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What is a vaccine and What are the different types of vaccines

Different types of vaccines
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Vaccines are biological preparations containing antigenic substances intended to develop active acquired immunity against some infections. The antigenic substances used may be whole pathogenic organisms or some parts of it. Nowadays there are different types of vaccines.

Vaccines usually produce small infection-like conditions in the body which train our immune system to get ready for real infection by making corresponding antibodies. Next time when the real infection comes, the body reacts quickly by creating enough antibodies to protect our body from harm.

In history humans have tried different methods to  attain active immunity against certain infections like smallpox. Most of the time it may be by introducing real infecting organisms or similar organisms to the body which is known as variolation. This method is scientifically explained and tested by British physician Edward Jenner, who is considered as the founder of vaccinology. He found that people who got infected with cowpox viruses ( vaccinia virus ) get immunity against smallpox. He collected pus from a milk maid who was infected with cowpox and inoculated a boy. After six weeks he proved that the boy acquired immunity against smallpox. since the name of the virus introduced is vaccinia virus the method got the name Vaccination. That was the beginning of the vaccine era. Later scientists introduced different types of vaccines against different infectious organisms. Now we have effective vaccines against more than 18 deadliest infections.

We can check some benefits and limitations of vaccines

Benefits of vaccines

  1. Vaccines prevent infection before it comes. If we are infected with a disease we have to suffer different health and other issues.
  2. Vaccination is cheaper than treating disease. It decreases hospitalization, use of different medications, diagnostic costs etc.
  3. By taking vaccines we can contribute to protection of our community from certain infectious diseases. By vaccinating a particular percentage of the members of a community that community gets herd immunity. That means even though some of the people with different reasons have not received vaccines we can protect them from infection.
  4. Vaccination helps prevent antibiotic resistance. By preventing spread of different infecting organisms and reducing the widespread use of antibiotics we can prevent antibiotic resistance development which is a great issue the medical field is facing.
  5. Vaccines provide peace of mind to persons and to the whole society. 

Limitations of vaccines

  1. No vaccine provides 100% protection to infections. Even though it is proven that vaccines protect humans from a number of infections there are chances of failure in small percentages depending on different factors.
  2. Side effects of vaccines. There are some common side effects like headache fever weakness etc reported in all vaccines. These are not serious and most of the time it is because of the immune reaction happening in the body. But in rare cases serious adverse are also reported but it does not overweigh the use of vaccines considering its benefits to the community.
  3. One has to expose a medicine without having any disease. But usually authorities give approval to a vaccine only after studying all the impact that it has on humans.
  4. Vaccines are available for only a limited number of infections
  5. There are chances of vaccine failure if mutation happens to the infectious organisms.

Now we can check different types of vaccines

Different types of vaccines

We know that the first vaccine developed is against smallpox and is made by using live virus collected from a person infected from cowpox. This method was very successful in that case but it is risky to use a live active virus to develop active immunity. Now scientists have developed different types of vaccines using different technologies. 

  1. Live-attenuated vaccines
  2. Inactivated vaccines
  3. Toxoid vaccines
  4. Subunit, recombinant, polysaccharide and conjugate vaccines
  5. mRNA vaccines
  6. Viral vector vaccines

We can explain different types of vaccines in details

  1. Live-attenuated vaccines:

This type of vaccine is prepared using living viruses or bacteria. Here the normal disease producing organisms are cultured in different mediums like embryonic egg yolk for a long time so that they lose their infection producing capacity in the original host cells. These organisms will not make any infection or harm to the human body and at the same time they retain their antigenic capacity so that they stimulate production of antibodies. 

When these vaccines are administered they multiply in the body and produce a small infection like condition. Our immune system recognizes them and produces antibodies to kill them. This type of vaccine creates more powerful and long lasting immunity.

Even Though it is an ideal type of vaccine it also has disadvantages. One is it cannot be given to any immunocompromised patients like HIV infected patients or patients on cancer chemotherapy because they cannot produce enough antibodies to deactivate organisms present in the vaccines. 

Another disadvantage is that there is a small chance of mutation happening to these attenuated organisms to convert to wild ones. 

Because these live attenuated vaccines contain live organisms an effective cold chain and careful handling is important. Otherwise it will lead to loss of effectiveness

Examples are:

  • Oral polio vaccine
  • Oral typhoid vaccine
  • MMR ( mumps, measles and rubella ) vaccine
  • Varicella vaccine
  • Yellow fever vaccine
  1. Inactivated vaccine:

In this type vaccines are made by using killed viruses or bacteria. Here disease causing organisms are cultured and killed by using different techniques like heat, chemicals or radiation in such a way that they retain their antigenic properties. Since these microbes are killed they will not multiply in the body but our immune system recognizes them as harmful things and produces antibodies against them.

Benefits of inactivated vaccines include

  • These vaccines can be given to immunocompromised patients because organisms present in the vaccines will not multiply like live vaccines.
  • Since these are killed vaccines they don’t need any strict storage conditions like live vaccines
  • It avoid the least chance of any mutation happening to the organism and converting to wild ones

But there is also some limitations to this vaccine

  • It is not do not produce a powerful and long lasting immunity like live vaccines
  • It needs booster dose in specified time intervals like 6 months, one year etc
  • It needs special adjuvants to be added in order to produce desired effects 

Examples of Inactivated vaccines

  • Rabies vaccine 
  • Hepatitis A vaccine
  • Injectable polio vaccine
  • Typhoid vaccine
  • Cholera vaccine
  • Pertussis vaccine
  • Some of the Covid-19 vaccines like Sinopharm, Covaxine etc
  1. Toxoid vaccine:

Toxoids are antigenic substances prepared from toxins released by some bacteria. Toxins produced from these types of bacteria are inactivated by treating with heat or chemicals in such a way that their antigenic properties are retained. In toxoid vaccines these toxoids act as antigenic materials and stimulate the immune system to generate antibodies against original toxines. Which protects the body during further infection with those bacteria. 

As like in the case of inactivated vaccines, toxoid vaccines also need booster vaccination in time intervals. 

Examples are 

  • Tetanus toxoid
  • Diphtheria toxoid
  1. Subunit, recombinant, polysaccharide and conjugate vaccines:
  • Subunit vaccine:

In this type of vaccine instead of using whole bacteria or viruses some parts of them like proteins, peptides, polysaccharides etc are used to produce an immune response in the body. These parts are known as sub units which are the most antigenic parts of the organisms. They are carefully separated and mounted to adjuants before adding to the vaccine. 

Their advantages include

  • They can be given to immunocompromised individuals safely
  • There are no chance organism converting to its wild form
  • They are more stable and no need of stringent storage conditions

Disadvantages of subunit vaccines include

  • Needed a complicated process to separate specific parts of microbes
  • It will not produce a long lasting immunity so booster doses required

Examples are

  • Hepatitis B vaccine
  • Pneumococcal vaccine
  • Meningococcal vaccine
  • Recombinant vaccine:

This type of vaccine is prepared using recombinant DNA technology. Here DNA segments which produce proteins or outer membrane particles of pathogenic organisms are separated and inserted in the DNA strands of other bacteria or yeast. When these modified bacteria or yeast are cultured in suitable medium they produce such antigenic proteins or membrane particles, which are purified and incorporated into the vaccines.

Here the advantage is that there is no need to culture pathogenic viruses and bacteria in large quantities. It saves a lot of time and prevents any mutation happening to the pathogenic organism during culturing. 

Other advantages and disadvantages are similar to that of subunit vaccines. 

Examples are:

  • Hepatitis B vaccine
  • Influenza vaccine
  • Conjugate vaccines:

Conjugate vaccine is actually a modified form of subunit vaccine. This is usually prepared for some bacteria which produce outer layer coatings like polysaccharide layer. The vaccines prepared by using these coating materials act as weak antigens and do not produce the required immunological response in the body. In order to increase antigenicity these weak antigens are covalently bound with strong antigens like proteins or peptide chains. These types of vaccines are called conjugate vaccines. This will improve recognition of antigen by the body immune system and produce enough antibodies.

Example for conjugate vaccine include

  • Haemophilus influenzae type B vaccine ( hib vaccine )
  • Pneumococcal conjugate vaccine
  • Meningococcal conjugate vaccine
  • Typhoid conjugate vaccine
  • Soberana 02, a conjugate vaccine against Covid-19
  1. mRNA vaccines

mRNA vaccine technique is a milestone in the history of vaccines. Here instead of introducing any foriegn organism or any part of the organism into the body as antigenic material we induce our own body to produce some spike proteins of pathogenic organisms. 

Here scientists prepare a genetic material known as messenger RNA ( mRNA ) in the lab. This mRNA will be a blueprint for the production antigenic spike protein of the required pathogenic organism. When these mRNA is injected into the human body they get inside to body cells and induce cell ribosomes to produce spike proteins of pathogenic organisms. These spike proteins are then attached to cell surfaces.

Body’s immune system recognizes these cells as foriegn particles and produces antibodies against it. These antibodies remain in the body when the original infection occurs and act quickly to destroy pathogens and protect the body. This is the mechanism by which mRNA vaccines work.

Advantages of mRNA vaccines:

  • Since this technology doesn’t use any living or attenuated organism there is no  chance of developing infection by any means like mutation, mishandling etc.
  • These vaccines are completely laboratory produced and it doesn’t need culturing of real organisms. So the bulk production easy and less time consuming
  • The immunity produced is stronger. It produces both cell mediated and humoral immunity
  • It can be given to immunocompromised individuals
  • These mRNAs do not enter into the nucleus of the cells and are destroyed within a certain period so it doesn’t produce any effect on human DNA structure.

Disadvantages of mRNA vaccines:

  • Even though mRNA vaccine technology has undergone study for decades, successful mRNA vaccines are introduced only recently to general use. So long term efficacy and adverse reactions details are not available.
  • Some of the mRNA vaccines need special storage conditions like ultra cold storages.

Examples of mRNA vaccines

  • Covid 19 vaccines like Pfizer-BioNTech, Moderna, Curevac, Walvax vaccines etc
  1. Viral vector vaccines

This type of vaccine uses modified non harmful viruses as vectors to deliver genetic codings to the human body. Here genetic coding for the production of antigenic proteins of infection producing viruses are incorporated into the genome of non harmful viruses. When these viruses are introduced to the human body they enter cells and induce the production of antigenic particles of required pathogenic organisms. 

The antigenic proteins produced are mounted on the cells. Our immune system recognizes these proteins as foriegn and produces antibodies against that. In this way viral vector vaccines train the human immune system to recognize infectious viruses and rapidly produce antibodies against them. 

The vector viruses may be modified in such a way that they will not multiply and be destroyed after some period of time. Examples of vector viruses used include adenoviruses, retro viruses, influenza virus, vaccinia virus etc.

Advantages of viral vector vaccines

  • It produces strong immunity, produces both humoral and cell mediated immunity
  • It doesn’t contain any original infecting virus or its parts so it avoids any chance of infection.

Disadvantages of viral vector vaccines

  • The manufacturing process of this types of vaccines is complicated
  • There are chances of developing immunity against vector viruses by individuals having early exposure to the same virus. This will lead to loss efficacy in those persons

Examples for viral vector vaccines:

  • Covid 19 vaccines like Oxford-AstraZeneca vaccine, Sputnik V vaccine, Johnson & Johnson Janssen vaccine etc
  • Ebola vaccines like rVSV ZEBOV vaccine

Conclusion

Vaccines are biological preparations containing antigenic substances intended to develop active acquired immunity against some pathogenic organisms. Invention of vaccines is one of the milestones in human history which saved millions of lives. Nowadays there are different types of vaccines which use different types of techniques.

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