The very first vaccines for COVID-19 to complete phase 3 testing are an entirely new type: mRNA vaccines. Never before have mRNA vaccines — such as the Pfizer/BioNTech vaccine that just received emergency use authorization from the FDA, and the Moderna vaccine — been approved for use in any disease. How do they differ from traditional vaccines, and what makes them so exciting?
How traditional vaccines work
The main goal of a vaccine for a particular infectious agent, such as the virus that causes COVID-19, is to teach the immune system what that virus looks like. Once educated, the immune system will vigorously attack the actual virus, if it ever enters the body.
Viruses contain a core of genes made of DNA or RNA wrapped in a coat of proteins. To make the coat of protein, the DNA or RNA genes of the virus make messenger RNA (mRNA); the mRNA then makes the proteins. An mRNA of a specific structure makes a protein of a specific structure.
Some traditional vaccines use weakened virus, while others use just a critical piece of the virus’s protein coat. In the case of COVID-19, a piece called the spike protein is the critical piece.
Traditional vaccines work: polio and measles are just two examples of serious illnesses brought under control by vaccines. Collectively, vaccines may have done more good for humanity than any other medical advance in history. But growing large amounts of a virus, and then weakening the virus or extracting the critical piece, takes a lot of time.
Early steps toward mRNA vaccines
About 30 years ago, a handful of scientists began exploring whether vaccines could be made more simply. What if you knew the exact structure of the mRNA that made the critical piece of a virus’s protein coat, such as the spike protein of the COVID-19 virus?
It is relatively easy to make that mRNA in the laboratory, in large amounts. What if you injected that mRNA into someone, and the mRNA then traveled through the bloodstream to be gobbled up by immune system cells, and then those cells started to make the spike protein? Would that educate the immune system?
Overcoming obstacles in creating mRNA vaccines
While the concept seems simple, it required decades of work for mRNA vaccines to overcome a series of hurdles. First, scientists learned how to modify mRNA so that it did not produce violent immune system reactions. Second, they learned how to encourage immune system cells to gobble up the mRNA as it passed by in the blood. Third, they learned how to coax those cells to make large amounts of the critical piece of protein. Finally, they learned how to enclose the mRNA inside microscopically small capsules to protect it from being destroyed by chemicals in our blood.
Along the way, they also learned that, compared to traditional vaccines, mRNA vaccines can actually generate a stronger type of immunity: they stimulate the immune system to make antibodies and immune system killer…