Vaccines teach your body's defense system to recognize and fight off serious diseases before you get sick.

Key idea: Vaccines are the most effective way to prevent infectious diseases, and they have successfully eliminated some diseases and greatly reduced others.

Vaccines can be used in different ways. Most vaccines are 'prophylactic,' meaning they are given to prevent you from getting sick in the future. For example, the flu shot helps prevent you from catching the flu. But some vaccines are 'therapeutic,' which means they help fight a disease that has already started, like some vaccines being developed for cancer.

Some vaccines offer 'sterilizing immunity,' which means they completely stop you from getting infected at all. Getting vaccinated is the best way to prevent infectious diseases. Thanks to widespread vaccination, diseases like smallpox have been completely wiped out worldwide. Other serious diseases, such as polio, measles, and tetanus, are now much less common in many parts of the world. The World Health Organization (WHO) says there are licensed vaccines for 25 different preventable infections.

Key idea: The practice of preventing disease through inoculation dates back centuries, with Edward Jenner developing the first modern vaccine against smallpox.

The idea of vaccination has a long history. People in China tried a form of smallpox prevention as early as the 10th century. They would blow powdered scabs from smallpox patients into people's noses to give them a milder form of the disease, hoping to protect them from a more severe one. This practice was called 'variolation.'

The modern concept of vaccines began in 1796 with Edward Jenner. He noticed that milkmaids who caught cowpox, a mild disease from cows, seemed to be protected from smallpox. Jenner took pus from a cowpox sore and put it into an 8 year old boy. Later, when he exposed the boy to smallpox, the boy did not get sick. Jenner called the protective substance 'vaccine,' from the Latin word for cow, 'vacca.' In 1881, Louis Pasteur suggested using this term for all new protective inoculations being developed. The study of vaccines is now called vaccinology.

“The modern concept of vaccines began in 1796 with Edward Jenner.”

Key idea: Vaccines work by training the immune system to remember specific germs, allowing for a rapid and effective response if the real threat appears, and widespread vaccination creates 'herd immunity' protecting entire communities.

There is a lot of scientific agreement that vaccines are a very safe and effective way to fight and get rid of infectious diseases. When your immune system meets the vaccine, it recognizes the vaccine's parts as foreign. It then destroys them and 'remembers' them. If you later encounter the real, dangerous germ, your body recognizes its outer coating and is ready to act. It first neutralizes the germ before it can get into your cells, and then it finds and destroys any infected cells before the germ can multiply and make you very sick.

For example, in 1958, there were over 763,000 cases of measles in the United States, leading to 552 deaths. After new vaccines were introduced, the number of cases dropped to fewer than 150 per year. The measles vaccine is estimated to prevent one million deaths each year. Vaccines helped completely get rid of smallpox, which was one of the most contagious and deadly human diseases. Thanks to widespread vaccination programs, diseases like rubella, polio, measles, mumps, chickenpox, and typhoid are much less common than they were a hundred years ago.

When most people in a community are vaccinated, it becomes much harder for a disease to spread. This protection is called herd immunity. Polio, which only spreads among humans, is now found in only a few parts of three countries (Afghanistan, Nigeria, and Pakistan) because of a huge effort to vaccinate everyone. Vaccines also help prevent bacteria from becoming resistant to antibiotics. For instance, by greatly reducing pneumonia caused by a certain bacteria, vaccine programs have also reduced infections that are hard to treat with common antibiotics.

Key idea: Vaccines are not always 100 percent effective due to factors related to the vaccine, the disease, or the individual's immune response, but they still significantly reduce illness severity and spread.

Even though vaccines are very effective, they do have some limits. Sometimes, a vaccine might not work perfectly. This can happen for reasons related to the vaccine itself, like if it wasn't stored correctly or if the vaccination schedule (when you get the shots) wasn't followed properly.

Sometimes, the problem is with the person receiving the vaccine. About 2 to 10 percent of people might not respond well to a vaccine because their immune system doesn't react strongly enough. This can be due to their genes, their overall health, age, or nutrition. For example, older people often have a weaker immune response to vaccines. In these cases, special ingredients called adjuvants are sometimes added to vaccines to boost the immune response, especially for older individuals.

It also takes time for your body to build full protection after a vaccine, usually one to two weeks. During this time, you could still get infected. Also, some vaccines offer only partial protection, meaning they reduce your risk but don't completely prevent infection, or temporary protection, meaning the immunity fades over time and you might need booster shots. Even with partial or temporary protection, vaccines still make a big difference by reducing how severe the illness is, lowering death rates, and helping people recover faster.

If a vaccinated person does get sick with the disease they were vaccinated against (called a 'breakthrough infection'), the illness is usually much milder and less likely to spread to others compared to someone who is not vaccinated. Many factors affect how well a vaccine works, including the specific disease, the type of vaccine, whether the vaccination schedule was followed, and individual differences like genetics or age.

Key idea: Vaccines are generally safe with mostly mild side effects, and serious reactions are extremely rare.

Vaccines given to children, teenagers, or adults are generally safe. If there are any side effects, they are usually mild. Common side effects include a slight fever, pain where the shot was given, and muscle aches. Some people might also have an allergic reaction to certain ingredients in the vaccine. Serious side effects are extremely rare.

Some people are more likely to have a weaker immune response to vaccines, such as the elderly, people with certain allergies, or those with underlying health conditions. For these groups, different vaccine technologies or repeated booster shots might be needed. Many countries have programs to provide financial help to people who experience very rare, severe side effects from vaccines. For example, the United States has the National Childhood Vaccine Injury Act.

Key idea: Different types of vaccines use various strategies, such as weakened germs, killed germs, toxins, or specific germ parts, to safely train the immune system.

Vaccines are made in different ways to best prepare your immune system. Here are some common types:

1. Attenuated (Live, Weakened) Vaccines: These contain live germs that have been weakened so they cannot cause serious disease. They create a strong, long lasting immune response. Examples include vaccines for measles, mumps, and rubella.

2. Inactivated (Killed) Vaccines: These use germs that have been completely killed by heat or chemicals. They are very safe because they cannot cause disease, but they might require booster shots. Examples include vaccines for polio and hepatitis A.

3. Toxoid Vaccines: These vaccines target the harmful toxins (poisons) produced by certain bacteria, rather than the bacteria itself. The toxins are made harmless but still teach your immune system to fight them. Tetanus and diphtheria vaccines are examples.

4. Subunit Vaccines: Instead of using the whole germ, these vaccines use only a small, specific part of it, like a surface protein. This part is enough to trigger an immune response. The hepatitis B vaccine is a subunit vaccine.

5. Conjugate Vaccines: Some bacteria have a sugary outer coating that the immune system doesn't recognize well. Conjugate vaccines link this sugar coating to a protein, making it easier for the immune system to see and fight. An example is the vaccine for Haemophilus influenzae type B.

6. Genetic Vaccines (DNA and RNA): These are newer types of vaccines that give your cells instructions (in the form of DNA or RNA) to make a specific protein from the germ. Your body then recognizes this protein and builds an immune response. The COVID 19 mRNA vaccines are a well known example of this type.

Key idea: Vaccines contain additional ingredients like adjuvants to boost immune response, preservatives to prevent contamination, and excipients to stabilize the vaccine and aid production.

Vaccines often contain more than just the active ingredient that triggers immunity. They also have other components:

Adjuvants: These are ingredients added to boost your immune system's response to the vaccine. They help your body react more strongly and for a longer time, meaning you might need a smaller dose of the vaccine. For example, tetanus vaccines often use aluminum salts as an adjuvant.

Preservatives: These are used to prevent bacteria or fungi from growing in vaccine vials, especially those that contain more than one dose. Thiomersal (also known as Thimerosal) was a common preservative, but it has been removed from most childhood vaccines in many countries as a precaution due to its mercury content, even though scientific evidence has shown it does not cause autism. Other preservatives include phenoxyethanol and formaldehyde.

Excipients: These are other inactive ingredients. They can include things like antibiotics to prevent bacterial growth during production, egg protein (if the vaccine was grown in eggs, like some flu vaccines), formaldehyde to inactivate viruses or toxins, and stabilizers like MSG to keep the vaccine effective when exposed to heat or light.