- Updated
What are vaccines and how do they help protect us?
Learn how vaccines work, the types available, and why they’re important for personal and public health.
4 min read
Medically reviewed by
Dr. David Weber · April 13, 2025
Key takeaways
Knowing the science behind vaccines can make it easier to understand how they work and why they are important for personal and public health.
What are vaccines?
Vaccines help protect against serious illnesses by teaching your body’s immune system how to recognize and prevent serious infections. They are an important tool in modern medicine.
Whether you're curious about how immunity works, or just looking to stay healthy, understanding vaccines is a smart step toward helping to protect yourself, your loved ones, and your community.
A brief history of vaccines
The first vaccine dates back to 1796, when Edward Jenner used material from cowpox sores to protect people against smallpox. This early discovery is the basis for modern immunology, which is the study of how the body’s immune system defends against infections. Since then, vaccines have helped fight the spread of diseases like polio, measles, and influenza.
According to the World Health Organization, it’s estimated that vaccines prevent 3.5–5 million deaths each year worldwide. Smallpox, once a deadly threat, was wiped out by 1979 thanks to widespread vaccination. Polio and measles deaths have dropped significantly in countries with high vaccine coverage.
How vaccines help prevent disease
Your immune system is your body’s defense system. When a germ—an invader such as a virus or bacteria that can make you sick—enters your body, your immune system responds by producing antibodies, that recognize and prevent serious infection.
Vaccines mimic this process. They introduce a type of germ that is either dead, weakened, or changed, so it can help prevent diseases, prompting your immune system to create antibodies producing cells. These cells “remember” the germ, so if you’re exposed in the future, your body can respond quickly and effectively—often before you even feel sick.
Think of it as a practice drill for your immune system. Once trained, your body can respond faster and more effectively to real threats.
What is community protection?
When a large portion of a population becomes immune to a disease—either through vaccination or previous infection—the germ has a much harder time spreading. This effect, where a natural protective barrier occurs, is called community protection, and it helps shield people who can’t be vaccinated, such as newborns, persons with allergies to vaccine, or those with weakened immune systems. When enough people are vaccinated, outbreaks slow down or may even stop altogether.
Before vaccines made it easier and safer to help build community protection against chickenpox, some parents held “pox parties” to purposefully expose their children at a young age to chickenpox. The idea was that it was better to get the disease early—when it was thought to be milder—than risk a more severe case later on in life. However, this approach carried serious risks, including problems like blood clots, infection of the brain, or even death. As vaccines became available, they offered a safer way to help build immunity without some of the dangers of natural infection.
Today, health organizations like the Centers for Disease Control and Prevention and World Health Organization recommend vaccination to help limit the spread of disease, provide community protection, and reduce disease severity. This is how diseases like measles and polio have been nearly stamped out in many parts of the world.
3.5-5
million lives saved
According to the World Health Organization, it’s estimated that vaccines prevent 3.5–5 million deaths each year globally.
Types of vaccines and how they work
There are several types of vaccines, each using a different method to help train your immune system:
- Live-attenuated (or weakened) vaccines: These use a weakened form of the germ (virus or bacteria) that has been altered so it can still produce protection against future infection, but without causing illness. Think of it like a fire drill; the weakened virus or bacteria sets off the alarm and gets your body’s emergency response team moving. This approach often leads to long-lasting immunity. However, these vaccines can only be used in healthy non-pregnant people.
Examples: MMR (measles, mumps, rubella), varicella (chickenpox) - Inactivated vaccines: These contain germs (viruses or bacteria) that have been killed so they can’t cause disease but still lead to immunity.
Examples: Hepatitis A, rabies, flu - Subunit and protein-based vaccines: These use specific parts of the germ (virus or bacteria) to produce an immune response.
Examples: Whooping cough, Hepatitis B - mRNA vaccines: These use a piece of genetic material from the germ that teaches your cells to make a piece of the germ and thus provide protection against infection or serious disease. Since only a piece of the germ is used it cannot cause infection.
Examples: some COVID-19 vaccines - Toxoid vaccines: Some germs make “toxins” that produce illness. Toxoid vaccines use an altered form of the toxin that has been made safe but still produce immunity.
Examples: Tetanus - Viral vector vaccines: These use a different, harmless virus (not the one that makes you sick) as a delivery vehicle—or vessel—to carry genetic instructions into the cell. This teaches the body how to fight off the target virus. Your body then produces a protein that helps build immunity.
While most vaccines are administered via injection into a muscle (a shot), some can be delivered through other routes—such as inhaled through the nose (e.g., nasal influenza vaccine) or ingested orally (e.g., typhoid fever vaccine).
Why do some vaccines require multiple doses?
Some vaccines require multiple shots to build immunity. The entire series must be administered to create immunity.
In addition, some vaccines need additional shots over time to maintain immunity; these additional shots are sometimes called "booster doses", help “remind” the immune system and maintain strong protection over time.
Why some vaccines require multiple doses?
Multiple doses are needed for several reasons:
- Initial series to build immunity: Some vaccines, such as those for hepatitis B or MMR (measles, mumps, and rubella), require multiple doses as part of the initial series. These doses work together to build up immunity. Skipping any dose in the series may result in incomplete protection.
- Decreasing immunity: Some vaccines, like those for tetanus or pertussis (whooping cough), provide strong protection at first, but that protection can decrease over time. An additional recommended dose helps reinforce the immune system’s memory.
- Changing viruses: Some viruses, like the flu, can change a lot in a short amount of time. Boosters are updated to match the latest versions of these viruses going around, so your body knows what to look for and is ready to fight.
- Improved vaccines: As new research is done, our understanding of how the body fights illnesses grows. This can lead to new models of vaccines that may offer wider or longer-lasting protection.
Some vaccines don’t need booster doses
Not all vaccines need booster doses. Why? That comes down to how your immune system responds, how tricky (or not) the disease is, and how likely you are to be exposed to it.
Take the MMR (measles, mumps, rubella) or varicella (chickenpox) vaccines, for example. These are live-attenuated vaccines and closely mimic a natural infection. Once the vaccination series is complete, there is a strong, long-lasting immune response that often provides protection for life.
Another reason some vaccines don’t need multiple doses is because the germs (viruses and bacteria) they help protect against don't change much over time, so the immunity you get from the vaccine stays effective over time.
Finally, in communities with high vaccination levels, community protection helps reduce the chance of infection. That means even if your personal immunity fades a bit over time, the chances of getting a particular infection are still fairly low.
Wrap up: Why vaccines matter
Vaccines help protect vaccinated people, reduce the spread of disease, and save millions of lives each year. Understanding how they work helps build trust in their safety and effectiveness.
If you have questions about vaccines, talk to your healthcare provider. Staying informed is one of the first steps to staying healthy—and to help protect those around you.
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Frequently
asked questions
How long does vaccine immunity typically last?
It varies. Some vaccines, like measles and hepatitis B, offer lifelong protection. Others, like the flu shot, need to be updated each year.
Can vaccines cause the disease they're meant to help prevent?
In most cases, vaccines do not cause the disease they’re meant to prevent because they use fragmented or inactivated forms of the germ. In extremely rare cases, some vaccines can turn back into a form of the germ that can cause disease, so they are typically not given to those with weakened immune systems (e.g., patients with cancer receiving therapy).
Why do some people experience side effects after vaccination?
Side effects are sometimes signs that your immune system is responding. Common side effects like soreness at the site of the shot, tiredness, or low fevers are normal. Serious side effects are rare.
Are natural immunity and vaccine immunity different?
Both provide protection, but vaccine immunity is different because it doesn’t require getting sick first. Natural infection can lead to severe illness or death.
How is the timing of vaccine dosing chosen?
Scientists study how the immune system responds at different ages, and how long protection lasts after getting a vaccine. Timing of shots are based on scientific studies and are regularly reviewed by public health officials.
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References
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Barron, M. Why do some vaccines work better than others? American Society for Microbiology. March 11, 2024. https://asm.org/articles/2024/march/why-some-vaccines-work-better-than-others
Desai AN, Majumder MS. What is herd immunity? JAMA. 2020;324(20):2113.
Different types of vaccines. History of Vaccines. The College of Physicians of Philadelphia. Accessed June 25, 2025. https://historyofvaccines.org/vaccines-101/what-do-vaccines-do/different-types-vaccines
Explaining how vaccines work. CDC Vaccines & Immunizations. August 10, 2024.
Accessed June 25, 2025. https://www.cdc.gov/vaccines/basics/explaining-how-
vaccines-work.html
Ginglen JG, Doyle MQ. Immunization. In: StatPearls. StatPearls Publishing; 2025.
Global immunization efforts have saved at least 154 million lives over the past 50 years. The World Health Organization. April 24, 2024. https://www.who.int/news/item/24-04-2024-global-immunization-efforts-have-saved-at-least-154-million-lives-over-the-past-50-years
Vaccines and immunization. The World Health Organization. 2025. Accessed June 25, 2025. https://www.who.int/health-topics/vaccines-and-immunization
Vaccines and immunization: what is vaccination? The World Health Organization. April 23, 2024. Accessed July 18, 2025. https://www.who.int/health-topics/vaccines-and-immunization
Why do vaccines require boosters? UCLA Health. March 14, 2022. Accessed June 25, 2025. https://www.uclahealth.org/news/article/why-do-vaccines-require-boosters