The innate immune response

The innate immune system is the body's first line of defence against any foreign substance. When a vaccine is injected into muscle, the innate immune system detects molecular patterns associated with the vaccine components and triggers an inflammatory response at the injection site. This is why the arm becomes red, warm, and sore after vaccination. Innate immunity is non-specific, rapid, and serves primarily as an alarm system that activates the adaptive immune response.

The adaptive immune response

The adaptive immune system is highly specific and has memory. When innate immune cells process the vaccine antigen and present it to T cells and B cells in the lymph nodes, these cells begin to proliferate and differentiate. B cells produce antibodies — proteins that bind specifically to the vaccine antigen and can neutralise the real pathogen. T helper cells coordinate the immune response. Cytotoxic T cells are trained to destroy cells displaying the antigen. This process takes one to two weeks to fully develop after vaccination.

Immunological memory: the key mechanism

After the initial adaptive response subsides, the immune system retains a population of long-lived memory B cells and memory T cells specific to the vaccine antigen. These cells persist in the body for years to decades. If the real pathogen is encountered in the future, these memory cells enable a rapid secondary immune response. While the primary response takes one to two weeks, the memory response can produce protective antibody levels within hours. This speed difference is the mechanism by which vaccines provide protection.

How antibodies neutralise pathogens

Antibodies produced after vaccination bind to specific structures on the pathogen surface. For respiratory viruses, this is typically the protein used to enter human cells. When antibodies coat the viral surface, they can directly neutralise the virus, preventing cell entry, and mark the virus for destruction by other immune cells. A person with high levels of pre-formed antibodies from prior vaccination can neutralise a pathogen immediately on exposure, before it has established significant infection.

Priming the immune system

Some vaccines require two or more doses because the first dose primes the immune system but produces a relatively modest initial response. The immune system needs to be exposed to the antigen at least once before it can mount a robust secondary response. The second dose, given weeks or months later, encounters a population of primed lymphocytes and triggers a dramatically amplified antibody response — sometimes 10 to 100 times the level produced by the first dose.

Waning immunity over time

Circulating antibody levels decline over time for most vaccine-induced immune responses, even when memory cells persist. For some vaccines, this decline is slow enough that protection remains high for decades. For others, circulating antibodies fall below protective levels within months to years, necessitating booster doses. The flu vaccine requires annual doses for a different reason: influenza viruses mutate rapidly enough that last year's vaccine may not match this year's strains.

The role of adjuvants

Many inactivated vaccines include adjuvants: substances added to enhance the immune response. Aluminium salts are the most widely used adjuvants, having been used safely in vaccines since the 1930s. Newer adjuvants including AS01 (used in Shingrix) produce substantially stronger immune responses than aluminium salts. Adjuvants work by creating a more pronounced innate immune response at the injection site, which in turn drives a stronger adaptive response.