Vaccines work by training the body’s immune system to recognise and remember the characteristics of a virus so that it can quickly respond when under attack. The vaccine not only prevents someone from being infected by a virus, it also stops the virus being passed on to others.
More than 35 teams across the world are working on a vaccine against the new coronavirus. At least five different approaches to vaccination are being explored.
Live-attenuated or weakened vaccines
One way to trigger an immune response is to introduce live coronavirus into the body. Examples of this type of vaccine are used against smallpox, yellow fever, chicken pox, and measles, mumps and rubella (MMR vaccine). This type of vaccine would use an ‘attenuated’ form of the new coronavirus, which has been weakened by growing the virus under specific laboratory conditions or making changes to its genome. Attenuated vaccines usually give strong and long-lasting protection, but they cannot be given to people with weakened immune systems because they risk making them ill.
These vaccines use an inactive or ‘dead’ version of the coronavirus, killed for example by exposing it to UV light. Inactivated viruses, like those used to protect against flu and hepatitis A, cannot cause infection but they do not generally provide immunity as strong as live vaccines.
Subunit vaccines are made from a specific part of the virus, such as a protein, which alone cannot cause disease. They produce an immune response that is targeted to these key parts, which will then prevent infection by the whole, live virus. Subunit vaccines are relatively safe and are currently used against whooping cough, shingles (after chickenpox), human papillomavirus (HPV) and hepatitis B. But they may not cause a strong immune response so that booster shots are required to maintain protection.
There are seven known human coronaviruses, including those causing severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). Attempts to develop vaccines against SARS and MERS have helped to identify the best options to take for a subunit vaccine against the new coronavirus. These include targeting single proteins, like the spike protein that the virus uses to enter human cells.
Another option is to use the whole external coat of a virus, excluding the virus genes that allow it to multiply.
DNA and mRNA vaccines
These vaccines, introduced around a decade ago, contain genetic material that instructs human cells to make a coronavirus protein. The body’s immune system recognises this protein as foreign and produces a response against it. DNA and mRNA (messenger RNA) vaccines are relatively easy to make in the laboratory and may be among the first to be tested in clinical trials. There are, however, currently no licensed vaccines for any human disease using these technologies.
These vaccines use other viruses, which do not cause human disease, as ‘vectors’ or ‘carriers’ of coronavirus protein into the body. Such viral vectors have been developed from a range of harmless viruses, including weakened or inactive versions of measles, poxviruses, and adenoviruses. The vector virus carries genes that make coronavirus protein on its surface, which stimulates protective immunity.
Why so many different approaches?
There are currently no licensed vaccines against the new coronavirus and, at this stage, researchers cannot predict which vaccines will work best. Working in parallel on multiple approaches increases the chance of finding a safe and effective vaccine quickly. Developments in vaccine technology and a better understanding of the new coronavirus genome have increased the speed with which a vaccine can be made. It used to take approximately ten years to get a vaccine to market but today, with the help of new technologies, that timeline has shortened to one or two years.