What’s in Pfizer’s vaccine? A look at the ingredients

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The newness of the novel coronavirus coupled with the quick development of a vaccine has put Pfizer and BioNTech’s shots under the proverbial microscope — particularly about the ingredients it uses.

While there’s no question this happened at warp speed, experts agree the listed ingredients are pretty standard.

In fact, they’re “almost simpler and more straightforward than any other vaccine,” according to Kyle Anderson, an assistant professor of biochemistry, microbiology and immunology at the University of Saskatchewan.

“That’s because it’s a revolution in vaccine making,” he said. “There’s nothing in this vaccine that rings any alarm bells.”

Pfizer’s vaccine was approved by Health Canada on Dec. 9. Canadians across the country have rolled up their sleeves this week to be among the first to receive the shot, which touts a 95 per cent effectiveness rate and no serious side effects.

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Its ingredients might look complicated, said Anderson, but they can be easily organized into four basic categories:

Active ingredient

  • 30 mcg of a nucleosidemodified messenger RNA (modRNA) encoding the viral spike (S) glycoprotein of SARS-CoV-2.

This is the only active ingredient in the vaccine.

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The mRNA — messenger RNA technology — sends tiny snippets of genetic code to cells, telling them to build proteins. In this case, it triggers the immune system to produce protective antibodies without using actual bits of the virus. “Old-fashioned” types of vaccines, required scientists having to express it in cells or chicken eggs to get the proteins right, Anderson said.

“And then we had to do all this clean-up,” he said. “If we had problems with cleaning and purifying it, we could get adverse reactions or add things like mercury compounds to stabilize that. … These are completely avoided with these mRNA-based vaccines.”

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The mRNA-based vaccines may have more science behind them, but they’re more efficient, Anderson said.

“Essentially, the message is telling your body how to make this one virus protein, and how to recognize it,” he said.

From there, the other ingredients form a “bubble” around the message, Anderson said, helping it become stable enough to get where it needs to go.


  • lipids (0.43 mg (4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate), 0.05 mg 2[(polyethylene glycol)-2000]-N,N-ditetradecylacetamide, 0.09 mg 1,2-distearoyl-sn-glycero-3- phosphocholine, and 0.2 mg cholesterol)

The lipid molecule in Pfizer’s vaccine contains four components.

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“Each is necessary for the mRNA — the medicinal ingredient — to gain entry into cells and be expressed,” said Roderick Slavcev, a professor at the University of Waterloo’s School of Pharmacy. Slavcev and a team are currently developing a DNA-based vaccine.

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The molecule essentially protects the mRNA. Without it, the vital active ingredient could be broken down before getting that genetic code to the cell.

This connection of fat is particularly delicate, experts say, which is why the Pfizer vaccine requires a subzero temperature for storage.

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Of all the ingredients, it’s the lipids that have been misunderstood, Anderson believes.

This is, in part, due to two vaccine recipients in the U.K. experiencing an adverse allergic reaction. It’s been suggested that the reactions may have been caused by polyethylene glycol, or PEG, which helps stabilize the shot and is used in some other vaccines.

But the ingredient is already widely used, Slavcevs said, and there is no reason to be concerned.

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PEG is found in a range of pharmaceutical, cosmetic and food products. It is also safely used at higher concentrations as a laxative. Slavcev said there have been some minor side effects recorded with PEG, but they’re often as a result of taking high concentrations.

The 0.05 mg of PEG in Pfizer’s vaccine is “such a small quantity,” Anderson said.

“It’s like the carrier of the message. It’s the envelope that your letter came in, and then you recycle the paper,” he continued.

“If you eat, say, a steak, you’re getting a lot of these fats and in far higher quantities, but they’re digested and you just don’t think about it.”

Lipids might also have a bad rap because “they have such long, complicated, really unpronounceable names,” he added.

“It kind of reads like a shampoo bottle,” he said with a laugh.

“All these Xs and Ys and Zs and numbers, it looks like an alien language. And it is to most of us. But it’s just essentially a fat droplet.”

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  • 0.01 mg potassium chloride
  • 0.01 mg monobasic potassium phosphate
  • 0.36 mg sodium chloride
  • 0.07 mg dibasic sodium phosphate dihydrate

Salts, like fats, are nothing new to vaccines, the experts agree. They’re there for two reasons — to balance and to stabilize.

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Having salt components allows cells to retain their proper shape, Anderson said.

“If you get an IV at the hospital, there are salts and sugars in there. It makes your cells a little happier. They’re there to help balance it out,” he said.

Components like potassium phosphate and dibasic sodium phosphate are there to make sure the pH — the acidity of a solution — is stable.

They’re actually variants of salt natural to the body that keep the pH balanced, Anderson said. The combination of these particular salts helps the body absorb the solution while also protecting the fat molecule and mRNA.

“It’s going to last longer from the time of thawing it to injecting it in you,” he explained.

“If there happened to be something slightly acidic in the syringe being using, that could destroy the vaccine. So this is really a protective measure that pH is going to be balanced across the time it actually reaches your cells.”

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  • 6 mg sucrose

The sugar-water mixture is there to protect and to stabilize the vaccine during freezing.

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Pfizer’s vaccine needs to be stored at an ultracold -70 C. The sugar helps keep things stable under this temperature, the experts said.

It is, again, another common addition to a vaccine and other drug and pharmaceutical products.

“We’re more likely to not trust something that we haven’t seen or heard of before,” Anderson said.

“Sucrose, we know it’s sugar. But we don’t necessarily think about all these fats in our body that have these complicated names, and there are hundreds more with complicated names.”

Sign of the times

Ultimately, there’s nothing in that ingredient list that is questionable or unnecessary, Slavcev said.

“Potentials for allergic responses or other adverse responses are always possible. It’s impossible to predict how every human will react,” he said, adding that all adverse reactions are and will continue to be documented.

As for the speed of the vaccine’s development? Get used to it.

“This is the future of vaccines,” Anderson said.

“There comes a time in the complexity of how things are made — whether it’s a computer program, a car, a drug, food — where you need to trust that there are experts who are critical enough to inspect this and ensure safety.”

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He added: “This is one of the most scrutinized vaccines ever made. It hasn’t been rushed, but there has been no delay.”

— with files from the Associated Press and Reuters

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