One of the most common questions parents have about vaccines is also one of the most reasonable: What’s actually in them?
It’s easy to feel overwhelmed when you see a list of ingredients you can’t pronounce or you’re scrolling Instagram and are inundated with strong opinions from both sides of the vaccine debate. The key is to understand what these ingredients are and why they’re used. After that, the picture becomes much clearer.
Here’s a breakdown of common vaccine ingredients and their purpose — in plain English.
1. The Antigen: What Your Body Learns To Recognize
This is the main ingredient. It’s a small piece of the virus or bacteria (or a blueprint of it) that trains your child’s immune system to recognize and respond to the real thing in the future — before it causes illness.
Different vaccines use different approaches like:
- Inactivated Viruses: ”Dead” or inactive version of the virus/bacteria that activates your immune response that will protect you if you are ever exposed.
- Protein Subunits: Purified pieces of proteins that stimulate an immune response.
- Live-attenuated: Weakened pathogens from a bacteria or virus that stimulate an immune response but don’t cause disease.
- Toxoid: Toxins made by the disease that target the toxic activity created by the bacteria instead of the bacteria itself.
- Viral Vector: Weakened or “harmless” versions of the virus that deliver the code to the immune system of the disease you want to fight to trigger an immune response.
2. Adjuvants: What Are They?
Adjuvants are ingredients added to vaccines to help the body build a stronger immune response. The most commonly used adjuvant is aluminum salts, which have been the topic of many conversations around vaccines recently.
Aluminum’s use in vaccines isn’t new — in fact, it’s been included since our grandparents or great-grandparents were children. The first study showing its effectiveness in boosting immunity was published in 1926, and aluminum-containing vaccines have been part of medical interventions ever since.
Despite recent attention online, the amount of aluminum in vaccines is extremely small — much less than what babies naturally ingest through food, water, or even breast milk.
The Children’s Hospital of Philadelphia has a great breakdown of how much aluminum is actually in vaccines, and what infants naturally ingest in their first few months of life.
While infants receive around 4.4 milligrams of aluminum from vaccines, they’ll naturally ingest more than that from food alone.
- Vaccines: 4-5 of 1,000 pieces
- Food via breastfeeding: 7 of 1,000 pieces
- Food via regular formula: 38 of 1,000 pieces
- Food via soy-based formula: 117 of 1,000 pieces
Aluminum is also the third most abundant element on Earth and is naturally present in our environment — including in the human body. So while the word may sound alarming, the numbers tell a different story.
Still curious? A large Danish study that followed 1.2 million children from 1997 to 2020 found no association between aluminum exposure (from vaccines or formula) and increased risk for more than 50 health outcomes including asthma, autoimmune issues, and neurodevelopmental delays.
If aluminum is something you’ve had questions about, you’re not alone. The goal isn’t to dismiss concerns — it’s to equip parents with accurate, well-researched information so you can make decisions with clarity and confidence.
3. Preservatives: To Prevent Contamination
Preservatives are used in multi-dose vials to stop bacteria or fungi from growing after the vial has been opened. One that’s received a lot of attention is thimerosal, which contains ethylmercury.
Most childhood vaccines today are thimerosal-free, and ethylmercury is different from the kind of mercury (methylmercury) that causes harm in large quantities — this is the kind that is found in fish like tuna, mackerel, and swordfish.
If you’re curious about what’s in each vaccine, you can ask your child’s doctor for the manufacturer’s insert — a document that lists all ingredients, including preservatives. And if you want to steer clear of thimerosal but still want your child to get a particular vaccine, just opt for single-dose vaccines.
4. Stabilizers: To Keep the Vaccine Potent
Stabilizers are added to vaccines to protect the active ingredients during manufacturing, shipping, and storage — especially if temperatures fluctuate. Common stabilizers include gelatin, sucrose (sugar), amino acids, and small amounts of MSG. These help prevent vaccines from breaking down or clumping, ensuring they remain safe and effective all the way to your child’s doctor.
For example, gelatin is often used in live-virus vaccines like MMR to maintain stability after freeze-drying. If your child has a known gelatin allergy, be sure to talk with your doctor — alternative options may be available. It’s also worth noting that the amount of these ingredients used in vaccines is extremely small, far less than what’s typically found in food. According to the FDA, vaccine stabilizers are widely studied and safely used in both medical and food applications, and serious reactions are very rare — less than 1 in 2 million doses, per the Children’s Hospital of Philadelphia.
When you know what’s in a vaccine, why it’s there, how long it’s been used, and what the science is behind its effects — you’re better prepared to ask thoughtful questions and make decisions with confidence. If your child has allergies or specific health concerns, it’s okay to ask your doctor about alternatives. That’s part of being an empowered parent.
Read more about the ingredients commonly found in vaccines, and check out our Vaccine Information Table for more info.