Initial clinical studies of medications do not usually include more than 5,000 people, nor extend longer than 5 to 8 years. Except for delaying availability of promising medications, post-marketing surveillance is the only way to study larger groups of people for longer periods of time. Vaccines receive this extra scrutiny, compared to other medications, because vaccines are complex biological mixtures that must be carefully manufactured to assure reproducible potency. Potency is assessed by survival of vaccinated laboratory animals after lethal challenge. Safety is confirmed by testing for weight gain and absence of fever in other animal tests using at least two species.
Sterility is tested using two kinds of culture media.
Purity is measured by chemical assays. Most vaccines are injected into the body, although a few can be swallowed by mouth. If injections are required, it is because stomach acid would prevent the vaccine from provoking a protective antibody response, or because not enough of the vaccine would get into the blood stream. Typically, vaccines administered by mouth correspond to a microbe that enters the body through the oral route. An influenza vaccine administered as a nasal mist showed promise in recent clinical studies.