How Vaccines Work: Immunity, Types, and Impact

The Immune System and Active Immunity

The immune system has two broad arms: the innate immune system, which responds rapidly and non-specifically to any foreign threat, and the adaptive immune system, which mounts a targeted, pathogen-specific response and—critically—retains immunological memory. Vaccines exploit this memory capacity by presenting the immune system with an antigen (a molecule that triggers an immune response, typically a protein on the surface of a pathogen) without the danger of a live infection.

Upon first exposure to a vaccine antigen, B lymphocytes produce specific antibodies while T lymphocytes learn to recognize and destroy cells infected with that pathogen. Some of these cells differentiate into long-lived memory B and T cells. If the vaccinated individual subsequently encounters the actual pathogen, these memory cells mount a rapid, high-amplitude secondary immune response—neutralizing the pathogen before it can cause disease. This is active immunity, in contrast to passive immunity, which involves transferring ready-made antibodies (as in immunoglobulin therapy), providing immediate but temporary protection.

Types of Vaccines

Live-attenuated vaccines contain a weakened (attenuated) form of the pathogen that replicates in the body but does not cause disease in immunocompetent individuals. They produce the strongest and most durable immunity, often requiring only one or two doses. Examples include the measles-mumps-rubella (MMR) vaccine, the varicella (chickenpox) vaccine, and the oral polio vaccine. Their limitation is that they cannot be given to severely immunocompromised individuals.

Inactivated vaccines contain killed whole pathogens or pathogen fragments. They are stable, safe for immunocompromised patients, and do not require cold-chain management as stringently as live vaccines, but they typically produce weaker immunity and require booster doses. Examples include the inactivated influenza vaccine and the hepatitis A vaccine. Subunit, recombinant, and conjugate vaccines contain only specific antigenic proteins or polysaccharides from the pathogen. Conjugate vaccines link polysaccharide antigens to carrier proteins to boost the immune response in infants. Examples include the Hib vaccine and pneumococcal vaccines.

mRNA vaccines represent a transformative technology. Instead of delivering an antigen directly, they deliver genetic instructions (messenger RNA) that tell the body's own cells to produce the antigen—typically the spike protein of a virus. The immune system then responds to this self-produced antigen. mRNA vaccines are rapid to design and manufacture, as the genetic sequence of a new pathogen's antigen can be encoded and produced within weeks. The COVID-19 vaccines developed by Pfizer-BioNTech and Moderna demonstrated the safety and efficacy of this platform at unprecedented scale. Viral vector vaccines, such as the Oxford-AstraZeneca COVID-19 vaccine, use a modified virus (the vector) to deliver antigen-encoding genetic material into cells.

Adjuvants: Amplifying the Immune Response

Adjuvants are compounds added to vaccine formulations to enhance and prolong the immune response. They stimulate the innate immune system at the injection site, creating a local inflammatory environment that recruits immune cells and provides the danger signals needed for a robust adaptive immune response. Without adjuvants, many subunit and inactivated vaccines would produce insufficient immunity.

Aluminum salts (alum) are the oldest and most widely used adjuvants, found in vaccines such as hepatitis B and HPV vaccines. Newer adjuvant systems, such as AS04 (used in Cervarix HPV vaccine) and AS01B (used in the shingles vaccine Shingrix), combine multiple components to activate specific immune pathways and produce particularly potent, long-lasting immunity. Adjuvant development is an active area of research aimed at improving vaccine efficacy, reducing the number of doses required, and enabling mucosal delivery.

Herd Immunity

Herd immunity (also called community or population immunity) occurs when a sufficient proportion of a population becomes immune to an infection—either through vaccination or prior infection—that transmission chains break down, protecting even those who are not immune. The threshold vaccination coverage required for herd immunity depends on the pathogen's basic reproduction number (R0), which describes how many secondary cases one infected person causes in a fully susceptible population.

Measles, with an R0 of 12–18, requires approximately 95% population immunity to achieve herd protection. Polio requires around 80–85%. COVID-19's original strain had an R0 of 2–3, suggesting a lower threshold, but the highly transmissible Omicron variant had an R0 exceeding 10, dramatically raising the herd immunity threshold. Herd immunity is particularly important for protecting individuals who cannot be vaccinated—newborns too young for certain vaccines, people with severe allergies, and immunocompromised patients who cannot mount adequate responses to vaccination.

Historical Impact of Vaccination

The history of vaccination is one of medicine's greatest success stories. Smallpox, which killed an estimated 300–500 million people in the 20th century alone, was declared eradicated by the World Health Organization in 1980 following a global vaccination campaign—the only human infectious disease to have been deliberately eradicated. Polio has been eliminated from all but a handful of countries, with cases reduced by over 99% since 1988.

The introduction of childhood vaccination programs in the 20th century dramatically reduced mortality from diphtheria, tetanus, pertussis, measles, and haemophilus influenzae type b meningitis. The WHO estimates that vaccines prevent 3.5–5 million deaths per year globally. The development of HPV vaccines has begun to show measurable reductions in cervical cancer incidence in vaccinated cohorts. The speed with which effective COVID-19 vaccines were developed and deployed—within a year of the pandemic's onset—demonstrated how far vaccine technology had advanced and set the stage for rapid vaccine development against future emerging pathogens.