Facts About Vaccines, Vaccine Ingredients, and Vaccine Safety

There are a lot of myths and misconceptions about vaccines, many of which are fueled by anti-vaccine conspiracies circulating on social media. These messages can be as convincing as they are scary, and education is your best defense against the confusion that vaccine misinformation can cause.

Parents holding baby girl while pediatrician prepares vaccination
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By understanding how vaccines work and what they can achieve, you can begin to separate fact from fiction and make informed choices for yourself and your family.

Here are 10 facts about vaccines and vaccinations that can help.

Vaccines Work

Fewer medical discoveries have had as much of an impact on public health as vaccines.

Before vaccination, diseases like diphtheria infected millions and killed tens of thousands each year. Those who survived were sometimes left with lifelong disabilities. To date, there have been only two cases of diphtheria reported in the United States, one in 2004 and the other in 2015.

Even as improved sanitation and interventions like antibiotics helped rein in diseases like rubella (German measles) and polio, it was vaccines that helped consign them to the history books. Today, fewer than 10 people get rubella in the United States per year, while the last case of polio originating in the U.S. was reported back in 1979.

A review of studies published in JAMA highlights how vaccines have turned the tide on many once-dreaded diseases.

Vaccines Protect Everyone

Vaccines work in two ways: by protecting the individual from disease and protecting the community as a whole from the spread of infection. When enough people are immune to a disease, germs can’t spread from person to person. This is referred to as herd immunity.

Achieving herd immunity is crucial, as not everyone in a community is a candidate for vaccination. Sometimes, a severe drug allergy might prevent someone from getting a vaccine. Others may be immunocompromised and advised to avoid the MMR (measles, mumps, and rubella), varicella (chickenpox), or rotavirus vaccines.

Similarly, babies may be too young to receive certain vaccines and are best protected when people around them are properly vaccinated.

By reducing the level of infectivity within a community with widespread vaccination programs, susceptible people—even those who refuse vaccines—are protected.

Vaccines Are Safe

Vaccines are among the most extensively tested medical products in the U.S., undergoing rigorous safety tests before being granted licensing by the Food and Drug Administration (FDA).

The process involves years of clinical trials conducted in phases with increasing numbers of participants of different ages, races, and health conditions.

Even after FDA licensing has been granted, researchers and regulating authorities continue to monitor for any report of adverse events. This includes issues lodged with the Vaccine Adverse Event Reporting System (VAERS).

If a pattern of adverse events is noted, the Centers for Disease Control and Prevention (CDC) and others investigate to confirm whether or not the issues were caused by the vaccine. If so, the recommendations for the vaccine might change or the vaccine may be pulled from the market.

Case in Point: Oral Polio Vaccine (OPV)

Such was the case with the oral polio vaccine (OPV) a live vaccine first introduced in the 1960s. In the late 1990s, after reports that a small number of children developed polio-related paralysis after receiving OPV, officials from the CDC and World Health Organization (WHO) launched an aggressive investigation.

It was soon found that the condition—referred to as vaccine-associated paralytic polio (VAPP)—was rare, affecting one out every 2.7 million OPV recipients. However, it was enough to compel the CDC to alter its recommendations and advise the use of the older inactivated polio vaccine (IPV) over OPV.

Checks and balances of this sort of help ensure that vaccines remain safe for the vast majority of people for whom they are recommended.

From 1955 to the present, the CDC has reported 10 incidences in which an adverse event triggered a major revision in vaccination recommendations or a recall.

Serious Side Effects Are Rare

Anti-vaccine proponents will often point to the list of side effects a vaccine can cause as a reason to avoid vaccination. And while these side effects are very real, the incidence of serious ones is extremely low.

In the end, any vaccine or drug can cause side effects, but those associated with FDA-approved vaccines tend to be mild and resolve within a day or so.

Common side effect of vaccines include:

  • Injection site pain, redness, swelling, or bruising
  • Fatigue
  • Headache
  • Nausea
  • Diarrhea

Severe ones like anaphylaxis are rare. In fact, this potentially deadly, whole-body allergy almost never happens.

According to a 2016 study in the Journal of Allergy and Clinical Immunology, only 33 cases of anaphylaxis were reported out of over 25 million doses of childhood vaccines.

Vaccines Keep Diseases at Bay

Because vaccines have been so successful, it is easy to forget just how dangerous vaccine-preventable diseases can be.

Even chickenpox—considered a rite of passage for some generations—is far from harmless. Before a varicella vaccine was available, the virus was responsible for roughly 11,000 hospitalizations and more than 100 childhood deaths in the United States each year.

Without high vaccination rates for chickenpox and other diseases, infections that have been brought under control could very well make a comeback.

Health officials have begun to see such an occurrence with measles, a disease that was declared eliminated in the United States back in 2000.

With the rise in anti-vaccine sentiments and a decline in vaccination rates, measles outbreaks have reemerged in pockets throughout the United States. The largest outbreak occurred in 2019, resulting in nearly 1,300 infections in 31 states.

Vaccine Schedules Protect Kids

Vaccination schedules in the U.S. are issued by a panel of experts within the CDC called the Advisory Committee on Immunization Practices (ACIP). The committee meets on a regular basis to review studies addressing the safety and efficacy of vaccines, including when to use a vaccine to ensure the greatest benefits.

Although some people will claim that the schedule is only a rough guideline—or worse yet, suggest that it exposes kids to "too many" vaccines—the ACIP recommendations are there for a reason.

For instance, hepatitis B vaccination is performed at birth because nine out of 10 newborns infected with the virus will remain infected for life (as opposed to only 2% to 3% of infected adults).

Similarly, the human papillomavirus (HPV) vaccine is given at ages 11 to 12 to prevent infection from a virus that is primarily sexually transmitted. By vaccinating children before most are sexually active, parents can dramatically reduce their child's risk of cervical cancer and anal cancer (90% of cases are caused by HPV).

Alternative Schedules

Over the years, experts like "Dr. Bob" (Robert Holbrook Smith, MD) have proposed alternative schedules based largely on supposition and unsupported claims. Others have suggested that it is better to expose a child to something like chickenpox so that the body can build immunity "naturally."

By modifying the ACIP schedule or skipping recommended doses, your child may not be fully immunized against vaccine-preventable diseases—some of which can cause serious, long-term harm.

Live Vaccines Are Safe

Live attenuated vaccines are vaccines made with live, weakened viruses. Although they are still alive, the viruses do not have the ability to cause disease.

Of the 15 childhood vaccines recommended by the ACIP, four are live attenuated vaccines:

  • Measles, mumps, rubella (MMR) vaccine
  • Varicella (chickenpox) vaccine
  • Influenza nasal spray vaccine (FluMist)
  • Rotavirus vaccine

Live attenuated vaccines have benefits over inactivated (killed) vaccines in that they exhibit the behaviors of the natural virus. As such, they target the same cells that they would otherwise infect and trigger a much stronger immune response.

Inactivated vaccines tend to be more passive and require regular boosters to maintain immunity.

Potential Risks

In rare instances, a live virus vaccine might revert to its natural virulent state and cause the disease it was meant to prevent. This typically occurs in people who are immunocompromised and aren't able to mount a robust defense.

Even so, the risk of this occurring is low—as evidenced by the low incidence of VAPP previously mentioned.

Some anti-vaccine proponents also claim that live vaccines can increase the risk of infection due to viral shedding. This is a very real phenomenon in which the body releases viral particles soon after receiving a live vaccine.

But, with rare exceptions, the level of shed virus is not substantial enough to establish an infection. The only exception may be the oral polio vaccine no longer used in the United States (and even then, the risk was considered negligible).

Live vaccines are typically avoided in immunocompromised people, including those with HIV, organ transplant recipients, children with primary immunodefiency, and people undergoing chemotherapy.

Vaccines Don't Contain Toxins

While some vaccine ingredients might sound scary, studies show that they are not only safe but can make the vaccines more effective. The ingredients are not unwanted byproducts of the manufacturing process but rather serve distinct purposes.


Some vaccines contain a mercury-containing substance called thimerosal that is used as a preservative. Prior to the introduction of thimerosal, multi-use vials were frequently contaminated with bacteria between injections, posing health risks to vaccine recipients.

As opposed to methylmercury, the toxic substance found in seafood, thimerosal is ethylmercury, a type of mercury that is quickly broken down and eliminated from the body.

Today, most vaccines (with the exception of certain flu and tetanus vaccines) are packaged in single-use vials or prefilled syringes that do not contain thimerosal.

Part of the reason for the change was the perception that thimerosal was "harmful." According to the CDC, ACIP, FDA, National Institutes of Health (NIH), American Academy of Pediatrics (AAP), and Institute of Medicine (IOM), thimerosal is not only safe but beneficial to vaccine delivery.


Aluminum salts are sometimes added to vaccines to make them more effective. When used in this way, they are referred to as adjuvants, meaning substances that amplify the immune response to bolster the vaccine's effects.

Aluminum salts have been used as adjuvants for over 70 years and have a proven record for safety. Among the vaccines that contain an aluminum adjuvant are the DTaP, Tdap, hepatitis A, hepatitis B, Haemophilus influenzae type b (Hib), and pneumococcal vaccines.

Like ethylmercury, aluminum is processed quickly by the body. Moreover, aluminum is ubiquitous in the environment. There is, in fact, more aluminum in breast milk and baby formula than there is in vaccines.


Formaldehyde is sometimes used to "kill" viruses to produce inactivated vaccines or to deactivate toxins to make toxoid vaccines (such as for the tetanus toxoid used to prevent tetanus).

Formaldehyde is a naturally occurring substance found in the air and environment. An average 2-month-old baby has around 1.1 milligrams of formaldehyde in its body, while older children and adults have far higher amounts.

In industrial settings, formaldehyde is considered safe at levels of up to 0.75 parts per million (ppm) per day. The highest amount found in vaccines after purification is 0.2 ppm per dose; most fall well below this.

Vaccines Don't Cause Disease

Even beyond the myth that the flu vaccine causes flu, there are vaccine detractors who have claimed that vaccines can cause everything from miscarriage to birth defects.

MMR Vaccine and Autism

One of the myths that has gained traction over the years is that the MMR vaccine causes autism. This myth has roots in a 1998 article published in The Lancet in which it was suggested that 12 children experienced colitis and the onset of autism after receiving the MMR vaccine.

In the wake of the publication, the lead author, Andrew Wakefield, publicly connected the MMR vaccine with autism, sparking a frenzy of media coverage. It was later discovered that the 12 children were handpicked to support the findings and that not all 12 had received the MMR vaccine.

Wakefield was soon after stripped of his medical license due to the manipulation of the research. But, even with the retraction of the Lancet article in 2010, the aftermath of Wakefield's claims still resonates today.

HPV Vaccine and Ovarian Failure

A similar claim was made against the HPV vaccine in 2014 when a team of researchers in Australia suggested that the vaccine caused ovarian failure in three girls, ages 16 to 18.

Despite failing to explore other possible causes for the condition, the researchers were quick to proclaim that the vaccine surveillance measures "do not accurately represent diagnoses" of adverse events.

The claims were debunked in 2018 when a study published in Pediatrics reported no increased risk of ovarian failure among 199,078 Australian women immunized with the HPV vaccine compared to the general public.

Some health experts argue that these well-publicized cases can alter the acceptance of vaccines among people who need them.

In the United States, fewer than half of teens and young adults are vaccinated against HPV, compared to Australia where 80% of girls and 76% of boys are vaccinated.

Most Parents Vaccinate

While anti-vaccine advocates get a lot of media attention, the truth is the majority of parents trust public health recommendations and vaccinate their children.

In 2019, the CDC reported that vaccine coverage in children 24 months of age and younger was high. Among the findings:

  • 92% received the oral polio vaccine
  • 90.8% received the MMR vaccine
  • 90.6% received the hepatitis B vaccine
  • 90.2% received the chickenpox vaccine
  • 81% received the pneumococcal vaccine
  • 80.7% received the DTP/DT/DTaP vaccine (four doses or more)
  • 72.1% received the rotavirus vaccine

Even so, only 68.2% received all seven vaccines, a drop from previous decades.

A Word From Verywell

Despite what some people may tell you, the harms caused by not getting vaccinated are far worse than any side effects a vaccine might cause.

With tetanus, for example, your likelihood of death is high once symptoms develop. Similarly, around one in four people hospitalized for meningococcal meningitis will die, while 20% to 40% of those who survive will experience some level of brain damage.

By getting vaccinated for these and other diseases, your risk of such harm can be reduced and, in some cases, eliminated.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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