Vaccination remains a major hope for controlling rapid spread of Covid-19 by imparting immunity to virus infection . Most of the countries are racing against the time to vaccinate majority of their population so that things can come back to normal. However, availability of vaccines in many countries is hampering this vaccination drive. In spite of record breaking time of roughly one year for developing vaccines against SARS-CoV2 / Covid-19, ramping up production of vaccine doses required for each country seems to be humongous task.
All vaccines provide immunity to virus infection by stimulating a immune response to an antigen. a molecule found on the outer surface of a pathogen. In the case of SARS-CoV2 , the antigen is the spike protein through which it attaches to human cells and invade.
Here we list few of the covid-19 vaccines approved in different countries and also understand how these vaccines are made and their mode of action.
Covaxin is indigenous vaccine of India, developed by Hyderabad based Bharat Biotech in collaboration with the Indian Council of Medical Research (ICMR) and the National Institute of Virology (NIV). This vaccine is developed by using whole virus which cause Covid-19. They first chemically inactivate the virus using a chemical beta-propiolactione, so that the virus can no longer replicate and cause disease. Even after inactivation the virus still retains the ability to trigger immune response in human cells to prepare a defense mechanism against active virus. This is possible as viral proteins remain intact even after chemical treatment.
This kind of vaccine development is one of the oldest in the field and many well known vaccines against diseases like Polio, Rabies, Pertussis, and Japanese encephalitis were development in this manner. Covaxin requires two doses with a gap of 28d ays between them and needs a storage temperature of +2°C to +8°C.
Sinopharm vaccine developed by China also follows a similar strategy.
Covishield ( Oxford / Astrazeneca ) :
Developed by Oxord and Astrazeca, Covishield is manufacture in bulk by Serum institute of India in Pune. Covishield is developed using a viral vector platform, a completely different technology when compared to that employed for Covaxin. A chimpanzee adenovirus – ChAdOx1 was modified to express the SARS-CoV2 spike protein. In this way a harmless virus is used as a vehicle to carry the information to produce antigen (spike protein of SAR-CoV2) in the form of double stranded DNA. Once the adenovirus is bound to our cell, DNA is injected into the cytoplasm and using the host protein machinery, viral spike protein is synthesized. This spike protein will now makes it’s way to the cell membrane where it triggers the immune response.
Covishield is also required in two doses to elicit robust immune response and storage conditions are +2°C to +8°C.
Johnson and Johnson vaccine is developed based on similar strategy but employs human common cold virus adenovirus 26 (Ad26). This a single dose vaccine.
Russian vaccine Sputnik V ( Gam-COVID-Vac,) is the first registered vaccine for covid-19 in the world with the efficiency over 90%, also uses a similar technology of viral vector but uses two different adenoviruses ( causes common cold in humans) as viral vectors to generate immune response. The first dose of Sputnik V uses adenovirus 26 (Ad26) to carry genetic information for producing spike protein of SARS-CoV2, while the second shot contains adenovirus 5 (Ad5). Our immune system reacts to the SARS-CoV2 spike protein by developing antibodies specific to the SARS-CoV-2 virus and by eliciting T-cell responses. These antibodies specific to SARS-CoV2 will protect our cells from getting infected in future. The company also recently launched a single dose vaccine called Sputnik light, which can significantly reduce the total number of doses required for full vaccination .
BioNTech / Pfizer : Developed by German Biotech company and Pfizer was one of the first covid-19 vaccine to approved and used in majority of European countries and USA. This vaccine is an example of “nucleic acid vaccine“, where Nucleic acids or genetic materials DNA/RNA of disease causing virus is used to trigger immune response. In the case of BioNTech / Pfizer vaccine, messenger RNA (mRNA) coding for spike protein of SARS-CoV2 is encapsulated in lipid layer. Once absorbed into the cells, our protein machinery is directed to produce viral spike protein, which eventually acts as antigen important for immune response. This vaccine is very effective in generating immune response but being an RNA vaccine , it requires ultra cold storage conditions such as -80°C. So storage and logistics could be a huge issue in poor or low income countries with not so specialized cold storage.
Moderna : Moderna vaccine is also a mRNA vaccine and majorly used for inoculations in many developed countries.
Immune response and elimination of virus:
Immune response can be defined as how our body recognizes different harmful pathogens and protects itself from them. Our body has the ability to differentiate between “self” and “non-self” molecules. If our cells encounters something which is non-self or foreign, our immune system responds by using chemical and cellular defense machinery to destroy it.
Antigens have the ability to trigger specific immune response. The specific antigens can now be recognized by special proteins called as antibodies. These antibodies can now recognize and bind to antigens directly, to initiate a process of elimination of the pathogens. This function of destruction requires humoral and cell mediated immunity involving B and T lymphocytes (white blood cells). After fighting a particular infection our body retains a few T-lymphocytes, called “memory cells,” that remembers the antigen it encountered and defeated. These memory cells will come handy when our body encounters the same virus again, by giving a signal to the B-lymphocytes to produce antibodies and neutralize the virus before causing the disease.
The main purpose of vaccination is to present the cell with a harmless form of antigen. Once our cells recognizes a non self molecule ( here viral spike protein of SARS-CoV2), it triggers a specific immune response and generate antibodies capable of destroying the viral particles. Two doses of vaccine against SARS-CoV2 produces sufficient antibodies and in future if our body encounters the virus, our body will be in best shape to defeat the virus.