Vaccines and immunization: What is vaccination?

Vaccination is a simple, safe, and effective way of protecting you against harmful diseases, before you come into contact with them. It uses your body’s natural defenses to build resistance to specific infections and makes your immune system stronger.

Vaccines train your immune system to create antibodies, just as it does when it’s exposed to a disease. However, because vaccines contain only killed or weakened forms of germs like viruses or bacteria, they do not cause the disease or put you at risk of its complications.

Most vaccines are given by an injection, but some are given orally (by mouth) or sprayed into the nose.

Vaccines reduce risks of getting a disease by working with your body’s natural defenses to build protection. When you get a vaccine, your immune system responds. It:

• recognizes the invading germ, such as the virus or bacteria; 
• produces antibodies. Antibodies are proteins produced naturally by the immune system to fight disease; and
• remembers the disease and how to fight it. If you are then exposed to the germ in the future, your immune system can quickly destroy it before you become unwell.

The vaccine is therefore a safe and clever way to produce an immune response in the body, without causing illness.

Our immune systems are designed to remember. Once exposed to one or more doses of a vaccine, we typically remain protected against a disease for years, decades or even a lifetime. This is what makes vaccines so effective. Rather than treating a disease after it occurs, vaccines prevent us in the first instance from getting sick.

Without vaccines, we are at risk of serious illness and disability from diseases like measles, meningitis, pneumonia, tetanus and polio. Many of these diseases can be life-threatening. WHO estimates that childhood vaccines alone save over 4 million lives every year.

Although some diseases may have become uncommon, the germs that cause them continue to circulate in some or all parts of the world. In today’s world, infectious diseases can easily cross borders, and infect anyone who is not protected

Two key reasons to get vaccinated are to protect ourselves and to protect those around us. Because not everyone can be vaccinated – including very young babies, those who are seriously ill or have certain allergies – they depend on others being vaccinated to ensure they are also safe from vaccine-preventable diseases.

Vaccines protect us throughout life and at different ages, from birth to childhood, as teenagers and into old age. In most countries you will be given a vaccination card that tells you what vaccines you or your child have had and when the next vaccines or booster doses are due. It is important to make sure that all these vaccines are up to date.

If we delay vaccination, we are at risk of getting seriously sick. If we wait until we think we may be exposed to a serious illness – like during a disease outbreak – there may not be enough time for the vaccine to work and to receive all the recommended doses.

If you have missed any recommended vaccinations for you or your child, talk to your healthcare worker about catching up.

Vaccines protect against many different diseases, including:

• Cervical cancer
• Cholera
• COVID-19
• Diphtheria
• Ebola virus disease
• Hepatitis B
• Influenza
• Japanese encephalitis
• Malaria
• Measles
• Meningitis
• Mpox
• Mumps
• Pertussis
• Pneumonia
• Polio
• Rabies
• Respiratory syncytial virus (RSV)
• Rotavirus
• Rubella
• Tetanus
• Typhoid
• Varicella
• Yellow fever

Some other vaccines are currently under development or being piloted, including those that protect against Zika virus, HIV or tuberculosis.

Not all of these vaccinations may be needed in your country. Some may only be given prior to travel, in areas of risk, or to people in high-risk occupations. Talk to your healthcare worker to find out what vaccinations are needed for you and your family.

Nearly everyone can get vaccinated. However, because of some medical conditions, some people should not get certain vaccines, or should wait before getting them. These conditions can include:

• chronic illnesses or treatments (like chemotherapy) that affect the immune system;
• severe and life-threatening allergies to vaccine ingredients, which are very rare; and
• if you have severe illness and a high fever on the day of vaccination.

These factors often vary for each vaccine. If you’re not sure if you or your child should get a particular vaccine, talk to your health worker. They can help you make an informed choice about vaccination for you or your child.

All the ingredients in a vaccine play an important role in ensuring a vaccine is safe and effective. These ingredients, as well as the vaccines themselves, have been thoroughly tested and are monitored to ensure they are safe.

These ingredients include:

• the antigen. Antigens can be a killed or weakened form of a virus or bacteria, or a part of it, or genetic instructions (RNA or DNA) that tell our cells to make the antigen. Antigens train our bodies to recognize the germ and respond if exposed in the future;
• adjuvants. These help vaccines work better by boosting our immune response.  Adjuvants like aluminium salts have been safely used in vaccines, helping them provide better and longer-lasting protection;
• preservatives. These help keep vaccines safe and effective by preventing contamination from germs that can grow in vaccine vials. An example is thiomersal, which has been safely used in some vaccines for decades; and
• stabilisers. These protect vaccines during storage and transport, supporting their safety and efficacy even when exposed to heat, light and other environmental changes. Common stabilisers include sugar and gelatin.

Vaccine ingredients may look unfamiliar when they are listed on a label. However, many of the components used in vaccines occur naturally in the body, in the environment, and in the food we eat. 

Yes, it is safe to receive several different vaccines during one visit. Combination vaccines are also as safe and effective as individual vaccines.

Receiving several vaccines or combination vaccines in one visit is important to protect children from various diseases as early as possible. This also makes it easier to complete the recommended doses on time. Combination vaccines (e.g. DTP for diphtheria, tetanus and pertussis) reduce the number of injections.

Receiving multiple doses also does not overwhelm the immune system. The antigens present in vaccines are a small fraction compared to what our bodies naturally encounter every day.

If you have questions about vaccines be sure to talk to your health-care worker. They can provide you with trusted advice about vaccination for you and your family, including the recommended vaccination schedule in your country.

When looking online for information about vaccines, be sure to consult only trustworthy sources. To help you find them, WHO has reviewed and ‘certified’ many websites across the world that provide only information based on reliable scientific evidence and independent reviews by leading technical experts. These websites are all members of the Vaccine Safety Net.