By KidsDirect Editorial Team
Every vaccine label lists a handful of extra ingredients beyond the actual germ fragment doing the work. None of them are there by accident. Preservatives keep bacteria from growing in the vial. Stabilizers keep the vaccine effective through shipping and storage. Adjuvants help the immune system respond more strongly, and trace antibiotics are sometimes left over from how the vaccine was made. Every one of these shows up in tiny amounts. A couple of them, thimerosal and aluminum in particular, get more attention than the rest. Here’s what’s actually going on with each category.
Antibiotics
Antibiotics keep unwanted bacteria from growing while a vaccine culture is being produced. Neomycin is the one used most often. Polymyxin B and streptomycin show up in smaller amounts in a few vaccines too. Penicillin and related antibiotics are never used in vaccine production, since so many people carry a penicillin allergy.
Preservatives
Multi dose vials need a preservative. Every time a needle goes back into that vial, there’s a small chance of introducing contamination. Thimerosal used to be the standard choice for this job, and it’s genuinely effective at the task. It’s also been the most debated ingredient on this list, mostly because close to half its weight is mercury.
Here’s where things actually stand today. Thimerosal was already removed from every routine childhood vaccine over two decades ago, with one exception. Some multi dose flu vaccine vials still used it. In the summer of 2025, federal health officials moved to eliminate it from flu vaccines too. That shifted the entire US flu vaccine supply to single dose, mercury free vials. At this point, thimerosal isn’t used in any vaccine given in the United States.
Formaldehyde, sometimes labeled formalin, plays a different role. It’s used during manufacturing to inactivate the virus in the polio vaccine, and to kill off bacteria in the cultures used for other vaccines. Only a trace amount ever makes it into the final dose. That amount is actually less than what your body already produces on its own through normal metabolism.
Stabilizers
Stabilizers protect a vaccine from losing effectiveness during shipping, storage, or exposure to light and heat swings along the way. Depending on the vaccine, that job gets done with sugars like sucrose, amino acids like glycine, or proteins like gelatin. Sulfites and MSG show up in a handful of vaccines for the same reason. All of these are ingredients your child has almost certainly already encountered plenty of times through ordinary food.
Adjuvants
Adjuvants help a vaccine trigger a stronger, longer lasting immune response, which sometimes means fewer doses are needed overall. Aluminum salts are the adjuvant used most often. They show up in vaccines like DTaP, Hepatitis B, and some pneumococcal and HPV formulations.
Aluminum has decades of safety data behind it. That includes a 2025 Danish study following over a million children, which found no link between aluminum containing vaccines and autism or food allergies. It’s also become one of the more actively discussed ingredients lately. Federal vaccine advisors have been reviewing the evidence around it more closely as of late 2025. As of now, no changes have actually been made to how aluminum is used in vaccines, and the established safety record hasn’t shifted. It’s worth keeping an eye on this one going forward, since it’s a live conversation rather than a settled one.
None of these additives show up in more than trace amounts. Each one is there to do a specific job rather than sitting around as filler. If you want the bigger picture on why vaccines work the way they do, our guide on how vaccines work is a good next stop. Our vaccine FAQ covers a lot of the other questions parents bring up most. For the full schedule of what’s recommended and when, see our complete vaccine timeline.
Some of the background here was first published in Vaccinating Your Child, an early 2000s parenting book by pediatrician Dr. Sharon G. Humiston and journalist Cynthia Good, and has been updated with current FDA and CDC information.
