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Frequently Asked Questions

Vaccines are one of the greatest public health success stories, but that doesn’t mean there aren’t important questions that parents have. We’re here to help with the frequently asked questions below.

General Vaccine Questions

There is so much information out there about vaccines that it can be hard to find facts that really answer a parent’s questions. Below you’ll find frequently asked questions about vaccines, and we encourage you to browse our science section to learn more about the science behind vaccines.

Today we are able to prevent more diseases through vaccines than we were thirty years ago. As a result, fewer children become ill or die from these diseases. Science and technology are constantly evolving so while children get more vaccines than they used to, they are actually getting fewer antigens (a foreign substance that helps the body create an immune response) total in all the vaccines they get today than they did from all the vaccines children got previously. 

Every disease we vaccinate against can be very serious and sometimes deadly. For example, even with treatment, tetanus kills at least 10% of victims.

But when a disease is not as deadly, complications from it can still be serious and lifelong. For example, one in every ten people who get measles will have complications. Those complications can be serious, like heart problems, vision loss, and infection of the membranes that surround the brain and spinal cord. For pertussis (whooping cough) about 1 in every 400 babies who survive pertussis will have permanent brain damage. Even for chickenpox, which is often thought about as only a nuisance can kill in rare occasions, but also lesions can become badly infected, and a chickenpox infection will leave you at risk for shingles later in life.

It’s not a good idea to skip vaccines. Vaccines prevent serious and sometimes deadly diseases, and without them, you leave your child and family vulnerable. Even if your child survives the disease, they can be left with a lifelong disability like heart or brain damage. If you have concerns about particular vaccines, talk to your doctor or ask us, we’re always happy to help.

Learn more:

The biggest problem with an alternative schedule is that it is difficult to predict if and when a vaccine preventable outbreak could occur which leaves a child vulnerable. The vaccine schedule was developed to provide children protection when they are most at risk of disease.

Science backs this up. There have been many clinical trials that show the schedule is both safe and offers the best protection for children. Aside from the scientific reasons, the vaccine schedule is also easier on a baby. Even though vaccines save lives, babies don’t understand that so well-baby visits can be stressful for them (and parents). By giving more than one vaccine at a visit, there are fewer visits and studies show this is less stressful for babies.

Key Resources to learn more:

No, in fact, the opposite is true. Countries with fewer recommended vaccines see increases in sickness and death from those diseases. Japan is a good example. In 1993 the Japanese government stopped recommending the MMR vaccination because of concerns about aseptic meningitis. However, aseptic meningitis actually increased among patients who contracted natural mumps (1.24%) compared to those who received the mumps vaccine (0.05%).

The World Health Organization states that not only do vaccines reduce disease and death, but they also reduce inequity which harms not just a child but the entire country. That is because children who are not protected from preventable diseases often fall sick and miss school. Additionally, if a child is permanently disabled from a preventable disease such as polio, it is harder for that child to find work as an adult. Learn more by reading the World Health Organization Bulletin.

There are a number of different types of vaccines that work in different ways, including;

  • Live “Attenuated” Vaccines: A vaccine containing a virus that is so weakened it cannot make you sick but your body can still recognize the virus and produce antibodies to fight it. Live vaccines include: rotavirus, MMR, chickenpox (varicella), nasal flu vaccines, and yellow fever.
  • Vectored vaccines: Sometimes a target antigen from a pathogen can be introduced to the body by using some harmless vehicle to get it there, such as another virus or bacterium which won’t cause disease in people. Vectored vaccines can also be made replication-incompetent so that once inside the body they cannot reproduce and thus cause disease or be passed to others. When vectored vaccines are replication-competent, they are considered a type of live attenuated vaccine. Examples of vectored vaccines include vaccines for Ebola and COVID-19.
  • Inactivated Vaccines: Vaccines made from a killed virus. These dead viruses look like the real thing to the body so that the immune system can remember how to fight off the real thing in the future. Because the virus is dead, it cannot make a person sick. Some inactivated vaccines are Polio (IPV), Hep A, rabies, and some of the flu shots.
  • Toxoid Vaccines: Toxoid vaccines create immunity to the toxins a bacteria release that cause the disease. That means the immune response is targeted to the toxin instead of the bacteria itself. Toxoid vaccines include the diphtheria and tetanus components of DTaP and TDaP.
  • Subunit/recombinant, polysaccharide, and conjugate Vaccines: These vaccines use specific pieces of a virus or bacteria — like their protein, sugar, or outside casing. Because they only use a fragment of the germ, they are considered very safe because these fragments are incapable of causing disease. They can also be given to almost everyone who needs them, including people with weakened immune systems and long-term health problems but often require boosters for long-term protection. These vaccines include: Hep B, Pertussis (part of DTaP and TDaP), Hib, Pneumococcal, meningococcal, and HPV.
  • mRNA Vaccines: mRNA teaches our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response protects us from getting sick if the real virus enters our bodies. mRNA vaccines do not affect or interact with our DNA in any way because they do not enter the nucleus of the cell (where DNA is stored). The cell breaks down and gets rid of the mRNA soon after it is finished using the instructions.

Different infectious diseases may require different approaches to keep people safe. The approach that works for one virus or bacterium antigen would not work for another.

Vaccine Development

Developing vaccines is a complex process that involves a number of steps. Read the most asked questions about vaccine development and then visit our vaccine development page for a primer to better understand the process.

Vaccines are some of the most tested medical interventions in the world. They go through a rigorous testing process that can take years with several phases of clinical trials and licensure. The process includes:

  • Exploratory Phase: This phase happens in research labs and works to identify potential antigens (a foreign substance that helps the body create an immune response that might prevent a disease) that the vaccine can use.
  • Pre-clinical Phase: In this phase, researchers take the antigen(s) they identified in the Exploratory Phase and test them in the lab (not on humans) to see if they work to produce an immune response.
  • Clinical Development
    • Phase I is a very small study (usually around 100 people) to determine if the vaccine is safe and to learn about what type of response the vaccine produces.
    • Phase II is a larger study with hundreds of participants that looks to learn more about safety, immunogenicity (how well the vaccine produces an immune response), how the vaccine fits within the immunization schedule, and the dose size.
    • Phase III is the largest of the phases, usually with thousands of participants that continue to measure the safety (rare side effects may not show up in small groups) and effectiveness of the vaccine. Despite the large size of these trials, exceptionally rare adverse events (those that occur every 1 per 100,000 doses or rarer) may still be missed.
  • Regulatory Review and Approval: If a vaccine is found to be safe and effective during the clinical development phase, then it is reviewed by the FDA for approval. The FDA will look at all the data compiled through the research phases to determine if it will approve the vaccine for use in the U.S.

In rare instances when there is a vaccine-preventable emergency, a vaccine will receive an Emergency Use Authorization (EUA) before licensure. Visit our vaccine development page or these key resources to learn more:

There is significant oversight for vaccine testing from many regulatory and government bodies. First, any clinical trial protocol must be approved by one or more Institutional Review Boards (IRBs). An IRB is a group of experts in an organization whose job it is to formally review and monitor biomedical research involving human subjects. In the U.S., the FDA sets the IRB process. In accordance with FDA regulations, an IRB has the authority to approve, require modifications, or disapprove research. There may be multiple reviews, for instance, each partner in a study may require its own IRB process and approval. The FDA also needs to approve a study. Additionally, there is independent monitoring of a clinical trial through a data safety monitoring board (DSMB) composed of experts that are not affiliated with either the pharmaceutical company sponsoring the trial or the FDA.

Key Resources to learn more:

When FDA temporarily allows a vaccine to be given to the general public before it has gone through the full licensure process. This can only be done during a public health emergency where the vaccine can reduce cases and death. The vaccine will still need to go through the full licensure process once the emergency is over if it will continue to be used.

Emergency Use Authorization takes a lot of time, money, and resources that are not sustainable in regular times. During an emergency, public health officials, researchers, and manufacturers will devote extra money and people to publishing and reviewing data, but these people are normally doing other important work and cannot be taken away from these tasks for non-emergency situations. When fighting a disease is important but not an emergency, we need to use our resources wisely and evaluate vaccines through the regular licensure process.

Vaccine Safety

Vaccines undergo a rigorous testing process but there are often questions about that process. Read the most asked questions below, and learn more on our vaccine safety page.

Every day a baby’s immune system comes in contact with thousands of antigens. Our immune system, even when we’re babies, is designed to handle lots of different germs at once. That is how a baby’s immune system learns to protect them from serious disease. In fact, more vaccines are recommended in infancy than any other time in a person’s life because that is the time when children are most at risk of getting sick or dying from preventable diseases.

But sometimes, a baby’s system cannot identify a disease and build protection against it fast enough and the baby can get very sick or die. That’s where vaccines come in. They allow a baby’s immune system to safely recognize a disease and build protection against it WITHOUT the risk of getting sick.

We have also been able to protect children against more diseases with fewer antigens over time, so we can be confident that the vaccine schedule can’t overwhelm an infant’s immune system, even if they feel uncomfortable for a short while after vaccination.

Key Resources to learn more:

Both vaccines and the vaccine schedule have been very well researched. Vaccine manufacturers have to do careful and thorough clinical trials on all vaccines. Then the FDA reviews all the data from the multiple clinical trials and decides whether or not to approve a vaccine. After FDA approval, ACIP experts look at studies to make sure the vaccine is safe to receive with other vaccines that are recommended at the same time. They will only recommend it as part of the schedule if that is true. Once they are added, vaccines continue to be monitored to keep an eye on any safety issues.

To understand more about the childhood schedule and safety you can read this study.

The Advisory Committee on Immunization Practices (ACIP) has 15 voting members who are responsible for making vaccine recommendations. The Secretary of the U.S. Department of Health and Human Services (HHS) selects these members based on a thorough application and nomination process. Fourteen members are medical experts. One member is a consumer representative.

ACIP also has 38 non-voting members. Eight are ex-officio members who represent other federal agencies involved in U.S. immunization programs. Thirty are representatives from liaison organizations with immunization expertise (like medical associations). There is only one non-voting liaison from the biotechnology industry.

All voting members have to file an ethics form and an annual financial disclosure report. At every ACIP meeting, each voting member must disclose any conflicts of interest. Members with potential conflicts cannot participate in discussions or decisions related to those issues.

Vaccines are very safe – the odds are approximately 1 in one million that someone will have a serious adverse reaction to a vaccine. But the Vaccine Injury Compensation Program (VICP) was created to make sure that, in the rare cases of serious vaccine adverse reaction, there is a way to file a claim and get compensation without having to go through the complex and costly legal system.

The VICP has paid out about $4 billion in compensation over the last 30 years. That might seem like a lot, however, it is a tiny amount compared to the number of vaccines that have been given since 1988. For every 1 million doses of vaccine distributed in the U.S., only about one recipient has been compensated (0.0001 percent). If we used the VICP number of cases compensated as proof, we could assume 99.9999% of vaccines that have been given did not result in serious adverse reactions. Key Resources to learn more:

Yes. Healthcare providers are required by law to report any adverse reactions that happen after immunization to the Vaccine Adverse Event Reporting System (VAERS). This system is used by the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) to keep an eye on vaccine safety and find possible safety problems as early as possible. Anyone can access the VAERS data (but no patient information is shared).

VAERS is not the only system used for vaccine safety monitoring in the US. The Vaccine Safety Datalink allows for tracking of adverse events among participating health centers and allows for direct comparisons between vaccinated and unvaccinated individuals. Because it contains private patient information it is not accessible to the public.

The CDC Immunization Safety Office regularly reports on vaccine safety to the Advisory Committee on Immunization Practices (ACIP). These reports are public, and anyone can see them.

When a vaccine produces a common, “expected” reaction, especially such as fever or sore arm at the injection site. Some vaccines are more reactogenic (meaning you can expect more side effects) than others due to, in part, what type of vaccine it is, and what antigens are in the vaccine. In addition, some vaccines include adjuvants – a substance added to vaccines to enhance the immune response, so less antigen is needed in each vaccine. Where and how a vaccine is administered also plays a part in a vaccine’s reactogenicity.

Vaccines are some of the safest and most effective ways to prevent serious illness but even so, there are some people who should not get certain vaccines. For instance, some live vaccines are not recommended for people with severely weakened immune systems (like people getting certain types of chemotherapy or those with certain genetic conditions). It’s important to remember:

  • If a person is not recommended to get a vaccine, they are still at serious risk from the disease which is why everyone around them should make sure they are vaccinated.
  • There are still some vaccines that are critically important for these people. Some vaccines are an absolute MUST to protect them from diseases that could be particularly dangerous for them, though they may not be as effective in them as in other patients.

Here are some people who should talk to their doctors before receiving certain vaccines:

  • People who have allergies to parts of vaccines (like eggs)
  • People with weakened immune systems
  • People who are pregnant
  • People on certain immune-suppressing medications

How Vaccines Work

How vaccines work can be a bit of a mystery, but we have answers to the most frequently asked questions below, and an overview of the immune system and vaccines work on the how vaccines work page.

Natural immunity develops when a person is exposed to disease and becomes infected. There are risks with this type of immunity because the disease can cause serious illness or death. In contrast, vaccine-acquired immunity occurs when a person receives a vaccine against a disease so their immune will respond if they are ever exposed to the pathogen that causes that disease. The vaccination will likely only cause mild side effects (like a sore arm) for a few days and go away on their own.

A baby is born with some immunity to diseases that they get from their mothers (maternal immunity), but that immunity wears off in the first few months of life. This leaves a child vulnerable to diseases that are very serious for babies, such as whooping cough, which is why the vaccine schedule recommends a number of infant and early childhood vaccines, as well as some vaccines in pregnancy as a first step towards protection of your child.

Hygiene and sanitation are important for controlling the spread of diseases such as typhoid or cholera. But looking at the actual rates of disease over the years, most vaccine-preventable diseases had a significant drop in cases only after vaccines were put into use, even in modern times.

For example, there were ups and downs in measles cases throughout the years, but the permanent drop only happened after the wide use of the measles vaccine beginning in 1963. Measles was declared eliminated in the U.S. in 2000, but we now have outbreaks again because of growing numbers of unvaccinated children.

Another example is the Hib vaccine, which was introduced in the early 1990s when we had similar sanitation to present day. After we started vaccinating babies with this vaccine, Hib disease went from affecting about 20,000 children every year in the U.S. to now almost none.

These experiences make it clear that disease cases would not be decreasing without vaccines. And, if we were to stop vaccinating, they would come back despite current sanitation standards. Key Resources to learn more:

Shedding happens when a recently vaccinated person transmits a live vaccine’s virus to an unvaccinated person. While shedding of this weakened virus after vaccination can occur, the risk of another person getting sick is very low. In fact, the Infectious Disease Society of America recommends that people living with someone who has a weakened immune system should still receive most recommended live vaccines to better protect their loved one.

Unvaccinated people, on the other hand, are vulnerable to vaccine-preventable diseases, which means they are more likely to get sick with viruses that can then infect and harm others. Several recent outbreaks of measles, whooping cough, and chickenpox have been traced to pockets of unvaccinated children:

Key Resources to learn more:

There are a number of reasons multiple doses of a vaccine may be needed:

  • Some vaccines (particularly inactivated vaccines) don’t provide as much immunity in the first dose. Subsequent doses help ensure your body has as much protection as possible.
  • Not all vaccines provide lifetime protection, so “booster” doses are needed to provide a reminder to the body to fight off the disease.
  • With live vaccines, some people don’t always produce enough antibodies with the first dose so a second dose is needed to ensure they are fully protected.
  • For the flu vaccine, a yearly dose is needed since the flu virus mutates every year so last year’s vaccine may not protect against this year’s flu.

You may have some protection if you do not finish the multiple dose schedule but not the best protection so your chances of getting sick are greater.

Vaccine Ingredients

Vaccines ingredients can be confusing. We have the most asked questions about ingredients below, and you can learn even more on our vaccine ingredients page.

Some vaccines contain aluminum salts because they are an effective adjuvant (a “booster” ingredient that helps a vaccine produce a stronger immune response so the vaccine works better). They have been used in vaccines for nearly a century without any apparent safety issues in that time.

Aluminum is naturally found in fruits, vegetables, meats, and fish – that’s because it’s the 3rd most abundant element on earth. The amount of aluminum found in vaccines is well within safe limits. For example, the amount of aluminum infants receive from vaccines in their first 6 months is about 4.4 mg. During the same six months, a breast-fed baby will typically receive more than twice the amount in a vaccine from their mother’s milk.

Some multi-dose vials of vaccines contain a type of mercury called ethylmercury, found in a compound called thimerosal, because it prevents dangerous microbes from growing in the vials once they are opened. In the past, before the use of thimerosal, there were tragic instances of people developing sometimes fatal infections after vaccination because of contamination of the vial. There has been concern about thimerosal because some people have confused it with a type of mercury (methylmercury) often found in fish, that can build up in your body and produce toxic effects at high doses. Ethylmercury, which is produced from thimerosal in the body, on the other hand, breaks down and is excreted from the body rapidly.

Many chemical compounds have complex and unfamiliar names that can confuse or concern people, even if they’re things we deal with on a daily basis. Thimerosal is actually an organic compound and has been used in vaccines, other drugs, and contact lens solution since the 1930s. Today, none of the childhood vaccines on the schedule contain thimerosal because vaccine manufacturers have found cost-effective ways to make single-use vials, but it is still found in some multi-dose flu vaccines. Thimerosal has been rigorously studied in the United States and many other countries and no apparent safety issues have been found, but for people who still have concerns about it, there are thimerosal-free vaccines available.

Formaldehyde is a naturally occurring compound. In fact, formaldehyde is essential in human metabolism and necessary for helping the body use DNA and amino acids. All people have detectable amounts of formaldehyde in their system.

Formaldehyde is also used when making vaccines to make sure the viruses or bacteria in the vaccine are inactivated and don’t cause sickness. Most of the formaldehyde is diluted during the process, but a trace amount may still be found in some vaccines. The amount of formaldehyde found in vaccines is safe and far less than what is found naturally in the body. An infant naturally has about 1500 times the amount of formaldehyde in their bloodstream than what is found in a vaccine.

Adjuvants help your body produce a stronger immune response, which means greater protection against a disease. Using adjuvants also means the immune system can produce more antibodies and longer-lasting immunity, so a smaller amount of the antigen needs to be used in the vaccine. Adjuvants are also critical in ensuring that the immune response is protective; if some antigens are introduced into the body without the addition of an adjuvant the immune system may instead learn to tolerate the antigen rather than work to combat it.

Adjuvants have been used in vaccines since the 1930s. Millions of Americans have safely received vaccines with adjuvants for decades. Like all vaccines, adjuvanted vaccines are rigorously tested in clinical trials to ensure they are both safe and effective. Once they are approved in the U.S. adjuvanted vaccines continue to be monitored by the CDC and FDA.

No, vaccines do not contain aborted fetus cells. The viruses used in vaccines are sometimes cultured in human cells, but there are two important things to know about these cells:

  • “Cultured in” is not the same as containing. Think about culturing as a garden. Often gardeners will use compost to help their vegetables grow. But when they eat the vegetables they have grown; they are only eating the vegetables and not the compost. Vaccines are similar to vegetables and what they are cultured in is similar to the compost. It helps them grow but is not in the vaccine.
  • The cells originally used for culturing some viruses came from two terminated pregnancies in the 1960s. The cells today are cultured in labs: a good way to think about them is that they are the great, great, great grandchildren of the original viruses that were grown in the 1960’s terminated pregnancies cell line.

Based on the concepts of “v’chai bahem” (which means you should live by the mitzvot or Torah commandments) and “pikuach nefesh” (holding saving a life as sacrosanct) most Hebrew scholars consider vaccines 100% kosher.

This question originally came up because of the gelatin used in some vaccines as a stabilizer. While this gelatin is porcine-derived (came from pigs), a vaccine is highly purified during manufacturing and is not taken orally. Jewish law has restrictions about non-kosher foods but does not forbid non-kosher products as long as they are not swallowed.

Some Muslim communities have questioned whether vaccine ingredients are halal. This concern has come up with some polio vaccines that are cultured in monkey cells. Think about culturing as a garden. Often gardeners will use compost to help their vegetables grow. But when they eat the vegetables they have grown; they are only eating the vegetables and not the compost. Vaccines are similar to vegetables and what they are cultured in is similar to the compost. It helps them grow but is not in the vaccine.

Most well-known Islamic religious scholars have issued Fatwahs (an Islamic scholar’s ruling) stating that vaccination is part of the Islamic duty to protect children.

False and Misleading Information

Misinformation is designed to deceive so the people who create untrue content work hard to make it believable. In addition to the frequently asked questions about disinformation below, visit our page on vaccine disinformation.

No. Vaccines are not big profit-makers for pharmaceutical companies. In fact, most of their profit is from selling drugs that treat diseases (including those prevented by vaccines).

Like any business, pharmaceutical companies must make enough profit from vaccine sales to justify investing the huge amounts of money, effort, and time it takes to develop vaccines. Vaccines can take up to 15 years to research and test. It costs a lot to produce and distribute vaccines, but they have limited demand (just a few doses) compared to other medicines. They are also heavily regulated. In the past, vaccines were so unprofitable that some companies stopped making them altogether.

Once global demand for vaccines began to grow in 2000, more pharmaceutical companies started making vaccines. The current global vaccine market is about $35 billion, which is less than 3% of the over $1.25 trillion global pharmaceutical market. To put this in perspective, the complementary and alternative medicine market (which includes mainly untested products and supplements) was $192 billion in 2018. Key Resources to learn more:

  • Lancet Article: Estimating the Cost of Vaccine Development
  • Atlantic Article: Vaccines Are Profitable, So What?

The US Government, specifically the Centers for Disease Control and Prevention, does not hold any patents on vaccines. This disinformation twists CDC’s altruism to ensure important biologics are accessible to scientists for free. Purdue University Biological Sciences Professor David Sanders explains it this way, “The CDC does hold some patents on life forms, but it generally does this for the common good, so a commercial company can’t come along and patent it. The CDC lets researchers work with the strain without fees.”

While there are some religious sects and leaders who have concerns about vaccination, no major religions prohibit vaccines. Some people use sincerely held religious beliefs to create concerns about vaccines. Some of these concerns span religions including Christianity, Islam, and Judaism. But the fact is that leaders across religions have all supported vaccination as part of their religion’s calls to care for others.

There are very few Christian sects that oppose vaccination because they believe only in faith healing, including Church of the First Born, End Time Ministries, Faith Assembly, and Faith Tabernacle.

Key Resources to learn more:

They aren’t. As the anti-vaccine movement started to grow in the 1980’s there was an increase of frivolous lawsuits against vaccine manufacturers. The time and money it took to fight these lawsuits made it difficult for the manufacturers to continue to invest in new vaccine development as well as manufacturing and distribution. The concern was companies would not be able to make vaccines, leading to dangerous outbreaks, particularly among the most vulnerable in our communities.

In order to protect vaccines as a critical public health intervention, the government passed the 1986 National Childhood Vaccine Injury Act and created the National Vaccine Injury Compensation Program (VICP). This program both protects vaccine manufacturers against frivolous lawsuits and provides an easier process for legitimate potential injuries. Now, any individual can file a petition without fees. They do not have to provide evidence of vaccine defects. There is a much lower bar to prove evidence of injury if that injury is included on what is known as the “table of injuries.” If someone is denied a claim through the VCIP, they may then sue the vaccine manufacturer directly. Additionally, a vaccine manufacturer can always be sued if there were problems with the manufacturing process or if the vaccines were improperly made.

No, doctors are not punished for speaking out against vaccines.

However, doctors do have ethical responsibilities that include providing medical advice based on accurate, scientific information and following evidence-based standards of care. Immunization is a standard of care supported by decades of scientific research and careful review by thousands of medical experts. If a doctor provides vaccine disinformation that leads to someone becoming seriously ill or disabled, or provides inappropriate medical exemptions for vaccination, they could face legal charges and be investigated by state medical boards.

Some insurance companies provide bonuses for high vaccine rates because preventing disease is much less expensive for them than treating disease. Knowing that doctors have limited time with patients, insurance companies want to encourage doctors to prioritize preventative care, like vaccines. Vaccine manufacturers do not offer any bonuses for high vaccination rates.

The immune system works in two ways: 1) by remembering diseases it has seen to fight them off quickly and 2) by triggering the body’s defense mechanisms (such as fever and sneezing) to get rid of a new invader in the body. Vaccines teach the immune system how to remember diseases, but no supplement can do this work. Additionally, there is no evidence to support that supplements trigger better defense mechanisms in the body.

There’s also something to be said for not boosting the immune system. People who eat well and live generally healthy lifestyles, and do not have any conditions that can weakened immune system already have well-functioning immunity that needs no boosting. An overactive immune system can cause issues such as allergies and autoimmune disorders.

Yes, all diseases we vaccinate against can cause serious illness, disability, and even death, especially for young children. There is no way to tell ahead of time if a person will get a mild or severe case.

Even diseases that people think are mild can be very serious:

  • Chickenpox (varicella) can have serious complications like pneumonia, encephalitis, and bloodstream infections. Before the vaccine, chickenpox sent 10,000 people in the U.S. to the hospital every year. It can also lead to shingles later in life, which can be very painful and cause long term nerve damage. It is additionally one of the most important causes of pediatric stroke.
  • Flu (influenza) can lead to pneumonia, bronchitis, heart attacks, and other infections. For people with chronic health problems, it can make their conditions worse. About 30,000 Americans die from flu every year.
  • Pertussis (whooping cough) can lead to pneumonia, seizures, brain damage, and death.

Visit Vaccines & Vaccine Preventable Diseases to learn more.

It can be difficult to know if a scientific finding is legitimate because unless you are a vaccine expert, the studies can be hard to understand and evaluate. Also, there are “junk” journals that publish sub-standard research – these journals may look legitimate but experts would not publish in them. The best way for parents to ensure the information they are looking at is good is to ensure the source is legitimate – a hospital, university, or government agency are all places for credible information.

Some helpful hints to determine if a website is trustworthy:

  • Is it free of advertising? Most legitimate sources will not “hawk” products.
  • Is it reviewed by experts in the field? Experts are scientists, doctors, and public health advisors who specialize in vaccines
  • Does it provide sources for its information? A credible website will tell you where it got its information and those sources will be experts in the field.

Key Resources to learn more:

Formaldehyde is also used when making vaccines to make sure the viruses or bacteria in the vaccine are inactivated and don’t cause sickness. Most of the formaldehyde is diluted during the process, but a trace amount may still be found in some vaccines. The amount of formaldehyde found in vaccines is safe and far less than what is found naturally in the body. An infant naturally has about 1500 times the amount of formaldehyde in their bloodstream than what is found in a vaccine.

Yes, the fact that vaccines are safe and effective is agreed upon by every credible medical authority in the United States and across the world. Almost every scientist who studies vaccines agrees with those authorities.

Vaccines are studied and tested more than other medical intervention. After they are approved for use, scientists continue to monitor vaccines in the rare case a serious adverse reaction should be discovered. To give a sense of how rare these cases are, only 1 in 1 million doses of vaccines might have a serious adverse reaction.

Because so much time and effort is devoted to studying vaccines and because scientists agree that they are safe and effective, we consider the science around vaccines to be settled.

Scientists can only prove what things ARE by studying what they observe. They cannot prove what things are NOT because things that are not, cannot be observed.

For example, a scientist cannot prove that something, say a unicorn, does not exist. They can only say that during their comprehensive research they have not observed any unicorns. But if a scientist DID see a unicorn they could definitely prove the existence of a unicorn. Now they may have millions of data points so they are confident that a unicorn doesn’t exist but they still won’t say definitively because you can prove the existence of what you can’t observe.

It’s the same for vaccines. In millions of instances, scientists have not observed any evidence that vaccines cause autism. So even though they are 99.999% certain, the rules of science mean that they will only say that they have no evidence to support that hypothesis.

This can be a good way to identify legitimate vs fake science. No legitimate scientist will say with 100% certainty that they can prove a negative (something they didn’t observe) and any “scientist” that claims to prove a negative will likely be criticized by their peers for breaking the rules of science.

Vaccine Side Effects

There is a lot of confusion about what side effects are and are not. Read the frequently asked questions below, and visit the vaccine side effects page to learn even more.

Some vaccines contain aluminum salts because they are an effective adjuvant (a “booster” ingredient that helps a vaccine produce a stronger immune response so the vaccine works better). They have been used in vaccines for nearly a century without any apparent safety issues in that time.

Aluminum is naturally found in fruits, vegetables, meats, and fish – that’s because it’s the 3rd most abundant element on earth. The amount of aluminum found in vaccines is well within safe limits. For example, the amount of aluminum infants receive from vaccines in their first 6 months is about 4.4 mg. During the same six months, a breast-fed baby will typically receive more than twice the amount in a vaccine from their mother’s milk.

Seizures in children are generally caused by two things:

  • Fever: Fevers can be common in children, and sometimes they result in temporary seizures, called febrile seizures. These are usually harmless and require no treatment. Some children get fevers after vaccination as part of the natural process of their body building immunity, and these febrile seizures can occur. Post-vaccine febrile seizures are rare. They tend to only occur in young children and infants, and do not cause lifelong seizure disorders. Children usually grow out of them by about age 5.
  • Genetic Disorders: Disorders such as Dravet Syndrome and other genetic epilepsies can also cause seizures. These can be lifelong disorders. In extremely rare instances vaccines can trigger Dravet Syndrome. But the reality is that unfortunately these children were likely going to get Dravets anyway due to their genetics.

The 1998 study that started this disinformation was investigated and found to be untrue. The doctor who led the study misrepresented the data and was found guilty of scientific misconduct. He lost his license to practice medicine, the paper was removed from the scientific record, and his co-authors took their names off the paper.

Since then, there have been hundreds of valid studies involving thousands of children in many countries looking at whether there is a connection between autism and vaccines. None of these studies has found any connection.

Two important studies include:

  • 1999 Brent Taylor study that compared children with autism and children without autism. The study found that the percentage of vaccinated children was the same in both groups.
  • 1991-1998 Denmark study by Madsen that included 537,303 children for a total of 2,129,864 person-years. The study found the risk of autism was the same in vaccinated and unvaccinated groups.

Here is a summary of studies looking at vaccines and autism sponsored or led by the CDC:

Ninety percent of SIDS deaths happen before a child is six months old. That is the same age when most children receive their DTaP series of vaccinations. Because of the similar timing, some people think vaccines cause SIDS. However, there is no link between the two. Many studies have been done in the last 20 years and none of them found a link between childhood immunizations and SIDS. In fact, several studies found children who recently received DTaP vaccines were less likely to die from SIDS. SIDS death investigations often find evidence of asphyxiation, meaning the child suffocated. The most common cause of SIDS-related asphyxiation is unsafe sleep practices such as blankets and toys in the crib, placing a child on their stomach to sleep, or bed-sharing with a parent. SIDS cases have dropped since safe sleeping campaigns (ABC: Alone, on their Back, in a Crib) began.

For a list of research and links to journal publications visit:

Pregnant people have been getting vaccinated for decades, keeping themselves and their unborn babies safe. There is a large body of research that has looked to see if there is any connection between vaccines and miscarriage and no link has been found. One study looked at flu seasons from 2012 to 2015 and found NO increased risk for miscarriage after influenza vaccination during pregnancy.

Not only are vaccines safe during pregnancy, but certain vaccines that those who are pregnant get can also provide immunity to their newborns. For example, if someone gets TDaP when they are between 27 and 36 weeks pregnant, their newborn will have some immunity to whooping cough, which is serious and sometimes deadly for babies. You can read the above referenced study here.

Recently, there has been some misinformation about vaccines and autoimmune issues. Some people incorrectly believe that because vaccines “train” the immune system to target pathogens, vaccines could also cause the immune system to target itself. But that’s not how vaccines work. Vaccines are “custom-made” to fit a specific pathogen to help the immune system recognize and fight it. They are like a puzzle piece–vaccines will not work unless they fit exactly.

While there have been some rare examples of Guillain-Barre Syndrome (GBS) following vaccination, studies generally show that there is no connection between vaccines and autoimmune diseases. The cause of GBS is not known but often occurs after a viral or bacterial infection. In some years, there have been GBS cases after the flu vaccine. However, it is important to understand the real risk. The studies show GBS after the flu vaccine occurs in 1-2 people out of 1 million vaccinated. But GBS after getting the flu occurs in 17 people out of 1 million infected. So, the risk of GBS is lower in vaccinated people compared to those who get the flu and because the flu vaccine prevents flu, a reasonable case could be made that it ultimately has a preventive effect against the development of GBS for most people. Key Resources to learn more:

This disinformation is another attempt to convince people that vaccines cause autism. The history of this false hypothesis stems from Andrew Wakefield’s discredited research. Wakefield falsely claimed that the measles vaccine virus was found in the intestines of autistic children. No scientist was ever able to recreate these findings and Wakefield’s paper has since been retracted and he lost his license to practice medicine because of his fraud.

What is true is that people with damage to their gut (from malnutrition, infectious disease, or overuse of antibiotics) are not always able to produce as strong an immune response from vaccines as healthy people. Additionally, there are vaccines that promote gut health. For instance, Hepatitis vaccines actually prevent some types of hepatitis that can damage the liver and affect gut health.

There have been a number of studies that show that good gut health is important in helping vaccines do their job. You can read one study here.

Vaccines are continually monitored even after approval, and the data show us that most vaccine side effects happen hours or days after vaccination, are minor, temporary, and go away on their own. In a very small number of cases, we have found extremely rare side effects after approval. We only find these side effects because they are so rare that millions of people must be vaccinated for even one of them to show up. If these side effects were long-lasting, we would find them through this ongoing monitoring.

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