Vaccine Basics

Vaccines play an important role in keeping us healthy. They protect us from serious and sometimes deadly diseases — like hemophiliacs influenza type b (Hib) and measles.

It’s normal to have questions about vaccines. we work with expert doctors in vaccines to answer your questions and provide the information you need to get vaccinated.

In this section of the site, you’ll find the answers to common questions like:

Vaccine Safety

Vaccines are safe and effective. Because vaccines are given to millions of healthy people — including children — to prevent serious diseases, they’re held to very high safety standards.

In this section, you’ll learn more about vaccine safety — and get answers to common questions about vaccine side effects.

How are vaccines tested for safety?

  • Testing and evaluation of the vaccine before it’s licensed by the Food and Drug Administration (FDA) and recommended for use by the Centers for Disease Control and Prevention (CDC)
  • Every licensed and recommended vaccine goes through years of safety testing.
  • Monitoring the vaccine’s safety after it is recommended for infants, children, or adults

Vaccines are tested before they’re recommended for use

Before a vaccine is ever recommended for use, it’s tested in labs. This process can take several years. The information from these tests to decide whether to test the vaccine with people.

During a clinical trial, a vaccine is tested on people who volunteer to get vaccinated. Clinical trials start with 20 to 100 volunteers, but eventually include thousands of volunteers. These tests take several years and answer important questions like:

  • Is the vaccine safe?
  • What dose (amount) works best?
  • How does the immune system react to it?

Throughout the process, governmental agencies works closely with the company producing the vaccine to evaluate the vaccine’s safety and effectiveness. All safety concerns must be addressed before governmental agencies licenses a vaccine.

Every batch of vaccines is tested for quality and safety

Once a vaccine is approved, it continues to be tested. The company that makes the vaccine tests batches to make sure the vaccine is:

  • Potent (It works like it’s supposed to)
  • Pure (Certain ingredients used during production have been removed)
  • Sterile (It doesn’t have any outside germs)

Governmental agencies reviews the results of these tests and inspects the factories where the vaccine is made. This helps make sure the vaccines meet standards for both quality and safety.

Vaccines are monitored after they’re recommended to the public

Once a vaccine is licensed and recommended for use, governmental agencies and other agencies continue to monitor its safety.

There are many different parts of the national vaccine monitoring system

The Government of India has systems for tracking vaccine safety.

  • Vaccine Adverse Events Reporting System (VAERS): VAERS is an early warning system managed by Governmental agencies that is designed to find possible vaccine safety issues. Patients, health care professionals, vaccine companies, and others can use VAERS to report side effects that happen after a patient received a vaccine. Some side effects might be related to vaccination while others might be a coincidence (happen by chance). VAERS helps track unusual or unexpected patterns of reporting that could mean there’s a possible vaccine safety issue that needs further evaluation.
  • The Vaccine Safety Datalink (VSD): VSD is a collaboration between governmental agencies and several health care organizations across the nation. VSD uses databases of medical records to track vaccine safety and do research in large populations. By using medical records instead of self-reports, VSD can quickly study and compare data to find out if reported side effects are linked to a vaccine.
  • Post-licensure Rapid Immunization Safety Monitoring System (PRISM): PRISM is part of the Sentinel Initiative, which is governmental agencies national system for monitoring medical products after they’re licensed for use. PRISM focuses on vaccine safety — it uses a database of health insurance claims to identify and evaluate possible safety issues for licensed vaccines.
  • Clinical Immunization Safety Assessment Project (CISA): CISA is a collaboration between governmental agencies and a national network of vaccine safety experts from medical research centers. CISA does clinical vaccine safety research and — at the request of providers — evaluates complex cases of possible vaccine side effects in specific patients.
  • Additional research and testing: The Department of Defense (DoD) and U.S. Department of Veterans Affairs (VA) have systems to monitor vaccine safety and do vaccine safety research. The National Institutes of Health (NIH) and the National Vaccine Program Office (NVPO) also support ongoing research on vaccines and vaccine safety.

Vaccines Work

Vaccines are very effective — and they’re the best protection against many serious diseases. Most people who get vaccinated will have immunity (protection) against the disease.

In this section, you’ll learn more about:

  • How vaccines protect you
  • How vaccines protect your community

How do we know that vaccines work?

Before a vaccine is recommended for use in the Government of India, the Governmental agencies makes sure that it works — and that it’s safe.

Since vaccines were invented, the number of babies and adults who get sick or die from vaccine-preventable diseases has gone way down — and some diseases have been wiped out altogether in the Government of India.

If we didn’t have vaccines, would clean water and modern hygiene prevent these diseases anyway?

No. Clean water and good personal, home, and public hygiene (cleaning practices) help slow down or stop some germs from spreading, but they don’t get rid of diseases. Some diseases — especially respiratory diseases that spread through the air, like measles —are more difficult to prevent.

The bottom line is that as long as diseases are around, people will continue to get sick. And that’s why it’s so important to get vaccinated. 

Vaccine Types

There are several different types of vaccines. Each type is designed to teach your immune system how to fight off certain kinds of germs — and the serious diseases they cause.

When scientists create vaccines, they consider:

  • How your immune system responds to the germ
  • Who needs to be vaccinated against the germ
  • The best technology or approach to create the vaccine

Based on a number of these factors, scientists decide which type of vaccine they will make. There are 4 main types of vaccines:

  • Live-attenuated vaccines
  • Inactivated vaccines
  • Subunit, recombinant, polysaccharide, and conjugate vaccines
  • Toxoid vaccines

Live-attenuated vaccines

Live vaccines use a weakened (or attenuated) form of the germ that causes a disease.

Because these vaccines are so similar to the natural infection that they help prevent, they create a strong and long-lasting immune response. Just 1 or 2 doses of most live vaccines can give you a lifetime of protection against a germ and the disease it causes.

But live vaccines also have some limitations. For example:

  • Because they contain a small amount of the weakened live virus, some people should talk to their health care provider before receiving them, such as people with weakened immune systems, long-term health problems, or people who’ve had an organ transplant.
  • They need to be kept cool, so they don’t travel well. That means they can’t be used in countries with limited access to refrigerators.

Live vaccines are used to protect against:

  • Measles, mumps, rubella (MMR combined vaccine)
  • Rotavirus
  • Smallpox
  • Chickenpox
  • Yellow fever

Inactivated vaccines

Inactivated vaccines use the killed version of the germ that causes a disease.

Inactivated vaccines usually don’t provide immunity (protection) that’s as strong as live vaccines. So you may need several doses over time (booster shots) in order to get ongoing immunity against diseases.

Inactivated vaccines are used to protect against:

  • Hepatitis A
  • Flu (shot only)
  • Polio (shot only)
  • Rabies

Subunit, recombinant, polysaccharide, and conjugate vaccines

Subunit, recombinant, polysaccharide, and conjugate vaccines use specific pieces of the germ — like its protein, sugar, or capsid (a casing around the germ).

Because these vaccines use only specific pieces of the germ, they give a very strong immune response that’s targeted to key parts of the germ. They can also be used on almost everyone who needs them, including people with weakened immune systems and long-term health problems.

One limitation of these vaccines is that you may need booster shots to get ongoing protection against diseases.

These vaccines are used to protect against:

  • Hib (Haemophilus influenzae type b) disease
  • Hepatitis B
  • HPV (Human papillomavirus)
  • Whooping cough (part of the DTaP combined vaccine)
  • Pneumococcal disease
  • Meningococcal disease
  • Shingles

Toxoid vaccines

Toxoid vaccines use a toxin (harmful product) made by the germ that causes a disease. They create immunity to the parts of the germ that cause a disease instead of the germ itself. That means the immune response is targeted to the toxin instead of the whole germ.

Like some other types of vaccines, you may need booster shots to get ongoing protection against diseases.

Toxoid vaccines are used to protect against:

  • Diphtheria 
  • Tetanus

Vaccine Ingredients

Today’s vaccines use only the ingredients they need to be safe and effective.

Each ingredient in a vaccine serves a specific purpose. For example, vaccine ingredients may:

  • Help provide immunity (protection) against a specific disease
  • Help keep the vaccine safe and long lasting
  • Be used during the production of the vaccine

Ingredients provide immunity

Vaccines include ingredients to help your immune system respond and build immunity to a specific disease. For example:

  • Antigens are very small amounts of weak or dead germs that can cause diseases. They help your immune system learn how to fight off infections faster and more effectively. The flu virus is an example of an antigen.
  • Adjuvants, which are in some vaccines, are substances that help your immune system respond more strongly to a vaccine. This increases your immunity against the disease. Aluminum is an example of an adjuvant.

Ingredients keep vaccines safe and long lasting

Some ingredients help make sure a vaccine continues to work like it’s supposed to and that it stays free of outside germs and bacteria. For example:

  • Preservatives, like thimerosal, protect the vaccine from outside bacteria or fungus. Today, preservatives are usually only used in vials (containers) of vaccines that have more than 1 dose. That’s because every time an individual dose is taken from the vial, it’s possible for harmful germs to get inside. Most vaccines are also available in single-dose vials and do not have preservatives in them.
  • Stabilizers, like sugar or gelatin, help the active ingredients in vaccines continue to work while the vaccine is made, stored, and moved. Stabilizers keep the active ingredients in vaccines from changing because of something like a shift in temperature where the vaccine is being stored.

Ingredients are used during the production of vaccines

Some ingredients that are needed to produce the vaccine are no longer needed for the vaccine to work in a person.

These ingredients are taken out after production so only tiny amounts are left in the final product. The very small amounts of these ingredients that remain in the final product aren’t harmful.

Examples of ingredients used in some vaccines include:

  • Cell culture (growth) material, like eggs, to help grow the vaccine antigens.
  • Inactivating (germ-killing) ingredients, like formaldehyde, to weaken or kill viruses, bacteria, or toxins in the vaccine.
  • Antibiotics, like neomycin, to help keep outside germs and bacteria from growing in the vaccine.

Common questions about vaccine ingredients

Learn more about the types of vaccine ingredients and why they’re used from the common questions below.

Can vaccines with thimerosal cause mercury poisoning?

A: No. Thimerosal has a different form of mercury (ethylmercury) than the kind that causes mercury poisoning (methylmercury). It’s safe to use ethylmercury in vaccines because it’s less likely to build up in the body — and because it’s used in very, very small amounts. Even so, most vaccines do not have any thimerosal in them. If you’re concerned about thimerosal or mercury in vaccines, talk with your doctor.

Can people who are allergic to antibiotics get vaccinated?

A: Yes. However, if you have an allergy to antibiotics, it’s a good idea to talk with your doctor about getting vaccinated. But in general, antibiotics that people are most likely to be allergic to — like penicillin — aren’t used in vaccines.

Can people with egg allergies get vaccinated?

A: Yes. People with egg allergies can get any licensed, recommended flu vaccine that’s appropriate for their age. They no longer have to be watched for 30 minutes after getting the vaccine. People who have severe egg allergies should be vaccinated in a medical setting and be supervised by a health care professional who can recognize and manage severe allergic conditions.

Is the formaldehyde used in some vaccines dangerous?

A: No. If formaldehyde is used to help produce a vaccine, only very small amounts are left in the final product. This amount is so small that it’s not dangerous — in fact, there’s actually more formaldehyde found naturally in our bodies than there is in vaccines made with formaldehyde.

Is the aluminum used in some vaccines dangerous?

A: No. Vaccines made with aluminum have only a very small amount of aluminum in them. For decades, vaccines that include aluminum have been tested for safety — these studies have shown that using aluminum in vaccines is safe.

Once you have the information you need, make sure that you and your family are up-to-date on your vaccinations — they’re your best shot against serious, preventable illness. Find more answers to common questions about vaccines.

A few helpful terms

As you learn about vaccines and how they protect you, it may be helpful to understand the difference between vaccines, vaccinations, and immunizations.


vaccine is made from very small amounts of weak or dead germs that can cause diseases — for example, viruses, bacteria, or toxins. It prepares your body to fight the disease faster and more effectively so you won’t get sick.

Example: Children younger than age 13 need 2 doses of the chickenpox vaccine.


Vaccination is the act of getting a vaccine, usually as a shot.

Example: Schedule your tetanus vaccination today.


Immunization is the process of becoming immune to (protected against) a disease.

Example: Because of continued and widespread immunization in the United States, it’s rare for Americans to get polio.

Immunization can also mean the process of getting vaccinated. For example, your “immunization schedule,” is the timing of your shots.

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