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.

• 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.

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.

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 against many different diseases, including:

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

Some other vaccines are currently under development or being piloted, including those that protect against Zika virus or malaria, but are not yet widely available globally.

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;

• 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 of a vaccine play an important role in ensuring a vaccine is safe and effective. Some of these include:

• The antigen. This is a killed or weakened form of a virus or bacteria, which trains our bodies to recognize and fight the disease if we encounter it in the future.

• Adjuvants, which help to boost our immune response. This means they help vaccines to work better.

• Preservatives, which ensure a vaccine stays effective.

• Stabilisers, which protect the vaccine during storage and transportation.

Vaccine ingredients can 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 foods we eat. All of the ingredients in vaccines – as well as the vaccines themselves - are thoroughly tested and monitored to ensure they are safe.

Vaccination is safe and side effects from a vaccine are usually minor and temporary, such as a sore arm or mild fever. More serious side effects are possible, but extremely rare.

Any licensed vaccine is rigorously tested across multiple phases of trials before it is approved for use, and regularly reassessed once it is introduced. Scientists are also constantly monitoring information from several sources for any sign that a vaccine may cause health risks.

Remember, you are far more likely to be seriously injured by a vaccine-preventable disease than by a vaccine. For example, tetanus can cause extreme pain, muscle spasms (lockjaw) and blood clots, measles can cause encephalitis (an infection of the brain) and blindness. Many vaccine-preventable diseases can even result in death. The benefits of vaccination greatly outweigh the risks, and many more illnesses and deaths would occur without vaccines.

More information about vaccine safety and development is available here.

Like any medicine, vaccines can cause mild side effects, such as a low-grade fever, or pain or redness at the injection site. Mild reactions go away within a few days on their own.

Severe or long-lasting side effects are extremely rare. Vaccines are continually monitored for safety, to detect rare adverse events.

Scientific evidence shows that giving several vaccines at the same time has no negative effect. Children are exposed to several hundred foreign substances that trigger an immune response every day. The simple act of eating food introduces new germs into the body, and numerous bacteria live in the mouth and nose.

When a combined vaccination is possible (e.g. for diphtheria, pertussis and tetanus), this means fewer injections and reduces discomfort for the child. It also means that your child is getting the right vaccine at the right time, to avoid the risk of contracting a potentially deadly disease.

There is  no evidence of any link between vaccines and autism or autistic disorders. This has been demonstrated in many studies, conducted across very large populations.

The 1998 study which raised concerns about a possible link between measles-mumps-rubella (MMR) vaccine and autism was later found to be seriously flawed and fraudulent. The paper was subsequently retracted by the journal that published it, and the doctor that published it lost his medical license. Unfortunately, its publication created fear that led to dropping immunization rates in some countries, and subsequent outbreaks of these diseases.

We must all ensure we are taking steps to share only credible, scientific information on vaccines, and the diseases they prevent.

Virtually all cervical cancer cases start with a sexually transmitted HPV infection. If given before exposure to the virus, vaccination offers the best protection against this disease. Following vaccination, reductions of up to 90% in HPV infections in teenage girls and young women have been demonstrated by studies conducted in Australia, Belgium, Germany, New Zealand, Sweden, the United Kingdom and the United States of America.

In studies, the HPV vaccine has been shown to be safe and effective. WHO recommends that all girls aged 9–14 years receive 2 doses of the vaccine, alongside cervical cancer screening later in life.

If you have questions about vaccines be sure to talk to your healthcare worker. He or she can provide you with science-based 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.