Vaccines prevent disease — I'll pull the evidence for how well they work.
Yes, vaccines are one of the most effective tools to prevent disease, according to major health authorities and decades of evidence.
Evidence of Effectiveness
•The World Health Organization (WHO) states: "Vaccination is one of the most effective ways to prevent diseases. Over the past 50 years, essential vaccines against just 14 diseases have saved at least 154 million lives"
•WHO estimates: "There are now vaccines capable of preventing more than 30 life-threatening diseases, and immunization prevents between 3.5 million and 5 million deaths each year from diseases such as diphtheria, tetanus, pertussis, influenza, and measles." Another WHO statement: "Immunization currently prevents between 3.5 million and 5 million deaths every year from diseases like diphtheria, tetanus, pertussis, influenza, and measles"
•The U.S. Centers for Disease Control and Prevention (CDC) reports: "Vaccines are available for these potentially dangerous or deadly diseases. Over the years, these vaccines have prevented countless cases of disease and saved lives. Research shows that routine vaccinations for children born during 1994–2023 will have prevented about 508 million illnesses and 32 million hospitalizations and saved over 1.1 million lives"
•Vaccines are described as "among the most cost-effective ways to protect against infectious diseases."
•Historical impact: "The introduction of a vaccine has, in many cases, led to a very large decrease in the incidence of the targeted disease — such as the 96% decrease in the incidence of polio that occurred within 7 years of the introduction of the Salk and Sabin polio vaccines"
•Eradication and control: "Vaccines have led to the eradication or control of fatal diseases globally, e.g., smallpox." and "Vaccines remain the most consistently effective intervention against infectious diseases such as smallpox, rabies, polio, and various childhood illnesses"
How They Prevent Disease
Vaccines work by introducing a harmless piece or imitation of a germ, prompting the immune system to develop defenses (antibodies) without causing illness, and preparing the body to fend off real infections in the future, often for years or even a lifetime.
They also reduce community spread:
•"Vaccination will prevent illness in an individual by providing artificial immunity... vaccines reduce the likelihood that an infected individual will spread the pathogen they have been vaccinated against to others... This is the principle behind the idea of herd immunity"
•"When enough people get vaccinated, it creates herd immunity, protecting individuals who can’t be vaccinated (like infants or those with weak immune systems)."
How Effective Are Specific Vaccines?
Effectiveness varies by vaccine and disease, but is high for most:
•Varicella (chickenpox) vaccine: "overall varicella vaccine effectiveness is 82% after 1 dose and 92 to 98% after 2 doses; effectiveness against severe varicella is 100% after even 1 dose"
•Flu vaccinations "reduce the risk of flu illness by 40%–60%."
•mRNA vaccines are "safe and highly effective at preventing infectious diseases like COVID-19"
For COVID-19, effectiveness is described as "not absolute prevention of infection but measurable reduction in severe outcomes—many studies and public-health analyses emphasize strong protection, especially against hospitalization and critical illness in older adults"
And vaccines have shown to be "somewhat effective in preventing transmission and very effective in preventing severe disease and mortality."
Safety and Limitations
No vaccine is 100% effective and protection can wane over time or against new variants, but serious side effects are rare:
•"Vaccines are very safe. Like any medicine, vaccines can cause side effects. However, these are usually very minor and of short duration, such as a sore arm or a mild fever. More serious side effects are possible but extremely rare."
•"A person is far more likely to be seriously harmed by a disease than by a vaccine." Vaccine-preventable diseases "can be deadly or cause severe illness and disability" including "blindness, deafness, paralysis, brain damage, infertility, cancer, birth defects and death."