How do the AstraZeneca, Johnson & Johnson, Sputnik, and CanSino vaccines work?
A group of vaccines that work in a similar way, based on a viral vector for the delivery of the vaccine information, are those developed by AstraZeneca and the University of Oxford (also named Covishield and Vaxzevria), Johnson and Johnson (J&J, Janssen), the in Russia developed vaccine called Sputnik V (Gamaleya), and the in China developed CanSino Vaccine.
What is a viral vector?
A viral vector is a virus that can deliver information to your cells. The virus that is used in these types of vaccines is a harmless virus to humans. This means it is a virus that does not cause disease and cannot replicate - or spread- in your body. AstraZeneca uses a modified Chimpanzee adenovirus, J&J uses the adenovirus vector type Ad26, Sputnik uses types Ad26 and Ad5 (one for each shot), and CanSino uses type Ad5. It is a virus that serves as a delivery system, a carrier, to provide your cells with an instruction manual. The instructions in these viral vectors come into the cell as dsDNA (double-stranded DNA). Viral vectors are like cars designed to arrive at a specific destination, the human cells. Once the viral vectors arrive at the cells they stick to them. This lets the cells know they need to open their doors so they can receive a message from the vector. In this case, this message is the instruction manual for the SARS-CoV-2 spike protein. This will give the immune system a heads up that this protein belongs to a virus that the body must eliminate. Once the spike protein code is within the cells, the cells fabricate this protein to show it to the cells of the immune system so they can recognize it in the future in a very rapid manner.
What is the viral vector in these vaccines used for?
The information in the viral vectors in these vaccines is used to deliver a set of instructions to your cell to produce the spike (S) protein from the virus. The coronavirus is an enveloped virus. The envelope serves as the ‘coat’ of the virus. This means that the virus is packaged in an outer layer of oily lipid molecules. In between these lipid molecules, the coat has some proteins. One of these proteins in the coating is used for the vaccine: the spike (S) protein (see Figure). The spike protein is important as the virus needs it to bind to specific components on human cells. This binding allows the virus to enter and infect the cell. Blocking this protein can therefore stop the virus from infecting more cells and spreading in the body.
How do these viral vector vaccines trigger an immune response against the virus?
When you are vaccinated, the viral vector that is injected into your body is taken up by some cells in your body and they will use the dsDNA in the vector to produce lots of S protein. First, the virus travels to the nucleus of the cell, where the DNA can be read. From the DNA instructions, the cell then produces mRNA that instructs the protein machinery in the cell to produce the S protein. The S protein will be placed on the coating of these cells as a form of signal that your immune system can recognize and use (see also post on the immune response).
The immune system then comes into action in two ways:
Your B cells will start producing antibodies against the spike protein. The antibodies will bind to any spike protein it comes in contact with, blocking any future virus you may encounter from being able to bind your cells and infect other/more cells.
T cells will be trained to recognize S protein fragments (also called antigens). In the future, if you get infected with the coronavirus, your cells will show these fragments to your T cells. In that way, the trained T cells can recognize the cells that are infected. The T cells will kill these infected cells, which stops the coronavirus’s further spread in your body.
Good to know
The DNA in this viral vector is much less fragile than the mRNA used in the vaccines by Pfizer and Moderna. This means that these vaccines can be stored in the fridge and it is easier to distribute these around the world.
Contributed by: Text: Maartje Wouters, llustration: Armando Andres Roca Suarez
Voysey et. al. 2021 (The Lancet) Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK
Madhi et. al. 2021 (NEJM) Efficacy of the ChAdOx1 nCoV-19 Covid-19 Vaccine against the B.1.351 Variant.
Johnson & Johnson
Phase 1-2a : Sadoff et. al. 2021 (NEJM) Interim Results of a Phase 1–2a Trial of Ad26.COV2.S Covid-19 Vaccine
Logunov et. al. 2021 (The Lancet) Safety and efficacy of an rAd26 and rAd5 vector-based heterologous prime-boost COVID-19 vaccine: an interim analysis of a randomised controlled phase 3 trial in Russia
Zhu et al. 2020: Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind, placebo-controlled, phase 2 trial
NY Times vaccine tracker