Why do vaccines work

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Last updated: April 8, 2026

Quick Answer: Vaccines work by training the immune system to recognize and fight specific pathogens without causing the disease itself. They typically contain weakened or inactivated parts of a pathogen (antigens) that trigger an immune response, producing memory cells that provide long-term protection. For example, the measles vaccine introduced in 1963 has reduced global measles deaths by 73% between 2000 and 2018, preventing an estimated 23.2 million deaths. Modern mRNA vaccines like those for COVID-19 work by instructing cells to produce a harmless piece of the virus spike protein, which then triggers an immune response.

Key Facts

The first successful vaccine was developed by Edward Jenner in 1796 against smallpox
Vaccination programs have eradicated smallpox globally (last case in 1977) and reduced polio cases by 99% since 1988
The measles vaccine prevented approximately 23.2 million deaths worldwide between 2000 and 2018
COVID-19 vaccines reduced mortality risk by approximately 90% in vaccinated individuals during the pandemic peak
Herd immunity typically requires 70-90% of a population to be vaccinated depending on the disease

Overview

Vaccination represents one of humanity's most significant medical achievements, with origins dating back to 1796 when Edward Jenner developed the first successful smallpox vaccine using material from cowpox lesions. The 20th century saw rapid vaccine development, including Jonas Salk's inactivated polio vaccine in 1955 and Albert Sabin's oral polio vaccine in 1961. The World Health Organization launched the Expanded Programme on Immunization in 1974, which by 2019 was preventing 2-3 million deaths annually from diseases like diphtheria, tetanus, pertussis, and measles. The 21st century brought new vaccine technologies, including the first HPV vaccine in 2006 and mRNA vaccines for COVID-19 in 2020. Global vaccination coverage for three doses of diphtheria-tetanus-pertussis vaccine reached 86% in 2019, though disparities persist between high-income and low-income countries.

How It Works

Vaccines function by introducing a safe version of a pathogen's antigen into the body, triggering an immune response without causing illness. Traditional vaccines use weakened live viruses (like MMR), inactivated viruses (like polio), or purified components (like hepatitis B). These antigens are recognized by immune cells called B lymphocytes, which produce antibodies specific to the pathogen. Simultaneously, T lymphocytes are activated to destroy infected cells. This process creates immunological memory through memory B and T cells that remain in the body for years or decades. When the actual pathogen later invades, these memory cells rapidly recognize it and mount a swift, robust defense, often preventing infection entirely or reducing severity. mRNA vaccines represent a newer approach where synthetic messenger RNA instructs cells to produce viral proteins that trigger immune responses, as demonstrated with COVID-19 vaccines.

Why It Matters

Vaccination has profound real-world impacts, having eradicated smallpox completely and brought polio to the brink of elimination. Childhood immunization programs prevent approximately 2-3 million deaths annually worldwide from vaccine-preventable diseases. Economically, every dollar invested in immunization returns an estimated $16 in healthcare cost savings and productivity gains. During the COVID-19 pandemic, vaccines prevented approximately 14.4 million deaths globally in 2021 alone. Beyond individual protection, vaccines create herd immunity that protects vulnerable populations who cannot be vaccinated, such as immunocompromised individuals. They also reduce antibiotic resistance by preventing bacterial infections that would require treatment.