Vaccines work by stimulating a person's immune system to develop protections against a disease.
Sometimes getting a vaccine can lead to mild health impacts, such as a fever. The CDC advises that such minor symptoms are normal and should be expected as the body builds immunity.
Different vaccines can work in different ways and each way has its advantages and disadvantages. For example, some require just one dose and others may require multiple doses to be given. Further, vaccines that work by giving people a weakened form of the virus, which for most people causes no more than mild, if any, illness, may not be suitable for people with weakened immune systems. The CDC encourages people to talk with their doctor to decide what vaccinations are right for them.
For more detailed information about how vaccines work, check out this resource from the CDC.
Note: To date, career public servant-scientists at the US Food and Drug Administration (FDA) have granted Emergency Use Authorization (EUA) for the Pfizer-BioNTech COVID-19 vaccine. Before the EUA was granted, independent experts serving on the FDA's vaccine advisory committee had voted overwhelming that, for people 16 years old and above, the benefits of the vaccine outweigh the risks. Canada and Britain have also cleared the Pfizer-BioNTech COVID-19 vaccine for use. There are limited doses of the vaccine, and it is expected the vaccine will first be made available to those most at risk to COVID-19 - health care workers and residents of long-term care facilities - although this will differ by state. For more information on the Pfizer-BioNTech COVID-19 vaccine, check out this nice FAQ from NPR.
Updated 12 December 2020