A vaccine shows your immune system a harmless preview of a pathogen, so that it builds a memory of that pathogen before ever meeting the real one. The training is genuine. Only the threat is fake.
Your immune system already does this on its own. Catch a disease, survive it, and you are usually protected afterwards because your body kept a record. A vaccine gets you the record without the illness.
A brief tour of the immune system
Two layers, and the second is the one that matters here.
The innate layer is fast and generic. It reacts to anything that looks foreign, has no memory, and behaves identically the second time. Inflammation, fever and the first line of white blood cells all belong to it.
The adaptive layer is slow, specific, and remembers. It works from antigens, distinctive molecular shapes on the surface of a pathogen. When it encounters an unfamiliar antigen it takes days to spin up a response, and while it does, you are ill.
Two products come out of that response. B cells make antibodies, proteins that lock onto that exact antigen and mark it for destruction. T cells find and kill cells already infected. Afterwards, a population of memory cells stays behind, primed for that specific shape.
The delay is the whole vulnerability. A vaccine removes it by arranging for the memory cells to exist already.
The main kinds
| Type | What it contains | Examples |
|---|---|---|
| Live attenuated | A weakened live pathogen | Measles, mumps, rubella |
| Inactivated | A killed pathogen | Some flu and polio vaccines |
| Subunit | Just one protein from it | Hepatitis B, HPV |
| mRNA | Instructions for making one protein | Some COVID-19 vaccines |
The mRNA approach is the one people find strangest, and it is conceptually the simplest. Rather than delivering a protein, it delivers a short-lived instruction telling some of your cells to make that protein themselves. Your immune system sees the protein, responds to it, and remembers. The instruction is degraded within days. It does not enter the cell nucleus and does not alter your DNA, which are separate compartments and separate chemistry.
What efficacy percentages actually mean
This is the most misread number in public health.
"95 percent efficacy" does not mean 95 out of 100 vaccinated people are protected and five are not. It is a comparison between two groups in a trial. It means the vaccinated group had 95 percent fewer cases than the unvaccinated group.
If 100 people in the unvaccinated group fell ill and 5 in the vaccinated group did, that is 95 percent efficacy, regardless of how many thousands were in each group. It describes reduction in risk, not a share of people.
It also matters what was being counted. A vaccine can be modest at preventing any infection at all and excellent at preventing hospitalisation, and those are different figures from the same trial. Reading a protection percentage without asking "protection against what" is the same error as reading an SPF number without asking what it filters.
Efficacy is a comparison between two groups, not a headcount. And it always answers a specific question about a specific outcome.
Why side effects happen
A sore arm, tiredness, a mild fever for a day. These are not the vaccine harming you. They are your innate immune system doing exactly what it does when it meets something foreign: local inflammation, then a systemic response.
Feeling rough afterwards is a sign the process started. Feeling nothing is not a sign it failed; immune responses vary enormously between people and with age.
Serious adverse reactions exist and are rare, which is why surveillance systems track them after approval rather than only during trials. It is also why every trial includes a placebo group. Some of the reported side effects in any trial appear in the group that received nothing, and separating real effects from expectation requires exactly that comparison.
Why boosters
Immune memory fades at different rates for different diseases. Measles immunity is close to lifelong. Tetanus needs topping up every decade or so. Flu needs an annual shot for a different reason: the virus mutates fast enough that last year's memory no longer matches this year's shape.
Herd immunity, briefly
When enough of a population is immune, a pathogen struggles to find a chain of susceptible hosts and outbreaks stop propagating. The threshold depends on how contagious the disease is: around 60 percent for some, above 90 percent for measles, which is exceptionally transmissible.
The point of the threshold is the people who cannot be vaccinated: newborns, the immunosuppressed, those with specific allergies. Their protection is made of everyone else's.
The short answer
A vaccine is a rehearsal. It presents the immune system with the recognisable shape of a pathogen, without the pathogen's ability to make you ill, so that the response which would have taken days to assemble is ready in advance.
This is general information about how vaccines work, not medical advice. Questions about your own vaccination belong with a doctor.