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.
The substances in vaccines that trigger an immune response are called antigens. Scientists use several types of antigens to create vaccines, including: inactivated, or "dead," versions of the pathogen; live but weakened versions of the pathogen; components of the pathogen; and suppressed versions of toxins some pathogens produce. Researchers also are investigating new ways to make vaccines that deliver genetic material, DNA or RNA, that would stimulate the body's own cells to produce a desired antigen.
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.
When will there be a vaccine against COVID-19, the disease caused by the novel coronavirus?
In early March 2020, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, predicted that a SARS-CoV-2 vaccine could take at least 12 to 18 months. On December 11, 2020, the FDA issued emergency use authorization for the first vaccine for prevention of COVID-19.
The scientific community mobilized at unprecedented speed to develop a SARS-CoV-2 vaccine, both by building on past progress toward vaccines for similar viruses, SARS and MERS, and by pursuing new directions. As of April 9, 2021, the World Health Organization reported that 186 vaccines were in preclinical development (in the lab or in animals), and 87 vaccines were in clinical trials in humans.
During a pandemic, vaccine development timelines can be shortened by overlapping steps in the process. Many companies and institutions are willing to take on more financial risk during a pandemic, such as the novel coronavirus, and will invest in development and manufacturing before their candidate vaccine has proven results. In addition, the FDA is working with vaccine developers to expedite vaccine-development timelines.