Vaccines definition

The body’s immune system helps protect against pathogens that cause infection. Most of the time, it’s an efficient system. It either keeps microorganisms out or tracks them down and gets rid of them.

However, some pathogens can overwhelm the immune system. When this happens, it can cause serious illness.

The pathogens most likely to cause problems are the ones the body doesn’t recognize. Vaccination is a way to “teach” the immune system how to recognize and eliminate an organism. That way, your body is prepared if you’re ever exposed.

Vaccinations are an important form of primary prevention. That means they can protect people from getting sick. Vaccinations have allowed us to control diseases that once threatened many lives, such as:

It’s important that as many people as possible get vaccinated. Vaccinations don’t just protect individuals. When enough people are vaccinated, it helps protect society.

This occurs through herd immunity. Widespread vaccinations make it less likely that a susceptible person will come into contact with someone who has a particular disease.

A healthy immune system defends against invaders. The immune system is composed of several types of cells. These cells defend against and remove harmful pathogens. However, they have to recognize that an invader is dangerous.

Vaccination teaches the body to recognize new diseases. It stimulates the body to make antibodies against antigens of pathogens. It also primes immune cells to remember the types of antigens that cause infection. That allows for a faster response to the disease in the future.

Vaccines work by exposing you to a safe version of a disease. This can take the form of:

  • a protein or sugar from the makeup of a pathogen
  • a dead or inactivated form of a pathogen
  • a toxoid containing toxin made by a pathogen
  • a weakened pathogen

When the body responds to the vaccine, it builds an adaptive immune response. This helps equip the body to fight off an actual infection.

Vaccines are usually given by injection. Most vaccines contain two parts. The first is the antigen. This is the piece of the disease your body must learn to recognize. The second is the adjuvant.

The adjuvant sends a danger signal to your body. It helps your immune system to respond more strongly against the antigen as an infection. This helps you develop immunity.

Vaccines are very important for infants, but they’re not all given immediately after birth. Each vaccine is given on a timeline, and some require multiple doses. This table can help you understand the timeline of each vaccine:

Name of VaccineAgeHow many shots?
Hepatitis BBirthA second at 1–2 months, a third at 6–18 months
Rotavirus (RV)2 monthsA second at 4 months, a third at 6 months
Diphtheria, tetanus, and whooping cough (DTaP)2 monthsA second at 4 months, a third at 6 months, a fourth at 16–18 months; then every 10 years
Haemophilus influenzae type b (Hib)2 monthsA second at 4 months, a third at 6 months, a fourth at 12–15 months
Pneumococcal conjugate vaccine PCV132 monthsA second at 4 months, a third at 6 months, a fourth between months 12 and 15
Inactivated Polio Vaccine (IPV)2 monthsA second at 4 months, a third at 6–18 months, a fourth at 4 to 6 years
Influenza6 monthsRepeat yearly
Measles, mumps, and rubella (MMR)12–15 monthsA second at 4–6 years
Varicella12–15 monthsA second at 4–6 years
Hepatitis A12–23 monthsA second at 6 months after the first
Human papillomavirus (HPV)11–12 years old2-shot series 6 months apart
Meningococcal conjugate (MenACWY) 11–12 years oldBooster at 16 years old
serogroup B meningococcal (MenB)16–18 years old
Pneumococcal (PPSV23)19–65+ years old
Herpes zoster (Shingles—RZV formulation)two doses at 50 years old

Vaccines are considered to be safe. They’re rigorously tested and go through many rounds of study, examination, and research before they’re used with the general public.

The overwhelming bulk of research and evidence shows that vaccines are safe and that side effects are rare. Side effects that do occur are typically mild.

Indeed, the greatest risk for most individuals will come if you choose not to get a vaccine and potentially get sick after exposure to a disease. The illness may be far worse than the potential side effects of the vaccine. It could even be deadly.

You may have more questions about the safety of vaccines. This guide to vaccine safety can help.

When considering whether or not to be vaccinated, these factors may be important to consider:


  • Vaccines help prevent dangerous diseases that have killed, and can sicken or kill, many people.
  • Researchers thoroughly investigate each vaccine before presenting the data to the U.S. Food and Drug Administration (FDA). The FDA can approve or deny the vaccine. The overwhelming majority of research shows that vaccines are safe.
  • Vaccines not only protect you. They protect people around you, especially people who are not well enough to be vaccinated.


  • Each vaccine is made with different components, and each can affect you differently. People who have experienced allergic reactions to certain vaccines in the past may experience an allergic reaction again.
  • You could still get sick, even if you’re vaccinated.
  • Some people with weakened immune systems cannot be vaccinated or should be only under close supervision of a health care provider.

Read more about which people should avoid certain vaccines and why.

Most side effects from a vaccine injection are mild. Some people will experience no side effects at all.

When they do occur, side effects, some rarer than others, may include:

Some risk factors do increase your risk for experiencing side effects from a vaccination. These risk factors include:

  • having a weak or suppressed immune system
  • being sick at the time you receive a vaccine
  • having a family or personal history of vaccine reactions

Serious or life-threatening side effects or reactions from vaccines are rare. Indeed, most people are at a higher risk of becoming ill from diseases if they’re not vaccinated.

That’s the case with influenza, commonly referred to as the flu. Know what to expect with the flu vaccine before you get one, including what side effects might be possible.

Vaccines are highly effective, but no vaccine is 100 percent effective. The effectiveness rate for vaccines differs from one type to the next.

Flu vaccines are effective in lowering the risk of infection by 40 to 60 percent in people who get the shot. That may sound low, but keep in mind the flu vaccine is designed to match the strain of the flu scientists expect to be most abundant in a coming flu season.

If they’re wrong, the vaccine may be less effective. If they’re right, the rate of protection may be higher.

The measles vaccine, on the other hand, is 98 percent effective when used as recommended. Indeed, most childhood vaccines are 85 to 95 percent effective if administered properly, according to the World Health Organization (WHO).

Vaccines are given in childhood to help protect their young immune systems against a range of potentially deadly diseases. Infants have a natural immunity from their mothers in their earliest months. As that begins to wane, vaccines are given to take over and help keep babies from falling ill.

Vaccines help protect children against diseases that their friends, playmates, classmates, and family members may introduce to them. That’s why some vaccines require a booster, or a follow-up dose, as children near school age. The booster shot helps reinforce your child’s defenses against illness.

The U.S. Centers for Disease Control and Prevention (CDC) sets a recommended vaccine schedule. Many vaccines are delivered in a group or vaccine series. However, if you’d like to space your child’s vaccines out more, talk with your child’s doctor about your preference.

Vaccines teach your immune system to recognize a particular virus or bacterium so that it can defeat it should your body encounter the disease again.

Four types of vaccines are currently used:

  • Killed (inactivated) vaccines are made from a virus or bacterium that is not living.
  • Live virus vaccines use a weakened (attenuated) version of a virus or bacterium.
  • Toxoid vaccines come from a harmful chemical or toxin that is made by bacteria or viruses. Toxoid vaccines do not make you immune to the germ. Instead, they make you immune to the harmful effects from the toxin of a germ. The tetanus shot is a type of toxoid vaccine.
  • Subunit, recombinant, polysaccharide, and conjugate vaccines take a structural component from a virus or bacterium that can train your immune system to attack this part of the germ.

Other ingredients are used to keep the vaccines safe during production, storage, and transportation.

These ingredients may also help the vaccine work more effectively once it’s administered. These additives represent a very small portion of the vaccine, however.

These additives include:

  • Suspending fluid. Sterile water, saline, or other fluids keep the vaccine safe during production, storage, and use.
  • Adjuvants or enhancers. These ingredients help make the vaccine more effective once it’s injected. Examples include aluminum gels or salts.
  • Preservatives and stabilizers. Many vaccines are made months, even years, before they’re used. These ingredients help prevent the virus, bacterium, or protein pieces from breaking down and becoming ineffective. Examples of a stabilizer are monosodium glutamate (MSG) and thimerosal.
  • Antibiotics. Small amounts of a bacteria-fighting drug may be added to vaccines to prevent the growth of germs during production and storage.

Each of these ingredients is studied rigorously for safety and efficiency. See how these ingredients work together in the flu vaccine.

Vaccines are a lifelong defense against illness. While childhood vaccines are important, you may receive injections or boosters throughout your entire life.

Infancy and early childhood vaccinations list

By the time your child starts elementary school, they should have received:

Middle childhood vaccinations list

In addition to the most common childhood vaccinations, your doctor may recommend these vaccines for your child:

Young adult vaccinations list

As your child grows older, other vaccines may be recommended. These include:

Adult vaccinations list

Older adults should receive:

Other vaccines list

Your doctor may suggest you receive additional vaccines or boosters based on your sexual orientation, health history, personal hobbies, and other factors. These possible vaccines include:

  • Bacterial meningococcal disease is a bacterial illness that can cause inflammation in the protective layer of tissue surrounding your brain and spinal cord. This infection is passed through sharing respiratory and salivary secretions to those in close contact, such as through kissing or coughing. Two different Meningococcal vaccines exist. You’ll want to talk to your doctor to find out which one is right for you.
    • Meningococcal serogroup B vaccine. This vaccine protects against the serogroup B type.
    • Meningococcal conjugate. This traditional meningitis vaccine protects against serogroup types A, C, W, and Y.
    • Yellow fever vaccine. Yellow fever is a serious and potentially deadly viral disease that causes flu-like symptoms. It’s spread by mosquitoes. The CDC recommends anyone 9 months and older to be vaccinated against yellow fever if they plan to travel or live in areas of the world where yellow fever is present.
    • Viral hepatitis is a potentially dangerous infectious disease. The CDC recommends infants and children be vaccinated against hepatitis A and B before international travel. Unfortunately, there’s no vaccine for hepatitis C at this time.

There are, however, vaccines for these six types of diseases that you may have never heard of.

Most health insurance plans cover vaccinations at little or no out-of-pocket cost to you. If you do not have insurance or your insurance does not cover vaccines, you can look for low- and no-cost alternatives.

These include:

  • Community health organizations. Many organizations provide vaccine clinics for infants and children at a greatly reduced rate.
  • Vaccines for Children Program. This no-cost program provides recommended vaccines to children who do not have health insurance, are underinsured, are Medicaid-eligible, can’t afford the shots, or are Native Americans or Alaska Natives.
  • State health departments. These community-based offices can provide basic health services, including vaccines, on a low-cost basis.

The CDC provides a routinely-updated list of vaccine costs so that consumers may have an idea of the out-of-pocket cost of a vaccine. If you do not have insurance and don’t qualify for any of these cost reduction programs, this list may help you estimate your total out-of-pocket cost.

When you’re pregnant, vaccines don’t just protect you. They provide immunity to your growing baby. During these nine months, you and your baby need protection against serious diseases, and vaccines are a vital part of that.

The CDC recommends women who plan to become pregnant receive an MMR vaccine before becoming pregnant. These diseases, in particular rubella, can lead to serious issues, including miscarriage and birth defects.

During pregnancy, the CDC recommends women have a whooping cough (Tdap) vaccine and an influenza (flu) vaccine. After pregnancy, women can receive vaccines, even while breastfeeding.

Post-pregnancy vaccinations also help protect your infant. If you’re immune to a virus or bacterium, you’re less likely to share it with your child.

If you aren’t properly vaccinated, you and your infant could get sick. Read why that’s a serious problem with the flu.

Vaccines are highly effective and safe. They’re used worldwide to prevent illness and death. These statistics show how successful they’ve been — and how much more successful they could be with improved access.

Polio cases have decreased by more than 99 percent since 1988, according to the World Health Organization (WHO). Today, polio is routinely found in only three countries (Pakistan, Afghanistan, and Nigeria).

WHO also estimates vaccines prevent 2 to 3 million deaths each year. Another million could be prevented with expanded vaccine access. Between 2000 and 2016, the worldwide rate of measles deaths fell by 86 percent.

According to the CDC, 70.7 percent of American children receive the 7-vaccine series that is recommended to infants and children under 3 years of age. However, that does not mean children are not being vaccinated. As their research also shows, most vaccination rates for individual vaccines are higher.

Parents sometimes divided vaccines into smaller groupings. The rates show that 83.4 percent of children are vaccinated for DTaP, 91.9 percent are vaccinated for polio, and 91.1 percent are vaccinated for MMR.

Older adults also follow CDC recommendations. More than two-thirds of adults over the age of 65 have had a flu vaccine in the last year. More than one in two adults 65 or older have had a tetanus shot in the last decade.

Antibodies help the body recognize antigens of diseases. Protection from antibodies can be achieved in two different ways.

Active immunization is the immunity your body achieves when it’s triggered to produce its own antibodies against antigens of a disease you’re exposed to. It stimulates long-term protection against a disease. Active immunity can occur after an infection (natural immunity). It can also occur through vaccination (artificial immunity).

Passive immunization provides short-term protection against a disease. It occurs when someone receives antibodies instead of making their own. Passive immunity is transmitted naturally from mother to child during birth and breastfeeding. It can also be achieved artificially through the injection of immune globulins. These are antibody-containing blood products.

In recent years, vaccine opponents have challenged their safety and effectiveness. However, their arguments have generally been flawed. Vaccination generally is a very safe way to prevent disease.

There’s no good evidence that vaccination can cause autism. However, there’s a lot of evidence that vaccines can prevent serious disease and death.

Not all people avoid vaccinations because of safety concerns. Some simply don’t know that they should be vaccinated. For example, people should get the flu vaccine every winter.

However, according to the Centers for Disease Control and Prevention (CDC), around 50 percent of Americans did not get the annual flu shot during the flu season of 2011 to 2012. Many have no idea they should.

It’s important to talk to your doctor about what vaccines you need. Avoiding vaccination puts you, and potentially others around you, at risk of serious disease. It can lead to costly doctor’s visits and hospital fees.

Vaccines can reduce disease. For example, vaccination helped to eliminate polio from the Western hemisphere.

In the 1950s, before polio vaccines were available, polio caused more than 15,000 cases of paralysis each year in the United States. After vaccines were introduced, the number of polio cases fell to less than 10 in the 1970s.

Vaccination has also reduced the number of measles infections by more than 99 percent.

Ending vaccination could be very dangerous. Even today, around the world, many vaccine-preventable deaths still occur. This is because vaccines are not available to everyone. One of the missions of the World Health Organization (WHO) is to increase vaccine availability.

The WHO estimates that immunization prevents between 2 to 3 million deaths each year.