Vaccines are biological preparations designed to provide acquired immunity to a particular infectious disease. They contain an agent that resembles a disease-causing microorganism, made from weakened or killed forms of the microbe, its toxins, or one of its surface proteins. A vaccine prepares the body’s immune system to recognize and fight off future infections from the actual pathogen. This preventive measure has altered public health by significantly reducing the burden of many severe illnesses globally.
How Vaccines Protect Health
Vaccines introduce harmless forms of pathogens or their components to the body, initiating an immune response without causing the actual disease. Some vaccines use inactivated viruses, which are whole viruses that have been killed but remain structurally intact. Others use attenuated (weakened) viruses that can replicate but are too weak to cause illness, or specific protein subunits of a pathogen, like the SARS-CoV-2 spike protein. The immune system recognizes these introduced elements as foreign antigens, prompting an adaptive response.
Upon exposure to these antigens, specialized immune cells like B cells and T cells are activated. B cells produce antibodies, proteins that neutralize pathogens or mark them for destruction. Helper T cells assist B cells in antibody production and activate cytotoxic T cells, which directly kill infected cells. This initial encounter, facilitated by the vaccine, creates immunological memory.
Immunological memory means the immune system “remembers” the pathogen. If a vaccinated individual later encounters the actual pathogen, memory B cells and T cells rapidly proliferate and mount a much stronger, faster immune response. This swift action prevents the pathogen from establishing a widespread infection, averting severe symptoms, reducing hospitalization, and preventing death.
Global Health Impact
Vaccination campaigns have profoundly impacted global health, transforming the trajectory of numerous infectious diseases. A prominent example is smallpox eradication, a devastating disease that once caused millions of deaths annually. The last naturally occurring case was reported in 1977, leading to its official eradication in 1980 through a global vaccination effort. Similarly, polio, which caused paralysis and death, has seen a reduction in cases worldwide due to widespread vaccination, with efforts continuing towards its global elimination.
Vaccines have also significantly reduced the incidence and mortality of diseases like measles, diphtheria, tetanus, and pertussis (whooping cough). Before the measles vaccine, measles caused millions of cases and thousands of deaths annually; now, outbreaks are less frequent and severe in vaccinated populations. This broad protection extends beyond the individual through herd immunity, also known as community immunity.
Herd immunity occurs when a sufficiently large proportion of a population is immune to a disease, through vaccination or prior infection, making it difficult for the disease to spread. This collective immunity indirectly protects vulnerable individuals who cannot be vaccinated, such as infants, the elderly, or those with compromised immune systems. By reducing the pool of susceptible hosts, vaccination programs prevent epidemics, lower overall disease burdens, and contribute to improvements in life expectancy and public health infrastructure across the globe.