-Like Particle Vaccines

1. Non-Infectious: VLPs do not contain viral nucleic acid (DNA or RNA) and cannot replicate, ensuring they cannot cause disease.
2. Structural Mimicry: VLPs closely resemble the natural virus in shape and structure, allowing them to present viral antigens effectively to the immune system.
3. Stability: VLPs are generally stable, making them suitable for storage and distribution without the need for stringent cold-chain requirements in many cases.

Mechanism of Action

• VLP vaccines work by simulating a natural infection, prompting the immune system to recognize and respond to the viral proteins presented on the surface of the VLPs.

1. Vaccine Administration: The VLP vaccine is administered (usually via injection).
2. Antigen Presentation: Upon entering the body, VLPs are recognized by antigen-presenting cells (APCs) such as dendritic cells. These cells engulf the VLPs and process the viral proteins.
3. Activation of the Immune System:
   • T-cell Activation: The processed antigens are presented on the surface of APCs to T-cells, which become activated and help stimulate B-cells.
   • B-cell Activation: B-cells are activated to produce antibodies specifically targeting the viral proteins displayed on the VLPs.
4. Memory Formation: The immune system generates memory B-cells and T-cells, ensuring a rapid and robust response if the actual virus is encountered in the future.

Advantages of VLP Vaccines

1. Strong Immune Response: VLPs elicit a robust immune response comparable to live-attenuated vaccines due to their ability to mimic the natural virus.
2. Safety: As non-infectious particles, VLP vaccines carry a minimal risk of causing disease, making them suitable for immunocompromised individuals.
3. No Adjuvants Needed: VLPs can induce strong immune responses without the need for additional adjuvants, although some formulations may still include them to enhance responses.
4. Targeted Approach: VLPs can be designed to present specific viral proteins that are critical for generating an effective immune response.

Challenges and Limitations

1. Complex Production: Manufacturing VLP vaccines can be complex and costly, often requiring sophisticated biotechnological processes to produce and purify the proteins.
2. Limited Types of Viruses: While VLP technology has been successful for certain viruses, it may not be applicable to all viral infections, especially those with complex structures.
3. Regulatory Hurdles: As with all vaccines, VLP vaccines must undergo extensive testing and regulatory scrutiny, which can prolong development timelines.

Examples of Virus-Like Particle Vaccines

1. Human Papillomavirus (HPV) Vaccine:
   • Examples: Gardasil and Cervarix are VLP vaccines that protect against HPV types associated with cervical and other cancers. These vaccines utilize L1 protein VLPs to elicit a strong immune response.
2. Hepatitis B Vaccine:
   • The Hepatitis B vaccine is composed of VLPs that contain the surface antigen (HBsAg) of the virus. It effectively prevents hepatitis B infection and related liver diseases.
3. Novavax COVID-19 Vaccine:
   • This vaccine is based on VLP technology using the spike protein of SARS-CoV-2. It has shown promising results in clinical trials and has received emergency use authorization in several countries.
4. Influenza Vaccine:
   • Some influenza vaccines are developed using VLP technology, offering a potentially safer alternative to traditional egg-based vaccines.
5. Ebola Vaccine:
   • The rVSV-ZEBOV vaccine uses a VLP strategy to provide protection against the Ebola virus, helping to control outbreaks in affected regions.

Virus-Like Particle vaccines represent a promising and innovative approach to viral immunization. By leveraging the structural properties of viruses without their infectious capabilities, VLP vaccines offer a safe and effective means of stimulating robust immune responses. As research and technology advance, VLP vaccines are likely to play an increasingly important role in public health, offering protection against a variety of viral infections.

 5. Messenger RNA (mRNA) Viral Vaccines

• These are a newer class of vaccines that use messenger RNA (mRNA) to instruct cells to produce a viral protein, which triggers an immune response.
• Examples: Pfizer-BioNTech and Moderna’s COVID-19 vaccines.

How Viral Vaccines Work

• Introduction of Antigens: The vaccine introduces antigens (virus particles, proteins, or genetic material) into the body.
• Immune Response Activation: The immune system recognizes these antigens as foreign and mounts a response, producing antibodies and activating immune cells.
• Memory Formation: After the vaccine, the immune system “remembers” the virus, allowing for a faster and stronger response if exposed in the future.

Benefits

• Prevention of Viral Diseases: Vaccines have significantly reduced or eradicated diseases like smallpox, polio, and measles.
• Herd Immunity: When a significant portion of the population is vaccinated, it helps protect those who cannot be vaccinated, such as immunocompromised individuals.

Challenges

• Development for Certain Viruses: Developing vaccines for rapidly mutating viruses (like HIV or certain strains of influenza) remains difficult.
• Storage and Distribution: Some vaccines require specific temperature conditions, which can complicate distribution, especially in low-resource settings.