We know EXACTLY what’s in vaccines – because we put it there
Vaccine scientists and regulatory agencies know exactly what is in vaccines and why, no matter what anti-vaccine activists try to claim.
This newsletter is free, but it’s able to sustain itself from support I receive from a small percentage of regular readers. If you value science-based information, consider upgrading to a paid subscription:
As a biomedical scientist who’s worked in academia and industry, and is directly involved with regulatory processes, one of the most frustrating—and false—talking points in the anti-science disinformation world is that “no one knows what’s in vaccines.”
This claims started with people like RFK Jr—our likely next head of Health and Human Services—and gained traction among celebrities (thanks Jenny McCarthy, Joe Rogan, Jim Carrey, et al.), wellness influencers, and others who are not only unqualified to speak on biopharmaceutical manufacturing, but ironically, are often selling products that we actually don’t know what’s in them.
That even includes healthcare professionals who spread anti-vaccine rhetoric. Because while healthcare professionals should be immersed in science through their career of practicing medicine, they are not the scientific experts on vaccines.
Biomedical vaccine scientists know exactly what’s in vaccines.
That’s because we’re the ones who make them. We don’t throw a bunch of random substances in a bioreactor and hope for the best. Vaccine research, development, and manufacturing is one of the most regulated and scrutinized things in our society. (That’s true for all FDA-regulated medicines)
Every ingredient in a vaccine serves a purpose, is rigorously tested, and is there for a reason. Regulation of vaccines extends to more than just the final medicine—the entire process of making a vaccine is regulated.
That means everything: raw materials, lab equipment, reagents, notebooks, data recording, lab personnel, and more. There are requirements for safety and stability studies, data requirements if a change is made, and more. Labeling is regulated too: everything on those inserts that anti-vaccine activists try to weaponize is there for a specific, and regulated, reason.
I want to break it down so you understand how rigorous this process really is. I hope it alleviates skepticism you may have so you understand how we know vaccine ingredients are safe, effective, and necessary.
Vaccine ingredients are publicly available for every brand of every vaccine—they are not a big secret.
The annual flu vaccine is one long plagued with misinformation and misconceptions. We are in the thick of flu season and are trailing our historically low flu vaccine rates, so maybe this will encourage some folks to go get that vaccine.
If you want more information on influenza viruses, why we have a new vaccine every year, and the importance of the vaccine, read this article I wrote previously:
Let’s dissect the flu vaccine ingredients like a biomedical scientist:
The goal of a vaccine is to train your immune system before you are exposed to a pathogen that can cause illness. As such, vaccines must contain several ingredients that assist in the generation of that memory immune response, but also ensure the medicine is safe, stable, and deliverable.
Each year, our [injectable] flu vaccine includes key ingredient categories:
Active ingredients—the antigens themselves that are recognized by the immune system as foreign and facilitate the “training” of memory immunity
Adjuvants—non-antigen ingredients that amplify immune response without increasing the dose of active ingredient (for flu vaccines, this is only in the Fluad vaccine for adults 65+)
Preservatives, buffers, and stabilizers—these ensure the vaccine doesn’t get contaminated, that ingredients don’t degrade, that it is safe to be administered, and that the solution isn’t compromised in any way.
Residuals—trace leftovers from the many purification steps in the manufacturing process. Analytical chemistry instrumentation can detect the tiniest levels of these—and even those miniscule quantities are assessed and regulated for safety.
Injectable flu vaccines include inactivated flu viruses as the active ingredient
Vaccines include a safe version of a pathogen or components of a pathogen that elicit memory immunity—that’s the active ingredient.
These are antigens—molecules our immune system recognize that lead to antibody production by B cells and the creation of memory T and B cells. For influenza, key antigens are the proteins hemagglutinin (HA) and neuraminidase (NA) on the outside of the virus.
Inactivated flu vaccines use whole virus, but it is “killed” so it no longer functions. It cannot infect cells or replicate. That means we have to grow up lots of virus in host cells.
Influenza virus is grown in either cell culture (in vitro) or in chicken eggs (in ovo). Note: a protein-based flu vaccine, Flublok is less common, so we won’t focus on that today.
In ovo: Flu virus is injected into the embryo of a chicken egg, incubated for several days to allow the virus to replicate, and then fluid from inside the egg is harvested to extract virus.
In vitro: Flu virus is grown in mammalian cell lines in Petri dishes. Specifically, MDCK cells (Madin-Darby Canine Kidney cells) are used. These are cells isolated from a Cocker Spaniel kidney back in 1958. Similar to in ovo, flu virus is added to the media of MDCK cells, incubated, and allowed to replicate. As virus grows, it is released into the growth media which is harvested.
Once virus is harvested, they are rendered inert—preserving the structure of viral proteins but halting processes that allow the virus to infect and cause illness. While some say “killed,” viruses are not living in the traditional sense, so the most appropriate term is inactivated.
Either β-propiolactone (BPL) or formaldehyde are used, as they modify viral structures so that they can no longer replicate. BPL binds directly to the RNA of flu viruses and inhibits the functionality of the genetic material.
Formaldehyde is a cross-linker, meaning that it binds adjacent molecules—proteins, nucleic acids—and forms a bridge between them which inhibits their function.
Following inactivation, the viruses are extracted, purified and filtered to remove unwanted materials, including residual formaldehyde or β-propiolactone. In certain flu vaccines, harvested inactivated virus is broken apart into pieces using a detergent, octylphenol ethoxylate (also called Triton X-100), which is also purified out.
In every 0.5 mL flu vaccine, 15 micrograms (µg) of hemagglutinin (HA) protein is included per strain, for a total of 45 µg of HA in this years trivalent vaccine (since it contains three strains).
The flu virus strains are selected by scientific experts: we aren’t “guessing” at that, either.
Flu viruses for vaccine formulation are based on global surveillance of dominant flu strains for a given year. Scientists track the genetic prevalence of different flu strains over the course of several years to assess which strains are less common, which are more common, and which are growing in prevalence. This informs which strains should be included in the upcoming vaccine.
Every year, after strains are selected, vaccine manufacturer reports to regulatory agencies the following for approval:
Strain composition data.
Growth conditions and purification data showing exact production methods.
Inactivation process validation, proving that all live viruses have been killed.
Batch-to-batch consistency data, demonstrating each dose contains 45 µg of HA ± 10%. A deviation in purity or quantity would require additional FDA review.
ANY change from the approved vaccine process—even if it’s a tiny change in a lab condition—has to be studied, justified, and reported to regulatory agencies—with data for approval.
An example? If the flu virus inactivation process normally takes place at 35°C for 24 hours, but a vaccine manufacturing team finds that tweaking it to 36°C for 22 hours improves the purity of the virus that is harvested, the scientific team must conduct extensive testing, document the impact on the final product, demonstrate comparability (we call this equivalent and/or better) and submit a report to the FDA.
If there is ANY concern, FDA (or other regulatory agencies) can request additional data, comparability testing, or even clinical trials before approval.
So yes, not only do we know which active ingredients are in vaccines, we know every single little condition that was used to produce them.
Adjuvants are only in certain flu vaccines, like Fluad for older adults
Adjuvants are non-antigen substances that augment the immune response. They are not related to a pathogen the vaccine is eliciting protection against, but they increase the responsiveness of memory immunity.
Most flu vaccines don’t have an adjuvant, but Fluad, a flu vaccine for older adults, does. It contains MF59, which is a milky white emulsion of oil and water. We also know the composition of MF59, because it’s been studied, published, and approved for use.
99% of MF59 is water. MF59’s full ingredient list includes:
Squalene, 9.75 mg. Squalene is a lipid found in human sebum, the oily secretion produced by your sebaceous glands.
Tween 80 (also called polysorbate 80), 1.175 mg. Tween 80 is a commonly used emulsifier that keeps the lipids (squalene) mixed with the water.
Sorbitan Trioleate (also called Span 85), 0.5 mg. Span 85 stabilizes oil droplets, ensuring the emulsion stays intact, along with Tween.
Sodium citrate, 0.66 mg. Sodium citrate is an ionic compound that serves as a buffer. Buffers maintain a consistent pH of a solution.
Citric acid, 0.04 mg. This, with sodium citrate, buffers the pH of the solution.
Water, 495 mg, 99% of the solution. The vast majority of MF59 is water, which is used to make the solution and ensure the viscosity allows easy mixing with other vaccine components.
MF59 promotes the recruitment of immune cells to augment the immune response.
MF59 creates a temporary environment that promotes recruitment of innate immune cells (antigen-presenting cells, APCs) to the injection site. This leads to increased uptake of the flu antigens—which APCs present to B and T cells to initiate adaptive immune response (more detail here). MF59 amplifies the overall activation of the immune system without increasing the amount of active ingredient.
Pretty cool, right? The Fluad vaccine for adults 65 and older uses this to account for the normal aging of our immune system as we also age, called immunosenescence.
Older adults have a less robust response to vaccination. It also means they have a less robust response to pathogens too, which is why they are at higher risk for severe disease. By adjuvanting the vaccine, we can improve their level of protection to keep them safe against the flu. And yep, this ingredient is also intentional, and wasn’t added casually, because an adjuvant addition is a major regulatory hurdle.
To get approval for MF59, new clinical trials needed to be conducted to show MF59-containing vaccine was safe and effective—equivalent or better than existing flu vaccines. It also required stability testing to show that MF59 didn’t accelerate degradation of the vaccine compared to non-adjuvanted formulations. It also required interaction studies to show MF59 didn’t interfere with the antigens or the immune response in a way that negatively impacted protection generated by vaccines.
If anything were to change with an adjuvant, even the concentration, most of those safety and stability studies would need to be redone. All of that data would need to be resubmitted for new approval.
Preservatives, buffers, and stabilizers ensure the vaccine remains safe, effective, and shelf-stable during transport and storage.
We have to ensure vaccines—liquid medicines—still do what they’re supposed to do when they get into your body. That means we have to make sure that they remain intact and functional from when they are manufactured until when they are administered.
Other ingredients are included to accomplish this. These are called excipients—the inactive ingredients. These are essential to maintain the solution integrity, prevent contamination, and ensure stability.
Excipient ingredients in flu vaccines include:
Sucrose, typically less than 10 µg. That’s the same sugar as your cane sugar or granulated sugar! In this instance, sucrose is a cryoprotectant: it protects vaccine and antigens from degrading and forming aggregates during freeze/thaw cycles.
Sodium chloride (NaCl), around 4 mg. That’s salt as you know the word. Sodium chloride maintains osmolarity, or electrolyte balance. This prevents damage to your cells when the vaccine is administered, due to electrolyte imbalances (this is the same reason you receive saline in an IV bag, not pure water)
Buffers, including monobasic potassium phosphate (KH2PO4), monobasic sodium phosphate (NaH2PO4), and dibasic sodium phosphate (Na₂HPO₄), each included at quantities of 300 microgram (µg) or less. Buffers maintain the correct pH to ensure the active ingredient in the vaccines don’t degrade.
Thimerosal is used as a preservative, only in multi-dose flu vaccine vials. It is present at 25 µg per 0.5 mL dose. Thimerosal is a thiosalicylate salt containing ethylmercury that prevents bacterial and fungal contamination. Thimerosal has been extensively studied for safety, is cleared from the body quickly, and does not bioaccumulate like methylmercury (read more detail below)
Want to know how stringent vaccine regulation is?
If a manufacturer wants to switch from one brand of sucrose to another—even when sucrose is just a stabilizer—full documentation showing that the new brand of sucrose meets all the FDA standards, that the sucrose is chemically identical to the previous brand, and that it does not introduce any contaminants.
Even if nothing in the vaccine changes functionally—literally just trading one brand of medical-grade sucrose for another—that data is still required by the FDA.
We also know about trace levels of residual materials from the manufacturing and purification process.
A vaccine isn’t created out of thin air. We use reagents and biological systems to make raw materials, and then purification processes to finish them. That means there are always going to be trace amounts of substances used in the manufacturing process.
Depending on which culture system the flu virus was grown in, there may be:
Egg protein, ovalbumin (for egg-based vaccines), would be present at less than 1 µg per dose. This level is safe, even for people with egg allergies—that’s been studied too.
Cell culture residues (for cell-based vaccines grown in MDCK cells) might include fragmented DNA at less than 0.01 µg per dose. This is also a residual that has been thoroughly studied and assessed for safety.
Other trace residuals might include reagents used in the viral inactivation steps:
Formaldehyde, if used, might be present at less <50 µg per dose.
An adult produces 1.5 ounces of formaldehyde daily during our normal metabolic processes (that’s 42,520,000 micrograms, in case you’re wondering. 850,000-times more formaldehyde every day that what might be found as a residual in a vaccine).
Triton X-100, used during the virus inactivation and purification process, might be present at less than 250 µg per dose.
All residual levels must be below FDA-established safety thresholds, and we definitely know which residuals might be present and what those thresholds are. That is a critical step in quality control.
Even if a manufacturer wanted to introduce a purification method that lowered the residual quantity of formaldehyde from 25 µg to 10 µg, they would STILL need to conduct safety and stability studies to show it didn’t impact vaccine quality. Yep, even reducing residuals requires formal regulatory scrutiny and approval. That’s how much scientists know about what’s in vaccines.
There are no mystery ingredients in vaccines.
Every vaccine is carefully engineered, rigorously tested, and stringently formulated. That’s after years of discovery research, development, clinical trials, and regulatory oversight. There is no conspiracy or hidden agenda.
Contrary to what anti-vaccine activists say, when it comes to making vaccines, even the tiniest change requires a mountain of evidence.
That’s exactly why they’re so safe. Don’t let people try to claim otherwise.
If someone says “no one knows what’s in vaccines” they are lying.
Take that opportunity to correct them.
Individuals who make those claims are trying to undermine trust in scientific experts and medical interventions that have saved 150 million lives in the last 50 years.
Vaccines are the real MVP: they've been saving lives for hundreds of years.
Vaccines are objectively one of the most impactful scientific interventions in human history. Today, we have vaccines that protect against dozens of debilitating and deadly diseases, with new candidates under study all the time.
We cannot let anti-vaccine disinformation that actively harms individual and public health go uncorrected. We have a moral responsibility now more than ever, to call out false health claims that erode trust in scientific evidence, undermine health interventions, and damages our society.
I hope you can use this article as a tool the next time you encounter someone spreading this falsehood.
Now, more than ever, we all must join in the fight for science.
Thank you for supporting evidence-based science communication. With outbreaks of preventable diseases, refusal of evidence-based medical interventions, propagation of pseudoscience by prominent public “personalities”, it’s needed now more than ever.
More science education, less disinformation.
- Andrea
ImmunoLogic is written by Dr. Andrea Love, PhD - immunologist and microbiologist. She works full-time in life sciences biotech and has had a lifelong passion for closing the science literacy gap and combating pseudoscience and health misinformation as far back as her childhood. This newsletter and her science communication on her social media pages are born from that passion. Follow on Instagram, Threads, Twitter, and Facebook, or support the newsletter by subscribing below:
Excellent piece. I’ve had people say to my face “we don’t know what’s in them.” Which is when I get to say, hi, let me first explain what “CMC Regulatory Affairs” is, and then point out that it’s my actual profession. Sigh.
This is all amazing, the work you do is relentless and exhausting I’m sure, thanks for making me so much smarter.🙏