Why Add Something Else to a Vaccine? Getting the Immune System to Pay Attention
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A purified piece of a pathogen frequently produces a weak immune response by itself, and adding a second ingredient makes the response strong and lasting. What those ingredients do was a black box for most of a century.
The problem being solved
A vaccine works by presenting the immune system with something recognisable from a pathogen so that it learns to respond before the real thing arrives. Early vaccines used whole organisms, killed or weakened, which are recognisably dangerous and provoke a strong response. Modern vaccines increasingly use purified components, which are far safer and better defined and which the immune system frequently ignores, because a protein arriving without any of the signals that accompany an actual infection looks like an ordinary harmless molecule. An adjuvant supplies those missing signals, indicating that something is wrong and that a response is warranted, which converts a weak reaction into a strong and durable one.
What they achieve
The benefits go beyond simply increasing the response:
- •A stronger antibody response from the same quantity of antigen
- •A longer-lasting response, requiring fewer boosters
- •Fewer doses of antigen per person, which stretches a limited supply during a shortage
- •A response of the right type, since different adjuvants shape which arm of the immune system responds
- •Protection in populations that respond poorly, including older people
- •Broader protection against related variants in some cases
The long-standing puzzle
Aluminium salts were introduced as adjuvants in the 1920s and were the only ones licensed for human use in many countries for some seventy years, and how they worked was genuinely unknown for most of that period, which led to them being described as immunology's dirty little secret. Proposed explanations included forming a depot that releases antigen slowly, which measurement has largely undermined. Current understanding involves the particles causing local cell damage and releasing molecules that signal tissue injury, activating a sensor complex within immune cells, which triggers the inflammatory response the vaccine needs. That mechanism was worked out from the mid 2000s, and the general finding that the immune system responds to signals of damage as well as to foreign material reshaped the field.
The vaccines that need none
Not every vaccine requires an adjuvant, and the exceptions clarify what one is for. Live attenuated vaccines use a weakened but replicating organism, which produces all the signals of an actual infection by itself and needs nothing added, which is why the measles, mumps and rubella vaccine contains no adjuvant. Vaccines using an inactivated whole organism frequently retain enough bacterial or viral material to provoke a response. The genetic vaccines developed recently deliver instructions for a cell to make the antigen itself, and the delivery particles and the genetic material both trigger the innate immune system, so the adjuvant effect is built into the platform rather than added as a separate ingredient. That is part of why those vaccines produce noticeable reactions.
The newer ones and what they cost
Several adjuvants beyond aluminium are now licensed and each represents a trade. Oil in water emulsions produce strong responses and are used in some influenza vaccines. Molecules resembling bacterial components activate the specific receptors that detect infection, which is a deliberate application of the mechanism. Extracts from a South American tree bark are used in combination in several recent vaccines. Every one of them increases local reactions including pain, swelling and fever, because the response they provoke is exactly what causes those symptoms, so a more effective adjuvant generally means a more uncomfortable injection. Balancing that is a real regulatory judgement, and it differs between a vaccine for a severe disease and one for a mild one.
The takeaway
A purified protein arriving without the signals of an actual infection looks harmless, so an adjuvant supplies those signals and converts a weak response into a durable one. Aluminium salts were used for seventy years before anyone established how they work, which turns out to involve signals of cell damage. Stronger adjuvants produce more local pain and fever, because that reaction is the response.