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Common Questions About Vaccines

Medically reviewed by: Elana Pearl Ben-Joseph, MD

Why Are Vaccines Important?

Vaccines have protected millions of kids and adults from dangerous diseases and saved countless lives. They help prevent infections in people and communities, and can even prevent some types of cancer. Thanks to widespread vaccination in the United States, childhood illnesses that used to be common (like measles and diphtheria) are rarely seen. Rates of certain cancers (such as cervical cancer) have dropped greatly since the HPV vaccine became available.

Some diseases are so rare now that parents sometimes ask if vaccines for them are still needed. But most diseases that vaccines prevent do still exist in the world, even in the United States, so vaccination is needed until a disease has been eliminated worldwide. This is what happened with smallpox, which no longer requires vaccination.

How Do Vaccines Work?

Most vaccines contain either a dead or weakened germ (or parts of it) that causes a particular disease. Some help the body to make a protein that looks like it came from a germ.

After someone gets a vaccine (called vaccination), their body practices fighting the disease by making that recognize specific parts of that germ. These antibodies last a long time in the body. This means that if the person is later exposed to the actual disease, the antibodies are in place and the body knows how to fight the disease so the person doesn't get sick. This is called immunity. The process by which vaccines make us immune or resistant to diseases is called immunization.

  • How Vaccines Help

    How Vaccines Help

    Vaccines keep millions of people healthy each year by preparing the body to fight illness. Learn how vaccines help and get answers to your biggest questions about vaccines.

Are There Other Ways to Become Immune?

A person can also become immune to a disease after they get infected with a germ (sometimes called "natural" immunity). As it does in response to vaccination, their immune system will recognize the germ and make antibodies to fight it. But becoming immune this way can make a person very sick.

Will the Immune System Be Weaker By Relying on a Vaccine?

No, the immune system makes antibodies against a germ whether it encounters it naturally(through an infection) or through a vaccine. Being vaccinated against one disease does not weaken the immune response to another disease.

Can a Vaccine Give Someone the Disease It's Supposed to Prevent?

It's impossible to get the disease from any vaccine made with dead (killed) bacteria or viruses or just part of the bacteria or virus.

Only those vaccines made from weakened (also called attenuated) live viruses — like the chickenpox (varicella) and measles-mumps-rubella (MMR) vaccines — could possibly make a child develop a mild form of the disease. But it's almost always much less severe than if a child became infected with the disease-causing virus itself. However, for kids with weak immune systems, such as those being treated for cancer, these vaccines may cause problems.

The risk of disease from vaccination is very small. One live virus vaccine, oral polio vaccine (OPV), is no longer used in the United States. OPV can be found in the spit and poop of people who got the vaccine and can then spread to others. The success of the polio vaccination program made it possible to replace OPV with the inactivated polio vaccine (IPV), which contains a killed virus form. This change means that vaccinated U.S. children cannot spread polio disease. OPV is still used in many other countries, though, with great success in reducing the number of cases worldwide.

Does My Child Need Vaccines if All the Other Kids in School Are Vaccinated?

It is true that a single child's chance of catching a disease is low if everyone else is fully vaccinated. But your child is also exposed to people other than those at school. And if one person thinks about skipping vaccines, chances are that others are thinking the same thing. Each child who isn't vaccinated gives highly contagious diseases another chance to spread.

Vaccination rates are fairly high in the United States, but there's no sure way to know if everyone your child has contact with has been vaccinated. That's especially true now that so many people travel to and from other countries. During travel, someone could be exposed to germs that are more common in other countries, such as measles or polio. They also could be exposed to people who got OPV in other countries, which can be risky for anyone who didn't get all their polio vaccines. So the best way to protect your kids is through vaccination.

Can Getting So Many Vaccines at One Time Harm My Baby?

Babies have stronger immune systems than you might think, and they can handle far more germs than what they get from vaccines. In fact, the amount of germs in vaccines is just a small percentage of the germs babies' immune systems deal with every day.

Sometimes, kids can have a reaction to a vaccine like a mild fever or rash. But the risk of serious reactions is small compared with the health risks from the often-serious diseases they prevent, and don't happen because the baby got several vaccines at once.

A lot of consideration and research went into creating the vaccine schedule most doctors use, and it has been proven safe time and time again. Still, some parents choose to use alternative schedules (spreading or "spacing out" vaccines) because they're concerned about the number of shots their babies get at each checkup. Doing this actually makes a baby more likely to get sick. Studies show that many babies on alternative vaccine schedules never get all the vaccines they need.

Plus, alternative schedules can mean that you'll have to take your child to the doctor — and your child will have to get a shot — more often.

Why Should My Child Get a Painful Shot if Vaccines Aren't 100% Effective?

Few things in medicine work 100% of the time. But vaccines are one of the most effective weapons we have against disease — they work in 85% to 99% of cases. They greatly reduce your child's risk of serious illness (particularly when more and more people are vaccinated) and give diseases fewer chances to take hold in a population.

It can be hard to watch kids get a shot, but the short-term pain is nothing compared with suffering through a potentially deadly bout of diphtheria, whooping cough, or measles.

Why Do Healthy Kids Need to Be Vaccinated?

Vaccines are intended to help keep healthy kids healthy. Because vaccines work by protecting the body before disease strikes, if you wait until your child gets sick, it will be too late for the vaccine to work. The best time to vaccinate kids is when they're healthy.

What Side Effects Can Happen From a Vaccine?

The most common reactions to vaccines are minor and include:

  • redness and swelling where the shot was given
  • fever
  • soreness at the site where the shot was given

In rare cases, vaccines can trigger a serious problem, like severe allergic reactions. If your child has a history of allergies to food or medicine, or has had a problem with a vaccine before, tell the doctor before your child gets any vaccines. Every year, millions of kids are safely vaccinated and very few have serious side effects.

Do Vaccines or Thimerosal Cause Autism?

No. Many studies have found no link between vaccines and autism. And a 2004 report from the Institute of Medicine (IOM) found that thimerosal (an organic mercury compound used as a preservative in vaccines since the 1930s) does not cause autism. Recent studies have not shown any cognitive and behavioral problems in babies who have gotten thimerosal-containing vaccines. And now, vaccines for infants and young children contain no or very little thimerosal.

Do Vaccines Cause SIDS, Multiple Sclerosis, or Other Problems?

There are concerns, many of which circulate on the Internet, linking some vaccines to multiple sclerosis, sudden infant death syndrome (SIDS), and other problems. To date, studies have not shown any connection between vaccines and these conditions. SIDS cases have actually dropped by more than 50% in recent years, while the number of vaccines given yearly continues to rise.

Why Do Kids Need Vaccines for a Disease That's Been Eliminated?

Diseases that are rare or wiped out in the United States, like measles and polio, still exist in other parts of the world. Doctors continue to vaccinate against them because it's easy to come into contact with illnesses through travel — either when Americans travel abroad or when people who aren't fully vaccinated come to the United States.

In recent years, there have been measles outbreaks in many different states, even though measles was declared eliminated from the U.S. in 2000. (Elimination means that the disease has not spread continuously for over a year, but it doesn’t mean there aren't outbreaks.) These cases were mostly among people who did not get vaccinated. Other preventable diseases that had recent outbreaks include whooping cough (pertussis), mumps, and hepatitis A.

It's only safe to stop vaccinations for a particular disease when that disease has been eliminated worldwide.

How Long Does Immunity Last After Getting a Vaccine?

A few vaccines, like the two for measles or the series for hepatitis B, may make you immune for your entire life. Others, like tetanus, last for many years but require periodic shots (booster shots) for continued protection against the disease.

The whooping cough (pertussis) vaccine also does not give lifelong immunity, and that may be one reason why outbreaks still happen. Whooping cough isn't a serious problem for older kids and adults, but it can be for infants and young children. Because of this, teens and adults now get a pertussis booster along with the tetanus and diphtheria booster (Tdap) — an important step in controlling this infection, particularly for pregnant women and other adults who will be around newborn babies.

It's important to keep a record of vaccinations so the doctor knows when your kids are due for a booster. Also make sure your kids get the flu vaccine each year. Last year's flu vaccine won't protect someone from getting the flu this year because flu viruses constantly change. The vaccine is updated each year to include the most current strains of the virus. The COVID-19 vaccine is also sometimes updated to protect against the newest variants.

How Are Vaccines Studied and Improved?

The FDA's Center for Biologics Evaluation and Research is the government agency that regulates vaccines in the United States. Working with the CDC and the NIH, they continuously research and monitor vaccine safety and effectiveness.

New vaccines are licensed only after thorough lab studies and clinical trials, and safety monitoring continues even after they're approved. There continue to be improvements that ease potential side effects and ensure the best possible safety standards.

Where Can I Get Affordable Vaccines for My Child?

Vaccines are one of the best tools we have to keep kids healthy. But they work best when everyone gets them. Health insurance plans are required to cover vaccines at no cost to the patient. Some plans only cover vaccines when they are given by a doctor or at specific locations. So check with your insurance company to make sure. You can also get inexpensive or free vaccines through many local public health clinics and community health centers, and campaigns to vaccinate kids often hold free vaccination days.

The U.S. government's Vaccines for Children (VFC) Program covers routine vaccines for children who are Medicaid-eligible, uninsured, underinsured or from Alaskan or American Indian populations. The vaccines are provided by the government and given in a doctor's office. But the doctor's visit itself is not covered (unless the child has insurance, including Medicaid). Some public health clinics may cover both the visit and the vaccinations.

Where Can I Learn More About Vaccines?

Read this article for details about each recommended vaccine. You also can visit the CDC's Vaccines & Immunizations website for more information about vaccines.

And talk with your doctor about which vaccines your kids need. Working together, you can help keep your family healthy.

Medically reviewed by: Elana Pearl Ben-Joseph, MD
Date reviewed: July 2024