Immunization – side effects

Immunization keeps us safe from many infectious diseases, but, like any medication, vaccines can sometimes cause side effects. These are mostly mild, usually last one to two days, and affect roughly one to ten in every 100 people who are immunized.

Common side effects#

Depending on the vaccine, the most common reactions include:

• fever (a temperature over 38.5°C / 101.3°F)
• redness, swelling or soreness at the injection site
• sometimes a small hard lump (nodule) at the injection site that may last for some weeks or months

A small lump is nothing to worry about and needs no treatment.

Easing mild side effects#

A few simple steps can help:

• Drink extra fluids.
• Do not overdress a child or baby if they are hot.
• Place a cool, damp cloth on the injection site to ease discomfort.

Routine use of paracetamol (acetaminophen) after vaccination is not generally recommended, but if fever or pain at the injection site is present, paracetamol can be taken. Check the label for the correct dose, or ask your pharmacist — especially when giving it to children.

When to seek advice#

See a doctor if a reaction after immunization is unexpected, persistent or severe, or if you are simply worried. It is also important to seek advice if the person who was vaccinated becomes unwell, as this may be caused by an unrelated illness rather than the immunization itself.

If any reaction is severe and persistent, or you are concerned, contact your doctor immediately or dial your local emergency number (for example 911 in the US and Canada, 112 across the EU and many countries, 999 in the UK, or 000 in Australia) .

Serious reactions are rare#

There is a very small chance of a serious reaction after immunization. A serious reaction is an unwanted, unexpected symptom or sign linked to the vaccine. This is why you are advised to stay at the clinic for at least 15 minutes after a vaccine, in case further treatment is needed.

Examples of uncommon or rare reactions include:

• Anaphylaxis – an immediate allergic reaction. It is dramatic but rare (fewer than one in a million people will have anaphylaxis after a vaccination), and it is completely reversible if treated quickly.
• Febrile seizure – a seizure triggered when a child’s fever rises quickly. It usually lasts one or two minutes. It can be frightening but does not usually cause permanent harm or lasting effects. This is a rare side effect, occurring in about 1 in 17,000 babies.

Millions of people — many of them babies and young children — are immunized every year with no side effects at all.

Vaccines are tested for safety#

You can be confident that the vaccines used today are safe. Every vaccine must pass rigorous research and testing before it is approved, and vaccines continue to be monitored once they are in use. If a safety concern arises, a vaccine can be withdrawn from the market.

Think of it this way: a vaccine is a medicine we use to stop ourselves and those in our care from getting sick. We are much less likely to catch a disease if we are immunized. Like any medicine, vaccines can cause mild side effects and, in rare cases, more serious ones — but the chance of serious harm is very small. Choosing not to be immunized leaves you, your loved ones and the community at greater risk of catching infectious diseases that can be serious or even deadly.

Making an informed choice#

There is a lot of information about immunization, and it is easy to feel overwhelmed. Before you agree to any vaccine, your immunization provider should give you the information you need to make an informed decision. They can explain how vaccines work, discuss the benefits of immunization and the risks of disease, and outline your options.

Your consent must be voluntary, without any pressure, coercion or manipulation, and you do not have to consent until the risks and benefits have been explained. You can take your time — you do not have to decide on the spot, and you can always make another appointment to ask more questions or to go ahead.

Where to get more information#

A good first step is to talk to your doctor. You can also ask a clinic nurse, a specialist, or — for young children — a maternal and child health nurse or pediatrician. When looking for information online, stick to reliable, evidence-based health sources.

Key points#

• Side effects after immunization are mostly mild and usually last one to two days.
• A small lump at the injection site is harmless and needs no treatment; extra fluids and a cool cloth can ease discomfort.
• Serious reactions such as anaphylaxis are extremely rare and reversible if treated quickly.
• Vaccines are rigorously tested and monitored, and being immunized greatly lowers your risk of serious disease.

Where to get help and trusted information#

For evidence-based global health guidance, see Source: World Health Organization (WHO).