How vaccines build immune memory, the different vaccine types, herd immunity, efficacy, safety and how to judge claims.
Vaccines are one of the most effective public-health tools ever developed. They work by letting the immune system practise against a harmless version of a germ so that it can respond fast if the real one arrives. This article explains how that practice works, the different kinds of vaccine, what terms such as herd immunity and efficacy really mean, and how to judge claims about vaccines.
Written for students up to O-Level (grades 8 to 10), and useful for parents and teachers. This is general education, not medical advice. For decisions about your own or your child's vaccinations, talk to a doctor or nurse and follow your national immunisation schedule.
A vaccine is a medical product that exposes the immune system to an antigen (or the instructions for making one) from a pathogen, without causing the full disease. Vaccination is giving the vaccine; immunisation is the resulting protection. The idea is to build memory cells before you meet the real germ.
The word comes from vacca, Latin for cow: in 1796 Edward Jenner showed that deliberate exposure to cowpox could protect against smallpox. Smallpox was declared eradicated worldwide in 1980, the only human disease so far to be eliminated this way.
When an antigen meets matching B and T cells, they multiply and some become memory cells. On a second meeting, the response is faster, larger and longer lasting (the secondary response; see the immune system article for the graph). A vaccine is a safe way to cause that first meeting.
| Type | What it contains | Examples (general) |
|---|---|---|
| Live attenuated | A living but weakened form of the germ | Measles, mumps and rubella (MMR); BCG for tuberculosis; oral polio vaccine in some countries |
| Inactivated (killed) | Germs that have been killed | Injected polio vaccine; some influenza and hepatitis A vaccines |
| Subunit or conjugate | Only selected pieces of the germ, such as a protein or sugar | Hepatitis B; whooping cough component; some pneumococcal vaccines |
| Toxoid | A harmless, treated form of a toxin made by the germ | Tetanus and diphtheria |
| mRNA | Instructions that let body cells briefly make one antigen | Some COVID-19 vaccines |
| Viral vector | A harmless carrier virus delivering a gene for an antigen | Some COVID-19 and Ebola vaccines |
Different methods suit different germs. Live vaccines usually produce strong, long-lasting protection but are not suitable for everyone, for example people with seriously weakened immune systems. Vaccines made from pieces or inactivated germs cannot cause the disease they protect against, but may need extra doses or an adjuvant, a substance that helps the immune system respond.
A single exposure may not build enough memory. A second or third dose acts like the secondary response, increasing antibody levels and the number of memory cells. Some protection also fades over time, so booster doses are recommended for certain diseases, such as tetanus. Some germs, such as influenza viruses, change from year to year, so the vaccine is updated regularly.
Vaccines protect the person vaccinated, and they can also protect others. If enough people in a community are immune, the germ struggles to find new people to infect, so outbreaks die out. This indirect protection is called herd immunity (or community immunity). It matters for babies who are too young to be vaccinated and for people who cannot be vaccinated for medical reasons.
How many people must be immune depends on how contagious the germ is. This is measured by R0 (the basic reproduction number): the average number of people one case would infect in a population with no immunity. A simple estimate of the share that needs to be immune is:
threshold = 1 − 1 ÷ R0
A germ has R0 = 4 (a practice value). What share of the population must be immune to stop spread?
Check the logic: if each case infects 4 people, then 3 of every 4 contacts must be immune so that each case infects only 1 person on average, and the outbreak stops growing. Real germs have different R0 values in different places, and vaccines are not 100% effective, so real targets are set by health agencies.
Efficacy is how well a vaccine works in a controlled clinical trial. Effectiveness is how well it works in everyday use across a whole population. They can differ because real life includes older people, people with other illnesses, missed doses and new variants.
Vaccine protection is rarely all-or-nothing. A vaccinated person may still catch the disease, but is often much less likely to become seriously ill. 'Vaccine efficacy 90%' does not mean 10% of vaccinated people will get the disease; it means the vaccinated group had 90% fewer cases than the unvaccinated group in the trial.
Practice values: in a trial, 1000 unvaccinated people had 50 cases, and 1000 vaccinated people had 5 cases.
So the vaccinated group had 90% fewer cases, even though some vaccinated people still became ill.
New vaccines go through laboratory testing and several phases of clinical trials with increasing numbers of volunteers before approval. After approval, safety monitoring continues: health agencies collect reports of health events after vaccination and investigate whether a vaccine caused them, because some events happen by coincidence. Like all medicines, vaccines can cause side effects. Common ones are usually mild and short-lived, such as a sore arm or a mild fever. Serious reactions are rare, and a vaccine is recommended when its benefits clearly outweigh its risks. Anyone with a history of a severe allergic reaction should tell the vaccinator.
| Claim | What the evidence supports |
|---|---|
| Vaccines give you the disease | Inactivated, subunit, toxoid, mRNA and vector vaccines cannot cause the disease. Live attenuated vaccines can occasionally cause a very mild form. |
| Natural infection is always better | Infection can cause serious illness or death and complications that the vaccine does not. Immunity from some infections is also short-lived. |
| If vaccinated people still get ill, vaccines don't work | Protection is rarely complete; vaccinated people are usually much less likely to become seriously ill. |
| Too many vaccines overload the immune system | The immune system handles a huge number of antigens every day; vaccine antigens are a small fraction of these. |
| Diseases were already disappearing, so vaccines were unnecessary | Cases of several diseases fell sharply after the introduction of vaccines in many countries, and outbreaks return where coverage falls. |
Vaccination programmes have helped eliminate smallpox, have brought polio close to eradication, and have greatly reduced diseases such as measles, diphtheria and tetanus in many countries. Outbreaks of measles can return when too few people are immune, because measles is highly contagious and needs a very high immunity level to stop spread.
| Term | Meaning |
|---|---|
| Antigen | A molecule the immune system recognises |
| Memory cell | Long-lived lymphocyte that gives a fast response on re-exposure |
| Attenuated | Weakened so that it can no longer cause serious disease |
| Adjuvant | A substance that helps a vaccine produce a stronger response |
| Booster | An extra dose that raises or renews protection |
| Herd immunity | Indirect protection when enough people in a community are immune |
| R0 | Average number of people one case infects in a fully susceptible population |
| Efficacy | Protection measured in a clinical trial |
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