How Vaccines Work: The Brilliant Biological Trick That Has Saved More Lives Than Any Medicine
From Edward Jenner's cowpox experiment to mRNA technology, vaccines exploit the immune system's most powerful feature: its memory. Here is exactly how.
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The milkmaid's observation that changed medicine
In 1796, an English country doctor named Edward Jenner noticed something curious about milkmaids: they rarely contracted smallpox, the disease that was then disfiguring and killing roughly 10% of Europeans. Milkmaids did, however, frequently catch cowpox — a much milder illness acquired from infected cattle. Jenner hypothesised that cowpox infection was somehow protecting them against smallpox.
To test his idea, he took material from a cowpox lesion on the hand of a milkmaid named Sarah Nelms and inoculated it into eight-year-old James Phipps. The boy developed mild cowpox and recovered. Six weeks later, Jenner exposed him to smallpox. Nothing happened. The boy was immune.
Jenner had no idea why this worked. He did not know about viruses, antibodies, T cells, or B cells — none of this language existed. He simply observed, tested, and published. The word he coined for his procedure — from vacca, Latin for cow — gave us vaccination. Two hundred years later, smallpox became the first human disease ever eradicated from the planet.
To understand why Jenner's intuition was so profound, you need to understand what the immune system is actually doing.
Two immune systems in one body
Your immune system is not a single thing but two overlapping systems with fundamentally different strategies.
The innate immune system is your first responder. It is ancient, fast, and non-specific. When a pathogen enters your body, the innate system recognises broad molecular patterns that say "this is foreign, this is microbial" — patterns that bacteria and viruses share and that your own cells do not have. Within minutes to hours, it floods the infection site with inflammation, heat, and generalised killing power. This is why infections cause fever and redness: these are not the disease but the body fighting it.
The innate system buys time, but it cannot eliminate most pathogens alone. It also triggers the second system.
The adaptive immune system is slower, more targeted, and has one superpower that makes vaccines possible: memory. It is built around two types of white blood cells — B cells and T cells — that learn to recognise specific molecular shapes on pathogens, called antigens. The first time your adaptive system encounters a novel pathogen, it takes days to two weeks to mount a full response. That delay is why you get sick. But after clearing the infection, your body retains long-lived memory cells — both B memory cells and T memory cells — primed to respond immediately to the same antigen if it appears again.
This is the immune system's elegant trick: it learns. A second encounter with the same pathogen triggers an immune response so fast and powerful that you often never feel ill at all.
Vaccination is simply a way of triggering that learning process without the danger of actual disease.
How different vaccines create the same result
Vaccines use several different strategies to expose your immune system to antigens, each with trade-offs.
Live attenuated vaccines use a weakened version of the pathogen — alive but unable to cause serious disease in a healthy immune system. The measles, mumps, and rubella (MMR) vaccine is the most well-known example. Because the pathogen replicates (slowly) inside the body, these vaccines generate a strong, durable immune response that often provides lifetime protection after one or two doses. The trade-off is that live vaccines cannot be given to people with severely compromised immune systems, and they require careful cold-chain storage.
Inactivated vaccines — like the traditional flu shot, polio (IPV), and hepatitis A — use a pathogen that has been killed, usually with heat or chemicals. The antigens remain intact but the pathogen cannot replicate. These tend to require booster doses and often adjuvants (more on these shortly) because the immune stimulus is weaker than a live vaccine. They are safer for immunocompromised individuals.
Subunit, recombinant, and protein vaccines — including the hepatitis B vaccine and some COVID-19 vaccines — skip the pathogen entirely. They deliver only specific proteins from the pathogen's surface, enough to train the immune system without any risk of infection. The HPV vaccine Gardasil works this way, using virus-like particles — hollow protein shells that look like the virus but contain no genetic material.
mRNA vaccines — developed for COVID-19 by Moderna and Pfizer-BioNTech — represent the newest platform and deserve a careful explanation, because they are the most misunderstood.
The mRNA vaccine: what it actually does
An mRNA vaccine delivers a short strand of messenger RNA — essentially a temporary molecular instruction — wrapped in a lipid nanoparticle (a tiny fat bubble) that helps it enter cells. Once inside a cell, the mRNA is read by ribosomes, the cellular machinery that builds proteins. The ribosomes construct a specific protein — in the case of COVID-19 vaccines, the spike protein on the surface of the coronavirus. Your immune system sees this foreign protein, recognises it as non-self, mounts an immune response, and builds memory.
The mRNA itself degrades within a few days. It is chemically unstable by design — this is why mRNA vaccines require ultra-cold storage.
Can it change your DNA? No. Here is precisely why not: DNA lives in the cell nucleus, surrounded by a protective membrane. mRNA never enters the nucleus. The flow of biological information in cells runs in one direction — DNA is transcribed into mRNA, which is translated into protein. There is no mechanism by which mRNA becomes DNA without a specific enzyme called reverse transcriptase, which human cells do not produce. The mRNA from a vaccine cannot insert itself into your genome any more than a recipe card can rewrite a cookbook.
What adjuvants do
An adjuvant is an immune-stimulating substance added to many inactivated vaccines. Think of it as a signal that says to the immune system: this is important, pay attention. Without adjuvants, inactivated vaccines often fail to generate a strong enough response to create durable immunity.
Common adjuvants include aluminium salts (used since the 1920s and extensively studied for safety), oil-in-water emulsions like AS03 used in some flu vaccines, and the more modern toll-like receptor agonists that specifically activate the innate immune pathways that, in turn, amplify the adaptive response. The specific pattern of immune activation matters enormously — different adjuvants shape not just the strength of the response but its character, influencing whether antibodies or T cells dominate.
The mathematics of herd immunity
No vaccine is 100% effective, and not everyone can receive every vaccine — newborns, the immunocompromised, and people with certain allergies are among those who depend on others being vaccinated.
Herd immunity describes the indirect protection that occurs when a sufficient proportion of a population is immune, making it difficult for a pathogen to find susceptible hosts and spread. The threshold proportion needed depends on how contagious the pathogen is — specifically, its basic reproduction number, R0, which describes how many people an infected person would infect in a fully susceptible population.
The formula is straightforward: herd immunity threshold = 1 − (1/R0). For measles, R0 is approximately 12–18, which means around 92–95% of the population needs to be immune. For COVID-19's original strain, R0 was roughly 2–3, meaning herd immunity required 50–67% immunity. The Delta variant, with an R0 of 5–9, pushed that threshold to around 80–90%.
This is why high vaccination rates matter even for people who are vaccinated: the more immunity exists in a population, the less a virus circulates, which protects everyone, including those for whom vaccines provide partial protection.
Why flu vaccines are only 40–60% effective — and COVID vaccines were higher
The influenza virus is a serial shapeshifter. It mutates rapidly and exists in a vast animal reservoir (birds, pigs) from which new variants continuously emerge. Each year, the WHO analyses circulating strains and makes a prediction about which three or four variants will dominate the coming flu season. Vaccine manufacturers then design that year's vaccine accordingly. When the prediction is accurate, efficacy is higher. When a new variant emerges after production has begun, efficacy drops.
COVID-19 vaccines achieved dramatically higher initial efficacy (95% for Pfizer-BioNTech in clinical trials) in part because the spike protein was a relatively stable, well-defined target in the original strain — and because mRNA technology allowed extremely precise engineering of the immune response. As variants evolved, efficacy against infection dropped, but efficacy against severe disease and death remained high.
Polio: a near-eradication story
In 1988, wild poliovirus was endemic in 125 countries, paralyzing hundreds of thousands of children a year. By 2023, it remained endemic in only two: Pakistan and Afghanistan. Oral polio vaccine — a live attenuated strain developed by Albert Sabin — proved so cheap, stable, and easy to administer (two drops on the tongue) that it became the backbone of one of the largest public health campaigns in history.
The near-eradication of polio is a demonstration of both the power of vaccines and the fragility of elimination campaigns. The virus persists precisely where vaccination campaigns face the greatest operational obstacles — conflict zones, areas of vaccine hesitancy, and regions where cold-chain infrastructure is unreliable. Eradication requires reaching virtually everyone.
The bottom line
Vaccines work by deceiving your immune system in the most constructive way possible: they show it a threat without delivering one, so that your adaptive immunity can learn, build memory cells, and respond with devastating speed if the real pathogen ever arrives. From Jenner's cowpox experiment to mRNA technology, the underlying principle has never changed. What has changed is the precision, speed, and safety with which we can now deliver that lesson to your immune system — and the mathematical clarity with which we can see that when enough people receive it, entire diseases can be driven to extinction.