Generally speaking, vaccine development is a long and arduous process, one that often takes years. With COVID-19, however, scientists were able to complete the task in a matter of months. Two of the most effective vaccines – from pharmaceutical companies Moderna and Pfizer/BioNTech – are what's known in the industry as mRNA vaccines, and are a new type of vaccine to protect against infectious diseases. CGTN'S Wang Hao tells us more about them.