Candidate vaccines against SARS-CoV-2
Various types of SARS-CoV-2 vaccines are being developed. The Netherlands has reached agreements on six vaccine candidates so far. By now, contracts have also been signed with the manufacturers of all these candidate vaccines. Negotiations on contracts for Novavax, a seventh candidate vaccine, are ongoing. The BioNtech/Pfizer and Moderna vaccines have been improved for use in the European Uninon.
For current information go to Updates on vaccins and treatment against the coronavirus on the CBG website (in Dutch).
How do vaccines work in general?
A vaccine mimics an infection without causing illness. It teaches the immune system to fight the infection. As a result, your illness will be less severe, or you will not become ill at all.
Vaccines utilise the way that the immune system works to defend the body against diseases and pathogens. This system involves many different immune cells and immune substances. Part of the immune system is innate (existing from birth) and quickly takes action to provide the first line of defence and to kick-start a second line of defence. This second part is adaptive (‘learned’ over time) and is more precise. Both of these parts can detect and respond to molecules that are foreign to the body, and work together closely.
A cell of the body that has been infected by a virus creates new virus particles. When that happens, the molecules associated with that virus can be seen on the outside of the infected cell and on the virus particles that are released. When immune cells see those molecules that are foreign to the body, innate and adaptive immune responses are triggered in which the adaptive immune cells launch a specialised defence against proteins that are specific to the pathogen. That ‘learning’ process takes time, from a few days up to about two weeks.
The result is that the body produces antibodies that can bind to the pathogen’s protein coat, so virus particles are unable to infect other cells. The body also achieves cellular immunity, which cleans up infected cells. Antibodies fight the pathogen itself, while cellular immunity prevents the virus from replicating by clearing away the infected cells quickly. Both of these mechanisms are necessary for protective immunity.
It is also important that memory cells are formed. When these memory cells recognise the protein again later, such as during a subsequent infection with the virus, they can take action faster, using a ‘smarter’ response. This is because they have already ‘learned’ to identify the pathogen. As a result, your illness will be less severe, or you will not become ill at all.
A vaccine mimics an infection without causing illness. To do so, the vaccine puts both defence lines of the immune system to work. This results in the formation of memory cells as parts of the adaptive immune response that help the body launch a fast, smart response if necessary. Read more below about how RNA vaccines work.
2 types of COVID-19 vaccines
Two types of COVID-19 vaccines will be used in the Netherlands from January 2021 on: two different RNA vaccines (Moderna and Pfizer) and a vector vaccine (AstraZeneca). Read more below about how these types of vaccines work.
In these vaccines, the genetic material (DNA or RNA) in the vaccine contains the code for a protein that is specific to a pathogen, such as the ‘spike’ protein of the coronavirus. After vaccination with the DNA or RNA vaccine, the cells of the body read the code at the injection site and build the protein, the active substance of the vaccination. Since this new protein is foreign to the body, cells in the body’s immune system detect its presence and then launch an immune response. In that immune response, cells of the adaptive immune system, using instructions from cells of the innate immune system, build up a specific defence. That ‘learning’ process takes time, from a few days up to about two weeks. The result is that the body produces antibodies that can bind to the pathogen’s protein coat, and achieves cellular immunity to clear infections.
It is also important that adaptive memory cells are formed. When these memory cells recognise the protein again later, such as during an infection with the actual virus, they can take action faster and more effectively. This is because they have already ‘learned’ to respond. As a result, your illness will be less severe, or you will not become ill at all.
These DNA and RNA vaccination techniques are new. Until recently, no DNA or RNA vaccines were available for human infectious diseases. An RNA vaccine for COVID-19 has recently received marketing authorisation for the first time. A number of DNA vaccines have already been used successfully to vaccinate animals.
Researchers can modify existing viruses to act as vaccines. Once that happens, they are no longer viruses, but vectors. The viruses have been adapted in such a way that they do not display exactly the same behaviour as unmodified viruses. The difference compared to the real viruses is that vector viruses:
- can no longer make someone ill;
- (often) cannot replicate themselves, and;
- not only contain their own RNA or DNA, but also have a piece of RNA or DNA from another virus within them. All pieces of RNA or DNA can work as an antigen, so the cells in our immune system will react to the vector virus as well as to part of the vaccine virus. This is how immunity is developed.
A category of viruses that are often adapted into a vector are the adenoviruses. Adenoviruses are a group of viruses to which people are often exposed, but which cause no or only mild illness. Because adenoviruses are so common, our immune system is very good at dealing with an adenovirus infection.