Vaccines and disease prevention — I'll pull the evidence for your question.
Yes, vaccines are an effective tool to prevent disease. Extensive scientific evidence and decades of public health data support this conclusion.
Effectiveness at the population level
•Vaccines remain one of the most effective public health interventions, particularly where it significantly reduces morbidity and mortality associated with infectious diseases.
•The World Health Organization (WHO) considers vaccination one of the most effective and cost-effective public health interventions, and asserts that immunizations are the most cost-effective health interventions with vaccines available for more than 20 life-threatening diseases that help to prevent and control infectious disease outbreaks. They are described as arguably the most cost-effective health intervention ever developed.
•Vaccines are safe, effective and life-saving, and it is important that people continue to trust in vaccines as one of the most effective tools to prevent diseases.
Estimates of impact include:
•According to the WHO, 2 to 3 million deaths are prevented each year thanks to vaccination. Other WHO estimates cite 3.5 to 5 million lives every year and 4–5 million deaths annually.
•A landmark 2024 WHO study modeling five decades of the Expanded Programme on Immunization found that vaccination against 14 key pathogens has saved an estimated 154 million lives worldwide since 1974. Other summaries note vaccination has already helped avert about 154 million deaths since 1974.
•Between 2000 and 2023, measles vaccination prevented an estimated 60 million deaths. The widespread use of the measles vaccine has led to a 73% decrease in measles deaths between 2000 and 2018.
•Vaccinations have wiped out smallpox, reduced global polio by 99%. Smallpox is the only human infectious disease that has been eradicated, was done so through effective vaccination. Throughout history, humans have successfully developed vaccines for a number of life-threatening diseases, including smallpox, meningitis, tetanus, measles and wild poliovirus, building on the success of smallpox eradication – certified by WHO in 1980 after global vaccination and surveillance efforts.
Vaccination not only prevents disease caused by acute infection but also protects against long-term immune-related sequelae that can follow an acute viral infection.
How vaccines prevent disease
Vaccines help the immune system recognise and fight harmful bacteria and viruses and prepare the immune system to respond to the infection more effectively and quickly when you encounter the infection in the future.
•Vaccines contain weakened or inactive parts of a particular organism (antigen) that triggers an immune response within the body. Regardless of whether the vaccine is made up of the antigen itself or the blueprint so that the body will produce the antigen, this weakened version will not cause the disease in the person receiving the vaccine, but it will prompt their immune system to respond much as it would have on its first reaction to the actual pathogen.
•Vaccines work by producing an immune response in the body without causing illness.
•They work by exposing us to harmless versions of viruses, triggering antibody production and memory cell formation for long-lasting protection. Once the body produces antibodies in its primary response to an antigen, it also creates antibody-producing memory cells, which remain alive even after the pathogen is defeated. If the body is exposed to the same pathogen more than once, the antibody response is much faster and more effective.
Community protection - Herd immunity
•Herd immunity, or community immunity, means there are enough people immunised in the community to slow or stop the spread of disease.
•When a lot of people in a community are vaccinated, the pathogen has a hard time circulating because most of the people it encounters are immune. So, the more that others are vaccinated, the less likely people who are unable to be protected by vaccines are at risk.
Efficacy and effectiveness
Effectiveness is measured in two ways:
•Vaccine efficacy is measured in controlled clinical trials. It is based on how many people who got vaccinated developed the ‘outcome of interest’ compared with how many people who got the placebo developed the same outcome. Vaccine efficacy tells us how much the vaccine lowers the risk of the outcome in a trial setting.
•Vaccine effectiveness refers to how well vaccines work in the real world. Effectiveness can differ from efficacy because we can’t predict exactly how effective vaccination will be for a much bigger and more variable population.
Typical levels:
•Most vaccines are over 85% effective.
•Vaccine effectiveness is usually 85 to 95%.
•The measles, mumps, and rubella (MMR) vaccine is the most effective tool we have for preventing measles. Two doses of the MMR vaccine are about 97% effective at preventing measles.
Limitations
Vaccines can stop most people from getting sick, but not everyone. Even after someone receives all of the recommended doses and waits a few weeks for immunity to build up, there is still a chance that they can get infected. Vaccines do not provide full (100%) protection because of many factors, including our individual differences in immunity and response to infection.
If vaccinated people do get sick, they are likely to have milder symptoms, in general it is very rare for someone vaccinated to experience severe illness or die.
For example, effectiveness for COVID-19 vaccines is not absolute prevention of infection but measurable reduction in severe outcomes— many studies emphasize strong protection, especially against hospitalization and critical illness in older adults.
In summary, while no vaccine is 100% effective for every individual, the evidence shows vaccines are highly effective at preventing disease, reducing severe complications and deaths, and providing community-wide protection through herd immunity.