4. Mode of Administration
Most available vaccines are delivered by injection, either into the fatty tissue just below the skin, or into muscle. A number of oral vaccines have also been developed. These are easier to administer, but because they pass through the digestive tract this method is not suitable for all vaccine types, because antigens may be altered by the body’s digestive system. The influenza vaccine for children is, as of early 2014, currently the only vaccine that can be administered through the nasal passage. It is given in the form of a spray that is directed into the nose and inhaled through normal breathing. Jet-injectors, which shoot a high-pressure stream of liquid medication through the skin to underlying tissues, were first used for medical applications in the 1940s. These were further developed in the late 20th and early 21st centuries and are still being optimised for use with vaccines now.