As the COVID-19 vaccines roll out around the world, there is plenty of discussion about what is IN these vaccines.
These are the ingredient lists for both the Pfizer/BioNTech and Moderna vaccines. Some of these may be really unfamiliar, or familiar in a confusing way—like...sugar? So, let’s go one by one and see why they’re all in there. And don’t worry, none of them are microchips.
The key, active ingredient in both Moderna’s and Pfizer’s vaccine is messenger RNA, what we call mRNA. mRNA is a molecule containing the genetic code that tells your cells how to make ONE kind of protein. That’s the spike protein on the surface of the virus. Your cells read those instructions and then make that spike protein—which can’t infect you on its own — and your immune system learns how to recognize it.
You start to build an army of antibodies, those are immune proteins that bind to the REAL virus and clear it away if you get infected. Then, after a while, your cells get rid of that mRNA, that genetic material, but your body remembers how to defend itself. It’s like showing the picture of a bad guy around town so everyone knows who to look out for if they ever show up.
Now, we can’t just inject straight mRNA into someone’s body, because your body is actually really good at chewing up and getting rid of foreign genetic material that’s not supposed to be there. That’s where the other vaccine ingredients come in.
Both Pfizer and Moderna contain a variety of lipids. The word ‘lipid’ is basically just the scientific name for a fat or fat-like molecule. You can even see the word cholesterol in the lipid list for both vaccines, same as the kind that’s in your body naturally. All of these lipids together form tiny little protective bubbles around the mRNA. One of the lipids sticks to the mRNA, others form the structure of the bubble and help it cross your cell membrane into your cells where it can be used, and other lipids keep the bubbles from clumping together.
In both of these vaccines, this whole complex is called a lipid nanoparticle, or LNP. Think of it like, if the mRNA is the letter, with all of the information, the LNP is the envelope you put the letter in so it can reach its destination safely. And although LNPs are currently the most advanced way of getting vaccine mRNA into a cell, so it can do its job, these nanoparticles may potentially be what’s causing allergic reactions.
Actually, just one lipid component of the nanoparticle, a form of polyethylene glycol, called PEG for short. PEGs are super common in everything from laxatives to cosmetics, but some people may have an immune reaction to them, and we still don’t totally know why. It may actually be that not the RNA vaccine itself, but the substances that are encapsulating it, the fats and oils that are used that may actually be generating a very important immune response.
Moving on to the next category of ingredients, we’ve got salts! These help balance the pH of the whole mixture, making it the same pH as your body. Salts balance pH by redistributing charges. A basic salt like sodium acetate helps balance out any acidity. This group of chemicals in the Pfizer vaccine when taken together is known as PBS, and I’ve actually used it in the lab myself to keep cells I’m studying alive and well and pH balanced.
In both lists you’ll see this familiar ingredient: sucrose. Yup, exactly the same kind as you have in your baking cupboard, but extra pure, of course. And honestly, between the sugar and the cholesterol, these vaccines actually sound kinda tasty.
But the sucrose isn’t in there because it tastes yummy...it’s there to keep everything stable at really cold temperatures. You wonder, "Why were those RNA vaccines stored at the temperature of dry ice?" It's because RNA has a problem with degradation. With the newer formulations, perhaps not. And the way they're packaged may also protect them. But having worked with RNA, you know that if you do make a mistake and there's contamination, the RNA very rapidly can be turned into pieces. And you don't want that.
Sugar essentially packs in around all the proteins and lipids in the other vaccine ingredients, keeping them from losing their shape and therefore, their properties. Think of the sugar kind of like molecular packing peanuts that protect the other stuff in the vaccine from cracking under the icy cold temperatures required by the mRNA. And the cool thing about how cold these vaccines are is that they don’t need any preservatives. Because everything is kept at such cold temperatures, nothing funky is going to grow in these vaccine vials — the only thing needed to keep them from going bad is ice. And that’s all the ingredients on the list! Pretty simple, right?
When it comes to actually USING these shots, the Pfizer vaccine needs to be mixed with saline right before it’s injected, to dilute it to the correct concentration — that’s just the way it’s formulated. The problem is, after you add the saline, the vaccine ingredients only have six hours before they start to degrade — you have to use it fast. This is where we see how differences in ingredients can make for differences in application.
For example, Moderna’s vaccine doesn’t have to be kept QUITE as cold as Pfizer’s because its lipid nanoparticles are more resilient. This, and the fact that there’s no dilution step, makes Moderna’s a little simpler on the logistical level… so easier to get to more people.
I hope learning more about these ingredients has made you feel more comfortable and empowered during your vaccine decision-making. mRNA vaccines are new in the sense that we’ve never used them before, but the technology has been around for years, and lots of research has gone into making it work… we just haven’t ever needed it — or at least not as urgently — until now. And their very nature makes them hugely exciting, not only for COVID, but for tackling many other diseases in the future too.