How vaccines work
A vaccine contains a weakened (attenuated), killed, or partial form of a pathogen, or its isolated antigens. This material cannot cause the disease but still carries the antigens that the immune system recognises.
After vaccination, lymphocytes detect the antigens and produce antibodies specific to them. The lymphocytes also produce memory cells. If the actual pathogen enters the body in the future, these memory cells recognise its antigens immediately and produce antibodies much faster and in larger quantities, usually preventing the person from becoming ill. This is active artificial immunity.
Herd immunity
When a large proportion of a population is vaccinated, the pathogen cannot spread easily because most potential hosts are immune. Even unvaccinated individuals (such as those too young or too ill to be vaccinated) gain indirect protection because the chain of transmission is broken. This population-level protection is herd immunity.
The threshold for herd immunity varies by disease. Highly contagious diseases like measles require about 95% vaccination coverage to maintain herd immunity. If vaccination rates drop below the threshold, outbreaks can occur even in previously protected populations.
Limitations and considerations
Vaccines may not provide 100% protection: some individuals may not produce enough memory cells. Pathogens like influenza mutate frequently, changing their surface antigens, so new vaccines must be developed each year. The original vaccine's antibodies may not recognise the mutated antigens.
Some people cannot be vaccinated due to allergies to vaccine components or because they are immunocompromised. These individuals rely on herd immunity for protection, which is why maintaining high vaccination rates in the wider community is important.