The human immune system has evolved to remember the pathogens, such as viruses and bacteria, it has fought off in the past. Through this immunological memory, the immune system can often recognize familiar pathogens and attack them quickly enough to prevent another bout of illness. Vaccines work by stimulating an immune response and creating immunological memory without causing harmful symptoms.
Vaccines make use of antigens, which are substances that trigger immune responses. Vaccines may contain one of several types of antigens, including: inactivated, or "dead," versions of the pathogen; "attenuated," or live but weakened versions of the pathogen; parts of the pathogen; or suppressed versions of toxins produced by the pathogen. Alternatively, vaccines may deliver genetic material, such as mRNA or DNA, which instructs the host's cells to produce a desired antigen. Researchers are investigating new ways to make and improve vaccines that use engineered virus particles to deliver this genetic material.
The immune system's response to pathogens is complex, and testing in animals and humans is necessary to ensure that a vaccine is both safe to administer without serious side effects and effective at preventing infection.
After a candidate vaccine has proved safe and effective in animals, it is tested in humans. This stage, clinical development, occurs in three phases, each including increasingly larger groups of people.
After successful clinical trials, the vaccine goes through the U.S. Food and Drug Administration (FDA) multistep approval process, and, finally, manufacturing and quality control.
Sometimes, when a new vaccine is discovered, new manufacturing techniques must be developed to make enough of the vaccine to distribute widely. This adds even more time to the process. However, during an emergency such as a pandemic, vaccine development timelines can be shortened by overlapping steps in the process. Many companies, institutions, and governments are willing to take on more financial risk during an emergency and will invest in development and manufacturing before their candidate vaccine has proven results.