In this article
Vaccines are among the most studied medical interventions in history. Their principle is simple: prepare the immune system in advance, so it can respond quickly when the real disease appears.
Key facts
- 154 million lives saved by vaccination over 50 years, according to a 2024 study led by the World Health Organization.
- 101 million of those were infants.
- 88% fewer measles deaths between 2000 and 2024; measles vaccination alone saved nearly 59 million lives.
- About 95,000 people, mostly young children, still died of measles in 2024.
How the immune system learns
Your immune system recognizes germs by features on their surface called antigens. The first time it meets a new germ, it takes days to work out a response, which is why the first infection can make you seriously ill. Afterward, it keeps memory cells that recognize that germ and can respond much faster next time.
A vaccine gives the immune system that first lesson safely. It presents antigens without causing the disease, so the body builds memory without having to survive the illness.
Types of vaccine
| Type | What it contains | Examples |
|---|---|---|
| Live weakened | A weakened form of the germ | Measles, mumps and rubella (MMR) |
| Inactivated | A killed form of the germ | Polio (injected), hepatitis A |
| Subunit or protein | Selected pieces of the germ | Hepatitis B, HPV, whooping cough |
| Toxoid | An inactivated toxin made by the germ | Tetanus, diphtheria |
| mRNA | Instructions for your cells to make a harmless piece of the germ | Some COVID-19 vaccines |
| Viral vector | A harmless virus carrying instructions | Some Ebola and COVID-19 vaccines |
Why boosters are needed
For some diseases, protection fades over time, so a booster reminds the immune system. For others, such as influenza, the germ itself changes, so the vaccine is updated regularly. That is why you may need a flu vaccine every year but only one course of some others.
Protecting the community
When enough people are immune, a disease struggles to spread, which protects those who cannot be vaccinated, such as newborns or people with weakened immune systems. The share needed depends on how contagious the disease is. Measles is one of the most contagious diseases known, so around 95 percent of people need two doses to stop outbreaks.
What the evidence shows
A study published in The Lancet in 2024 and led by the World Health Organization estimated that vaccination against 14 diseases has saved at least 154 million lives over the past 50 years, 101 million of them infants. Measles vaccination accounted for about 60 percent of the lives saved.
But progress is fragile. In November 2025 the WHO reported that measles deaths fell by 88 percent between 2000 and 2024, yet cases are surging in many regions where vaccination rates slipped, and about 95,000 people died of measles in 2024.
Are vaccines safe?
Vaccines are tested in large clinical trials before approval and monitored continuously afterward. Most side effects are mild and short-lived, such as a sore arm or a slight fever. Serious reactions are rare, and health authorities investigate reports through dedicated safety systems.
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From trial to your arm
- Laboratory research identifies a promising vaccine design.
- Phase 1 trials test safety and immune response in a small group of volunteers.
- Phase 2 trials expand to hundreds of people to refine the dose.
- Phase 3 trials involve thousands or tens of thousands of people to confirm the vaccine prevents disease and to detect less common side effects.
- Regulatory review by national medicines agencies examines all the data before approval.
- Ongoing monitoring continues after approval, so rare problems can be spotted and investigated.
Frequently asked questions
Can a vaccine give you the disease it protects against?
Inactivated, subunit, toxoid and mRNA vaccines cannot, because they do not contain a live germ that can cause the disease. Live weakened vaccines can rarely cause a mild form of illness in people with seriously weakened immune systems, which is why doctors check before giving them.
Why are several vaccines given at once?
Giving vaccines together means fewer visits and earlier protection. The immune system handles many antigens every day, and combined schedules are tested for safety.
Health note: this article is general information. For advice about a specific vaccine or your own health, speak to a doctor, pharmacist or your national health service.
Sources
- WHO: Global immunization efforts have saved at least 154 million lives over the past 50 years
- The Lancet: Contribution of vaccination to improved survival and health, 50 years of the Expanded Programme on Immunization
- WHO: Measles deaths down 88% since 2000, but cases surge (November 2025)
- WHO: Measles fact sheet
- CDC: How vaccines work
