Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The curves and the outbreak are simplified models to show the idea; they are not medical tools.
Tap the buttons and see what happens.
Drag the days. When do germs win, and when do your defences catch up?
Scrub through a primary infection: pathogen load vs. immune response.
Slide along. The germs grow first. Then your body catches up!
Illustrative shapes, no real units.
Tap the virus to copy its spikes onto the poster.
Tap the virus: its surface spike proteins are the antigen the immune system learns.
Tap the germ. Put its bumps on the poster!
Tap each kind of vaccine to see how it makes the poster.
Tap each platform: what's inside, examples, and trade-offs.
Deliver a message, then move time forward. Where does the mRNA go?
Deliver an mRNA, then advance time: spike made, mRNA degraded, nucleus untouched.
The fade-out time is approximate: "within a few days".
Switch memory cells on or off, then replay the infection.
Toggle immune memory and replay: compare time to control and the time above the "sick" line.
Turn memory on and off. Which one stops the germ fast?
Set how many people are vaccinated, then tap someone to start an outbreak.
Set vaccine coverage, then seed an outbreak. Watch where transmission chains break.
Give more people their vaccine. Then tap someone. Does the germ get stuck?
A toy model: real spread depends on the disease and how people mix.
Tiny germs are all around us. Most of them never make us sick. But when a brand-new germ gets in, your body has to learn how to fight it. That takes time, and you might feel sick while it learns.
Your body knows germs by their shape. A vaccine shows your body what a germ looks like, like a wanted poster. It is not the sick-making germ, so your body can learn safely.
Your body's helper cells make tiny grabbers, called antibodies, that fit the germ. Your arm might feel a bit sore for a day or two. That just means your body is busy learning.
A few helper cells stay on guard for a long time. If the real germ ever comes, they know it right away and fight it fast.
Germs jump from person to person. When lots of people have their vaccines, the germ gets stuck. That helps keep tiny babies safe, because they are too little for some vaccines.
More body explainers: rabies, antibiotics and allergies.
Note: This page explains how vaccines work in general; it isn't medical advice. Talk to your doctor about which vaccines are right for you or your child.
A vaccine is a safe preview of a germ. It shows your immune system what a germ looks like, without the danger of a real infection.1 Your body reacts as if it had met the germ: it makes antibodies and trains special cells. Afterwards, a few memory B cells and T cells stay on watch.2 If the real germ ever gets in, they recognize it and launch a fast, strong defence, often before you feel sick at all.4
The CDC puts it in one line: vaccines work by imitating an infection to engage the body's natural defences.1 Getting protection from a vaccine is safer than getting it by being sick.7
You meet germs every day in the air you breathe, the food you eat and the things you touch. Most of them never make you sick.5 But the first time your immune system meets a brand-new germ, it has to work out how to fight it from scratch. This first reaction is called the primary immune response.4
While your defences are still getting organised, a virus keeps copying itself.5 That head start is when you can get sick. A vaccine lets your immune system have that first meeting in a safe, controlled way, so the head start never happens.5
Your immune system recognises germs by their shape, especially the molecules on their surface. Anything that makes your immune system start producing antibodies is called an antigen. The antigen is the active ingredient in every vaccine.1
Think of a vaccine as a wanted poster. It shows your immune cells the germ's "face", such as the spike proteins that stick out of a coronavirus, so they know exactly what to look for later.26
Here's what happens after the shot, step by step:
first meeting = slow vs with memory = fast
You might feel tired, have a sore arm or even a mild fever for a day or two after a shot. That's the immune system doing its training, and the protection that follows can last a lifetime.12
Every vaccine teaches the same lesson, but scientists have several ways to deliver it.37
| Type | What's inside | Examples | Good to know |
|---|---|---|---|
| Live, weakened (live-attenuated) | A living germ weakened in a lab | MMR (measles, mumps, rubella), chickenpox | Strong, long-lasting protection, often after 1–2 doses; not for people with a severely weakened immune system13 |
| Inactivated | A germ killed with chemicals, heat or radiation | Hepatitis A | Can't copy itself or cause disease; usually needs several doses and boosters38 |
| Subunit, recombinant, toxoid | Only a piece: a protein, sugar coat or a disarmed toxin | Hepatitis B, HPV, whooping cough, tetanus | Often includes an adjuvant to boost the response3 |
| mRNA | Instructions to make one harmless protein piece | COVID-19 | Contains no germ at all26 |
The hepatitis B vaccine was the first recombinant protein vaccine. The viral protein is made by yeast cells that were given the gene for it.3
In the United States there are currently two types of COVID-19 vaccine: mRNA and protein subunit. Neither contains live virus, so neither can give you COVID-19.2
mRNA (messenger RNA) is a natural molecule your cells use all the time. It's a short-lived copy of the recipe for one protein.6 An mRNA vaccine works like this:
Protein subunit COVID-19 vaccines skip the recipe and deliver the spike protein pieces directly, together with an adjuvant that helps the immune system respond.2
No. Your DNA is stored inside the cell's nucleus, and vaccine mRNA never goes into the nucleus. It can't change or interact with your genes.213 mRNA is a fragile molecule that the body breaks down quickly, within a few days of the shot.1617
Not really. Scientists had studied mRNA vaccines for decades before COVID-19, for diseases including flu, Zika, rabies and cytomegalovirus (CMV).26
New vaccines start in the lab. Then they go through clinical trials in people, in phases that get bigger each time:12
| Phase | Who | What researchers check |
|---|---|---|
| Phase 1 | Small groups (20 to 100 people) | Safety, side effects, whether it causes an immune response |
| Phase 2 | Larger groups | More safety data and how well it triggers immunity |
| Phase 3 | Thousands (1,000 to 3,000) | How well it works, common and less common side effects |
| Phase 4 | After approval | Long-term safety and effectiveness |
During Phase 3, the FDA also reviews how the vaccine will be made and inspects the factory.12
One dose of a vaccine often gives only partial protection. Live-weakened vaccines usually reach full protection after two doses. Non-live vaccines typically need at least three, and because their protection fades over time, booster doses top it up. For example, adults need a tetanus booster every 10 years.1 Some viruses keep changing, so the flu vaccine is updated every year.1
Protection is never perfect. Vaccinated people can still sometimes get infected, but they're far less likely to become seriously ill or die than someone whose immune system is unprepared.1
Germs spread from person to person. Every immune person makes it much harder for the germ to spread. When enough people in a community are immune, the germ struggles to find new people to infect, and outbreaks usually fizzle out. This is called community immunity or herd immunity.715
It matters most for people who can't be protected by a vaccine themselves: newborn babies who are too young for some vaccines, people with weakened immune systems, and people who are allergic to a vaccine ingredient.7
How many people need to be immune depends on how contagious the disease is.15 Measles is one of the most contagious diseases known. Up to 9 out of 10 unprotected people who are close to someone with measles will catch it.10 So almost everyone in a community needs to be immune to stop measles spreading.5 After the measles vaccine arrived in the US in 1963, measles cases dropped by more than 95%.9
The very first vaccine was made in 1796, when British doctor Edward Jenner used cowpox to protect people against smallpox.14 Almost two centuries later, a worldwide vaccination campaign left smallpox with nowhere to go. The last natural case was in 1977, and on May 8, 1980, the world was declared free of smallpox.11 It's one of only two diseases humans have ever wiped out. The other, rinderpest, infected cattle.4
A vaccine shows your immune system a harmless version or piece of a germ, or instructions to make one piece. Your body makes antibodies and memory cells against it without you getting sick. If the real germ shows up later, your immune system recognizes it and fights back much faster.
In the US there are mRNA and protein subunit COVID-19 vaccines. mRNA vaccines give muscle cells in your arm instructions to make a harmless piece of the virus's spike protein. Protein vaccines deliver that spike piece directly. Either way, your immune system learns to recognize the spike, and neither type contains live virus.
No. The mRNA never enters the nucleus, where your DNA is kept, so it can't change your genes. Your cells read it, use it to make the spike piece, and then break it down within a few days.
Inactivated, subunit and mRNA vaccines contain no living germ, so they cannot cause the infection. A sore arm, tiredness or a mild fever for a day or two is a normal sign that your immune system is building protection.
One dose gives only partial protection for many vaccines. Non-live vaccines usually need three or more doses, and their protection fades, so boosters top it up. Some viruses, like flu, change over time, so the vaccine is updated each year.
When enough people in a community are immune, a germ has trouble finding new people to infect, so outbreaks stay small or die out. This protects people who can't be vaccinated, such as newborn babies and people with weak immune systems. Very contagious diseases like measles need almost everyone to be immune.
British doctor Edward Jenner made the first vaccine in 1796, using cowpox to protect people against smallpox. Vaccination later wiped smallpox out: the world was declared free of it in 1980.