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How does the vaccine work?
When a pathogen invades the body, the immune system can respond by producing immune cells (macrophages, B-lymphocytes and T-lymphocytes) to fight back. The battle between immune cells and the pathogen during infection can last several days. After that, the body keeps some memory cells that can react immediately to the same pathogen. When similar pathogens are identified in the body, antibodies can be sent by B-lymphocytes to destroy them. What a vaccine does is to imitate the infection process. A high-quality vaccine is supposed to cause the immune system to develop immune cells and antibodies without causing any concerned diseases, though some minor symptoms like fever may occur in some cases.   

How to develop vaccines?

To develop a new vaccine, a rigorous series of human clinical trials (Phase I, Phase II and Phase III) are required. To begin with, Phase I clinical trials involve giving the candidate vaccine to a few of individuals (often less than 10) to exclude major safety concerns and find the right dose. If everything goes well, the vaccine will be tested in a larger group of volunteers (from 100 to 1000) to check the consistency of immunogenicity and side effects. That’s how a Phase II trial is conducted. After that, tens of thousands people will be involved in Phase III trials to examine if the candidate vaccine can prevent the natural infection.