Immunisations and Vaccines: How They Work, Schedules and Side Effects

Vaccines are among the most studied medical products in use, and yet many people have never had a clear explanation of how they work, why the schedules look the way they do, or what side effects are normal. This guide covers the essentials in plain language, with links to the public health bodies that publish the detail.

Immunisation and vaccination: the difference

The two words are often used interchangeably. Strictly, vaccination is the act of giving a vaccine, and immunisation is the process by which a person becomes protected against a disease, whether through a vaccine or, less safely, through infection. The CDC describes a person as immunised once their immune system has built that protection.

How vaccines work

Your immune system learns to recognise germs by their surface features, called antigens. The first time it meets a new germ, it takes days to mount a full response, and in that time the infection can cause serious illness. A vaccine gives the immune system a safe preview. It contains an antigen, or instructions for your cells to make one, without the ability to cause the disease. The immune system responds by producing antibodies and memory cells, so that if you meet the real germ later, it can respond quickly and strongly.

The World Health Organization explains that vaccines train the immune system in this way and that most are given by injection, with some given by mouth or nasal spray.

The main types of vaccine

• Live attenuated vaccines use a weakened form of the germ. Examples include the measles, mumps and rubella (MMR) vaccine. They usually give long-lasting protection but are not suitable for some people with weakened immune systems.
• Inactivated vaccines use a killed version of the germ, as with many flu injections.
• Subunit, recombinant and conjugate vaccines use specific pieces of the germ, such as a protein or sugar coating. The HPV and hepatitis B vaccines are examples.
• Toxoid vaccines target a toxin made by the germ rather than the germ itself, as with tetanus and diphtheria.
• mRNA and viral vector vaccines give your cells instructions to make a harmless antigen, which the immune system then learns to recognise.

Why some vaccines need several doses

A single dose is not always enough to build strong, lasting memory. Some vaccines need a primary series of two or three doses, and some need boosters years later because protection fades. Others, such as flu vaccines, are updated regularly because the virus itself changes. Measles is a good example of why the full course matters: the WHO measles fact sheet states that children should receive two doses of the vaccine to ensure they are immune.

Schedules differ between countries because disease patterns and health systems differ. In the UK, the NHS vaccination schedule lists what is offered from infancy through to older age. In the US, the CDC publishes immunisation schedules for children and adults. If you have moved country, a nurse or doctor can compare your records with the local schedule and suggest any catch-up doses.

What vaccines have achieved

The NHS notes that since vaccines were introduced in the UK, diseases like smallpox, polio and tetanus that used to kill or disable millions of people are either gone or now very rarely seen. Globally, the WHO estimates that measles vaccination averted nearly 59 million deaths between 2000 and 2024.

These gains are not permanent. When vaccination rates fall, diseases return. The NHS reports that measles and mumps have started to reappear in England, even though the MMR vaccine is the best protection against both.

Community protection

Vaccination protects the person vaccinated, and it also reduces how easily a disease spreads through a community. When enough people are immune, an infection struggles to find new hosts. This matters for people who cannot be vaccinated or who respond poorly to vaccines: newborn babies, some people having cancer treatment, and people with certain immune conditions. For very contagious diseases like measles, a high proportion of the population needs to be immune for this effect to hold.

Side effects: what is normal

Most side effects are mild and short-lived, and they are a sign that the immune system is responding. The CDC notes that the immune response to a vaccine might cause tiredness and discomfort for a day or two. Common reactions include:

• soreness, redness or swelling where the injection was given
• a mild fever
• tiredness, headache or aching muscles
• in babies, some irritability for a day or so

Serious reactions, such as a severe allergic reaction (anaphylaxis), are rare. Vaccination staff are trained to recognise and treat them, which is why you may be asked to wait for a short time after some vaccines. Vaccine safety continues to be monitored after approval; in the US this is described on the CDC vaccine safety pages.

Vaccines for adults

Immunisation does not stop in childhood. Depending on your age, health and circumstances, you may be offered:

• a yearly flu vaccine, particularly if you are older, pregnant or have a long-term condition
• tetanus boosters, especially after a dirty wound if your record is incomplete
• shingles vaccination in older age; the NHS shingles page explains who is eligible
• pneumococcal vaccination for older adults and people at higher risk
• vaccines in pregnancy, such as whooping cough, which help protect the baby in its first weeks
• travel vaccines, depending on the destination

Questions to ask before a vaccine

It is reasonable to ask questions. Useful ones include: what the vaccine protects against, how many doses are needed, what side effects are common, and whether any of your medicines, allergies or health conditions make a difference. Tell the person giving the vaccine if you have had a serious reaction to a vaccine before, if you are pregnant, or if your immune system is weakened by illness or treatment.

When to see a doctor

Contact a doctor or nurse if a fever is high or lasts more than a couple of days, if redness or swelling at the injection site keeps spreading, or if you are worried about any symptom after a vaccine. Call emergency services straight away for signs of a severe allergic reaction, such as swelling of the face or throat, difficulty breathing, or feeling faint soon after vaccination. If you are unsure whether you or your child are up to date, your practice or clinic can check your records.

This article is general information, not medical advice. Speak to a doctor, nurse or pharmacist about the vaccines that are right for you.