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Vaccines
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Infections

Vaccines

Vaccines are essential tools that teach your immune system to recognize and defend against harmful germs, providing a safer path to immunity than contracting the disease itself.

What are vaccines?

Vaccines are injections (shots), liquids, pills, or nasal sprays that you take to teach your body's immune system to recognize and defend against harmful germs. For example, there are vaccines to protect against diseases caused by:

  • Viruses, like the ones that cause the flu and COVID-19
  • Bacteria, including tetanus, diphtheria, and pertussis

What are the types of vaccines?

There are several types of vaccines:

  • Live-attenuated vaccines use a weakened form of the germ.
  • Inactivated vaccines use a killed version of the germ.
  • Subunit, recombinant, polysaccharide, and conjugate vaccines use only specific pieces of the germ, such as its protein, sugar, or casing.
  • Toxoid vaccines use a toxin (harmful product) made by the germ.
  • mRNA vaccines use messenger RNA, which gives your cells instructions for how to make a protein (or piece of a protein) of the germ.
  • Viral vector vaccines use genetic material, which gives your cells instructions for making a protein of the germ. These vaccines also contain a different, harmless virus that helps get the genetic material into your cells.

These vaccines may work in different ways, but they all spark an immune response. The immune response is the way your body defends itself against substances it sees as foreign or harmful. These substances include germs that can cause disease.

What happens in an immune response?

There are different steps in the immune response:

  • When a germ invades, your body sees it as foreign.
  • Your immune system helps your body fight off the germ.
  • Your immune system also remembers the germ. It will attack the germ if it ever invades again. This "memory" protects you against the disease that the germ causes. This type of protection is called immunity.

What are immunization and vaccination?

Immunization is the process of becoming protected against a disease. But it can also mean the same thing as vaccination, which is getting a vaccine to become protected against a disease.

Why are vaccines important?

Vaccines are important because they protect you against many diseases. These diseases can be very serious. So getting immunity from a vaccine is safer than getting immunity by being sick with the disease.

For a few vaccines, getting vaccinated can actually give you a better immune response than getting the disease would.

But vaccines don't just protect you. They also protect the people around you through community immunity.

What is community immunity?

Community immunity, or herd immunity, is the idea that vaccines can help keep communities healthy.

Normally, germs can travel quickly through a community and make a lot of people sick. If enough people get sick, it can lead to an outbreak. But when enough people are vaccinated against a certain disease, it's harder for that disease to spread to others. This type of protection means that the entire community is less likely to get the disease.

Community immunity is especially important for people who can't get certain vaccines. For example, they may not be able to get a vaccine because they have weakened immune systems. Others may be allergic to certain vaccine ingredients. And newborn babies are too young to get some vaccines. Community immunity can help to protect them all.

Are vaccines safe?

Vaccines are safe. They must go through extensive safety testing and evaluation before they are approved in the United States.

What is a vaccine schedule?

A vaccine, or immunization, schedule lists which vaccines are recommended for different groups of people. It includes who should get the vaccines, how many doses they need, and when they should get them. In the United States, the Centers for Disease Control and Prevention (CDC) publishes the vaccine schedule.

It's important for both children and adults to get their vaccines according to the schedule. Following the schedule allows them to get protection from the diseases at exactly the right time.

Frequently Asked Questions

Severe allergic reactions, known as anaphylaxis, are extremely rare but can occur within minutes to a few hours after vaccination. If you experience symptoms such as difficulty breathing, swelling of the face or throat, hives, or a rapid heartbeat, seek emergency medical attention immediately. This is why it is standard practice to stay at the vaccination clinic for 15 minutes after receiving a shot, allowing healthcare providers to monitor you and treat any immediate adverse reactions if they occur.
No, you cannot get the actual disease from a live-attenuated vaccine. These vaccines contain a weakened form of the germ that is still alive but has been modified so it cannot cause serious illness in healthy people. While you might experience mild symptoms that resemble the disease, such as a low-grade fever or a mild rash, the vaccine is designed to stimulate your immune system without causing the full-blown infection. However, individuals with severely weakened immune systems should consult their doctor before receiving live vaccines, as there is a rare risk of the weakened germ causing illness in these specific cases.
Many vaccines are considered safe and are actually recommended during pregnancy to protect both the mother and the baby. For example, the flu and Tdap (tetanus, diphtheria, and pertussis) vaccines are routinely given during pregnancy. The antibodies produced by the mother can also pass to the baby, providing protection in the first few months of life when the infant is too young to be vaccinated. However, live vaccines are generally avoided during pregnancy, so it is essential to discuss your specific vaccination schedule with your obstetrician to ensure you receive the appropriate protections at the right time.
It typically takes two to four weeks after vaccination for your immune system to build up sufficient protection. During this period, your body is producing antibodies and training immune cells to recognize the specific germ. This means that if you are exposed to the disease during this waiting window, you are not yet fully protected. For some vaccines, a second dose is required to boost this protection, and full immunity is often not achieved until a few weeks after the final dose in the series.
No, you do not need to fast or make any special dietary preparations before getting a vaccine. You can eat and drink normally. It is generally recommended to stay well-hydrated and wear clothing with short sleeves to make it easier to access the injection site. If you are currently feeling unwell with a moderate or severe illness, it is best to wait until you recover before getting vaccinated, but mild illnesses like a cold usually do not require you to postpone your appointment.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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