How Vaccines Work — The Science Explained
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The immune system has two components relevant to vaccine function: the innate immune system which responds quickly and non-specifically to anything foreign, and the adaptive immune system which responds specifically and remembers. Vaccines work by training the adaptive immune system without causing actual disease.
When a vaccine introduces an antigen, a fragment or weakened form of a pathogen, B cells recognize it and some become memory B cells that persist for years or decades. T cells are also activated, some becoming memory T cells. When the actual pathogen enters later, these memory cells recognize it immediately and mount a rapid response before disease can establish itself.
Traditional vaccines use killed or weakened whole pathogens. The immune system sees multiple antigens from the pathogen and develops broad immunity. Live attenuated vaccines like MMR tend to produce more durable immunity than killed pathogen vaccines because they replicate briefly and provide more extensive immune stimulation.
mRNA vaccines, the type developed for COVID-19, introduce messenger RNA that instructs your own cells to produce a specific protein from the pathogen. Your immune system recognizes this protein as foreign and develops immunity to it. The mRNA is degraded quickly by normal cellular processes. Your DNA is never involved.
Why effectiveness varies between vaccines: the pathogen's mutation rate matters. Influenza mutates rapidly so last year's flu vaccine is less effective against this year's strains. COVID variants similarly reduce some vaccine effectiveness. Stable pathogens like smallpox and polio are eradicated or nearly so precisely because they do not mutate rapidly enough to escape immunity.
Adjuvants are added to many vaccines to increase immune response. Aluminum salts and other compounds cause mild local inflammation that recruits more immune cells to the injection site and increases antigen presentation. They make smaller doses of antigen effective and improve the durability of the immune response.
Herd immunity is a mathematical property, not a vague concept. When enough of a population is immune, transmission chains break. The threshold percentage varies by how infectious the pathogen is. Measles requires about 95 percent immunity for herd protection. COVID required a higher threshold due to higher transmissibility.
Vaccine safety: the approval process tests for safety in tens of thousands of participants before approval. Post-approval surveillance continues to detect rare adverse events that trial sizes cannot identify. The vaers database in the US collects adverse event reports which are sometimes misrepresented as proven causation.
The historical evidence for vaccine effectiveness is in the disease burden data. Polio, smallpox, diphtheria, tetanus, measles all had massive annual death and disability tolls. These essentially disappeared after widespread vaccination. The correlation is among the most clear in medical history.