Adult Vaccination

What do vaccines treat?

Vaccines are mostly used to prevent you from getting sick from viruses and bacteria. Some immunotherapies used to treat cancer are also called vaccines.

What’s the difference between vaccination and immunization?

Vaccination and immunization are sometimes used to mean the same thing, but they’re slightly different. Vaccination is when you actually get a vaccine. Immunization is the process that happens in your body that protects you from getting the illness you were vaccinated for.

How do vaccines work?

Vaccines work by training your immune system to recognize and destroy harmful invaders (pathogens) quickly, before they can make you sick. In some cases, they teach your immune system to recognize something already in your body that it didn’t know was harmful (like cancer cells) and destroy it.

Primary immune response

Vaccines trigger your primary immune response. When bacteria, a virus, or other pathogen gets into your body for the first time, your immune system has to recognize the threat and find the right tools to fight it off.

Your body first looks in its toolbox for a B-cell. Each B-cell is unique and fits a pathogen like a lock and key. The B-cell with the right “key” then makes a bunch of antibodies (immune system chemicals) that also have the key to that specific pathogen. Antibodies can use that key to grab onto harmful invaders so your immune system can destroy them. This is your primary immune response. While your immune system is mounting that response, you can get very sick.

Once your body has seen a pathogen, your body adds special immune cells (memory cells) to its toolbox that keep a lookout for the pathogen. The memory cells keep a kind of “wanted poster” of a harmful invader and know which tools (antibodies) to call in to fight off a future infection. Memory cells are very specific to the virus or bacteria they identify and destroy, which is why vaccines only give you protection against one specific illness.

Secondary immune response

The secondary immune response, or the second time your body sees the same invader, is much quicker. Your memory cells patrol your blood and are able to quickly respond to a repeat offender with a flood of antibodies. The invader is destroyed before it can make more copies of itself and make you sick.

Vaccines aim to trigger your primary immune response and generate memory cells without making you sick.

What are the types of vaccines?

There are several ways that vaccines train your immune system to fight off harmful invaders. These include using a weakened pathogen, an inactive pathogen, or parts of the virus or bacteria.

Live-attenuated vaccines

Live-attenuated vaccines use a weakened form of the entire bacterium or virus (whole agent). As it’s the closest to an actual infection, it causes the strongest response of all types of vaccines. You should only need one or two doses to have full protection for a lifetime.

Most people shouldn’t get sick from a weakened version of a bacterium or virus. But if you’re living with a condition that weakens your immune system, you should ask your healthcare provider before getting vaccinated with a live virus.

Examples of live-attenuated vaccines include:

• MMR (measles, mumps and rubella) vaccine.
• Rotavirus vaccine.
• Chickenpox (varicella) vaccine.

Inactivated vaccines

Inactivated vaccines contain dead bacteria or viruses. You can’t get sick from an inactivated pathogen. They’re safe for people with weakened immune systems, but don’t provide protection that’s as strong as a live vaccine does. You may need to get additional vaccinations over time to remain protected.

Examples of inactivated vaccines include:

• Polio shot (oral polio vaccines are live).
• Hepatitis A vaccine.
• Rabies vaccine.
• Flu shot (intranasal flu vaccines are live).

Subunit vaccines

Subunit vaccines use only part of a virus or bacterium. Similar to inactivated vaccines, subunit vaccines are safe for people with weakened immune systems but might require additional doses to keep you protected.

Subunit vaccines can take longer for scientists to create because they need to find the part of the pathogen that makes the most effective vaccine.

You might hear some subunit vaccines called recombinant vaccines. This describes how scientists made more copies of the part of the pathogen used in the vaccine.

Types of subunit vaccines include:

Protein subunit vaccines

The immune response to protein subunit vaccines is caused by specific proteins taken from a pathogen. Examples include:

• Shingles vaccine.
• Hepatitis B vaccine.
• HPV vaccine.

Polysaccharide vaccines

The immune response to polysaccharide vaccines is caused by chains of sugar molecules (polysaccharides) from a bacterium. Some pneumococcal vaccines are polysaccharide vaccines.

Conjugate vaccines

The immune response to conjugate vaccines is caused by a combination of a polysaccharide with a protein, like a toxoid (makes a stronger immune response). The Haemophilus influenzae type B (Hib) vaccine is a conjugate vaccine.

Toxoid vaccines

Toxoid vaccines don’t use any part of the pathogen to make the vaccine, but instead, use a weakened form of the toxin (toxoid) that some bacteria produce. Your body has an immune response to the toxin rather than the bacterium itself. You need booster shots to stay protected over time. Diphtheria and tetanus vaccines are examples of toxoid vaccines.

Nucleic acid vaccines

Nucleic acid vaccines use your own cells to make part of a virus or bacteria. They can use DNA or messenger RNA (mRNA) to do this, but current vaccines use mRNA. The vaccine contains the instructions that your body uses to make antigens (the unique part of the pathogen that your body recognizes as an invader). Your body has an immune response to the antigen and will remember how to attack it if it tries to infect you in the future. Just like the mRNA your body produces for its own instructions, the mRNA from the vaccines breaks down in your body in a few days. We have mRNA vaccines for COVID-19.

Now that scientists have the technology to create mRNA vaccines, they can be made for new pathogens fairly quickly — this is how the COVID-19 vaccines were made so fast. This is a huge advantage over vaccines that contain all or part of a pathogen.

Viral vector vaccines

Vector vaccines use a harmless virus (vector) to deliver the pathogen you want to be vaccinated against. Ebola vaccines and some COVID-19 vaccines are vector-based.

Cancer vaccines

Not all vaccines are used to prevent disease. Cancer vaccines, a form of immunotherapy, train your immune system to recognize and attack cancer that’s already in your body. As cancer cells often hide from your immune system, vaccines could be a way to get your immune system to notice them again and destroy them.

What is a vaccine made of?

Vaccines are made of the active ingredient (which causes the immune response that gives you protection), ingredients that help the vaccine work well and ingredients that prevent contamination and make the vaccine last longer. Sometimes, they’ll also have small amounts of products that were used to make the vaccine.

Vaccines can include:

• Antigen/active ingredient. These are usually a whole virus or bacterium (or part of it) or a weakened form of toxins made by a bacterium. Some vaccines use instructions (mRNA) for your body to make antigens themselves.
• Adjuvants. Adjuvants are substances that help the vaccine work well. The most common adjuvant is aluminum salts.
• Preservatives. Preservatives, like thimerosal, keep vaccines in multidose vials from getting contaminated. Most vaccines are preservative-free because they only have one dose in a container.
• Stabilizers. Stabilizers are used to make sure the active ingredient doesn’t break down or change during manufacturing and storage. Gelatin is a commonly used stabilizer.
• Products used to make the vaccine. Vaccines can contain small amounts of products used to grow a bacteria or virus, kill pathogens or toxins and keep vaccines from getting contaminated during production. Egg proteins and formaldehyde are both used to make some vaccines.

What vaccines do we use today?

Today the most commonly used vaccines include those against:

• Measles, mumps and rubella (MMR).
• Rotavirus.
• Polio.
• The flu.
• Meningococcal disease.
• Pneumococcal disease.
• Tetanus, diphtheria and pertussis (Tdap or Dtap).
• Chickenpox (varicella).
• Shingles.
• Haemophilus influenzae type b (Hib).
• Hepatitis A.
• Hepatitis B.
• HPV.
• COVID-19.

Procedure Details

Who should get vaccinated?

Almost everyone should get vaccinated against vaccine-preventable illnesses, unless you have a weakened immune system or your healthcare provider recommends that you don’t. If you have a weakened immune system, live vaccines can be dangerous. If you’re pregnant, live vaccines could pose a risk to the fetus. Ask your healthcare provider if it’s safe for you to get vaccines.

You usually get vaccines:

• During infancy and childhood.
• To maintain previous vaccinations (“boosters”).
• Seasonally, to protect against the flu.
• Over the age of 50, when you’re more likely to get certain diseases.
• When traveling to an area where certain illnesses are more common.

You can find out what vaccines are recommended for you based on age and risk factors from the Centers for Disease Control and Prevention/CDC (in the U.S.), the National Health Service/NHS (in the U.K.) or your local public health authority.

How do you get a vaccine?

Vaccines are often given as shots, but there are also oral and nasal vaccines.

• Injected vaccines. Your healthcare provider will clean your skin with an alcohol swab. They’ll give you a shot (inoculation) in your muscle, under your skin or, rarely, in between the layers of your skin. Most vaccines are intramuscular shots because some immune cells live in your muscle. They’re given in your arm because it’s close to lymph nodes, a key part of your immune system, in your armpit.
• Oral vaccines. Your healthcare provider gives you a liquid vaccine from a dropper in your mouth and you swallow it. Oral vaccines are useful for easily giving vaccines to large numbers of people, including those who are afraid of needles. They’re especially effective for diseases that infect your gut. But they’re difficult to make because they have to survive your stomach acid to cause a strong immune response.
• Intranasal vaccines. Your healthcare provider sprays a mist up your nose to deliver an intranasal vaccine. Intranasal vaccines cause an immune response in your entire body just like other vaccines, but can also give you extra protection with an immune response in the lining of your nose and airways. The only intranasal vaccine available in the U.S. is for the flu.

What’s a booster and why do I need it?

Some vaccines require more than one dose to be effective. This can be because:

• You need several doses, spread out by days or weeks, to reach full immunity.
• Some vaccines are very effective, but only for a limited time (usually a few years).
• Vaccines have been updated to be more effective.
• You have to get a flu shot every year because it’s made to prevent what experts think will be the most common type (strain) that year.

What is an immunization schedule?

An immunization schedule is a recommendation on when you should get vaccinated for each vaccine-preventable illness, usually based on age. You can find immunization schedules through the CDC (in the U.S.), the NHS (in the U.K.) or your local public health authority. The schedule will tell you how many doses you need and how far apart to get the doses.

Risks / Benefits

Why do we get vaccinated?

Most people get vaccinated so they won’t get sick with an infectious disease. Viruses and bacteria can not only make you feel terrible, but they can also cause serious complications, which can be life-threatening or long-lasting. But vaccines can also help protect the people around you — your loved ones, people in your community and around the world — by stopping the spread of an illness.

What are the benefits of vaccination?

There are personal, community and global benefits of vaccination. Vaccines:

• Protect you from infectious diseases. Infectious diseases can be mild or cause serious complications. Some are life-threatening.
• Protect others from infectious diseases. Some people, like infants and people with certain conditions, can’t get vaccinated against some diseases. They can get very sick if they get an infectious disease. If those of us that can get vaccinated, do, we can stop the spread of diseases and keep our loved ones and neighbors from getting sick.
• Protect you from serious illness. Sometimes, vaccines don’t keep you from getting sick altogether, but your immune system still mounts a quick response, keeping you from getting seriously ill.
• Reduce hospitalization. When many people get very sick all at once (an outbreak), hospitals can become overwhelmed with patients. That can mean that people who need medical treatment — for any reason — might not be able to get it. Vaccination reduces the number of people who get sick, spread illness and need to be treated in a hospital.
• Eliminate disease. If enough people are vaccinated or have had an infectious disease, it can’t spread and eventually won’t be able to find anyone to infect. This is called herd immunity. Even people who aren’t immune are protected by herd immunity. After a period of high vaccination, eventually, the disease won’t spread in an area or an entire country (though, you can still get the disease if you’re not vaccinated and travel to a place where it still spreads). This is called disease elimination. Polio, measles and rubella are considered eliminated in the U.S., but they’re still spread in other parts of the world. While a disease still exists, it can still spread in a place where it was eliminated if vaccination rates drop.
• Eradicate disease. If enough people are vaccinated or have had an infectious disease, it can’t spread and eventually won’t be able to find anyone to infect. This is called herd immunity. Even people who aren’t immune are protected by herd immunity. After a period of high vaccination, eventually, the disease won’t spread in an area or an entire country (though, you can still get the disease if you’re not vaccinated and travel to a place where it still spreads). This is called disease elimination. Polio, measles and rubella are considered eliminated in the U.S., but they’re still spread in other parts of the world. While a disease still exists, it can still spread in a place where it was eliminated if vaccination rates drop.

What are the risks of vaccination?

Vaccines are very safe. They go through extensive testing before they’re released to the public. But, like any medical treatment, vaccines can have side effects. Most side effects are mild and go away on their own. They include:

• Pain, redness or swelling at the injection site.
• Mild fever.
• Fatigue.
• Headache.
• Muscle aches.
• Joint pain.
• Nausea.
• Diarrhea.

Serious side effects are very rare. They include:

• Anaphylaxis (a severe allergic reaction).
• Guillain-Barre syndrome (a neurological disorder).
• Thrombosis with thrombocytopenia syndrome (TTS) (a blood clotting disorder).
• Myocarditis (inflammation of the heart muscle).
• Pericarditis (inflammation of the lining of the heart).

If you have a weakened immune system, you should talk to your healthcare provider before getting vaccinated. Some vaccines can make you sick if you have a weakened immune system.

What are the risks of not getting vaccinated?

If you don’t get vaccinated, you’re at risk of getting sick with an infectious disease. Some infectious diseases can be mild, but others can be serious or even life-threatening. If you don’t get vaccinated, you’re also at risk of spreading an infectious disease to others. This can be especially dangerous for people who can’t get vaccinated, like infants and people with certain conditions.

What are the risks of not getting a booster?

If you don’t get a booster, you’re at risk of getting sick with an infectious disease. Some infectious diseases can be mild, but others can be serious or even life-threatening. If you don’t get a booster, you’re also at risk of spreading an infectious disease to others. This can be especially dangerous for people who can’t get vaccinated, like infants and people with certain conditions.

What are the risks of not following an immunization schedule?

If you don’t follow an immunization schedule, you’re at risk of getting sick with an infectious disease. Some infectious diseases can be mild, but others can be serious or even life-threatening. If you don’t follow an immunization schedule, you’re also at risk of spreading an infectious disease to others. This can be especially dangerous for people who can’t get vaccinated, like infants and people with certain conditions.

What are the risks of not getting vaccinated for travel?

If you don’t get vaccinated for travel, you’re at risk of getting sick with an infectious disease that may be more common in the area you are visiting. Some diseases can be serious or even life-threatening. Additionally, you may be required to show proof of vaccination to enter certain countries, and failing to do so could prevent you from traveling or result in quarantine upon arrival.

When should I see my healthcare provider?

Talk to your healthcare provider about what vaccines they recommend for you. If you’re pregnant, are being treated for an ongoing health condition, or have a weakened immune system, ask your provider which vaccines are safe for you.

Contact your provider if you have any serious side effects after getting vaccinated.

Additional Common Questions

Why do I have to get vaccinated against uncommon diseases?

If you’ve never known anyone who’s had a particular disease, you might wonder why you should bother getting vaccinated for it. But think of it this way: The disease is uncommon because so many people have gotten vaccinated against it.

When enough people stop getting vaccinated or stop getting their kids vaccinated because they think they can’t get sick, the disease can begin spreading again. For instance, if someone who’s not vaccinated travels to a place where the disease still exists, they can bring it back to their community and cause an outbreak. Large outbreaks can overwhelm hospitals and put infants, older adults, and people with weakened immune systems at risk for life-threatening illnesses.

I got vaccinated and still got sick — why didn’t the vaccine work?

Vaccines don’t always fully prevent you from getting sick, but that doesn’t mean they didn’t work. Some reasons you might still get sick with a disease you were vaccinated for include:

• Some vaccines are very good at completely preventing disease. Others reduce the severity of illness instead.
• Pathogens can change (mutate) after you’ve been vaccinated, making it harder for your immune system to recognize and fight them off.
• Some people’s immune systems don’t develop a strong or long-lasting response to certain vaccines.

How long does it take to make a vaccine?

Vaccines can take decades to go from a theory in a lab to testing in humans. Once they get to human testing, it can take months or years to get approved. Vaccines go through the same testing that other drugs do. Several trials are required to find the safest dose of the vaccine, how effective it is, and whether there are serious side effects.

What was the first vaccine?

There’s evidence that people have been inoculating against diseases since as early as the 10th century, but the first modern version of a vaccine was for smallpox. Back in the 1700s, people started noticing that people who had cowpox didn’t get smallpox. They found ways to infect people with the pus from cowpox to prevent them from getting smallpox, a much more serious illness. From there, Edward Jenner created the first vaccines for smallpox. Smallpox no longer exists (it’s eradicated) because of vaccinations.