Technology - general aspects

Traditional vaccines consist primarily of a killed or a weakened version of a pathogen or of a subunit of the agent. Vaccines aim to prime the immune system to quash dangerous viruses, bacteria or parasites quickly, before the pathogens can gain a foothold in the body.

Standard vaccines vary. Those based on killed pathogens or on antigens isolated from disease-causing agents, cannot make their way into cells. They therefore give rise to primarily humoral responses and do not activate the killer “T-cells”. Such responses are ineffective against many microorganisms that infiltrate cells. Also, even when non-living preparations do block disease, the protection often wears off after a time; consequently, recipients may need periodic booster shots.

Attenuated live vaccines, usually viruses, do enter cells and make antigens. They thus spur attack by killer “T-cells” as well as by antibodies. That dual activity is essential for blocking infection by many viruses and for ensuring immunity. Live vaccines frequently confer lifelong immunity. For those reasons, they are considered the "gold standard" of existing vaccines.
Live vaccines can be problematic though. They can cause full-blown illness when the immune system is compromised.

Weakened viruses can at times mutate in ways that restore virulence.

Whole-organism vaccines, whether live or dead, have other drawbacks as well. Being composed of complete pathogens, they retain molecules that are not involved in evoking protective immunity. They can also include contaminants that are unavoidable by-products of the manufacturing process. Such extraneous substances sometimes trigger allergic or other disruptive reactions.

Most marketed vaccines are either based on killed or live agents; recombinant DNA techniques can be used in live and subunit vaccines.