What’s Actually In Pfizer’s COVID-19 Vaccine?

Photo: Liam McBurney/Getty Images.
After months of speculation, we've come to a point where a COVID-19 vaccine is actually within our reach. The first Pfizer vaccines, some 30,000 doses, arrived in Canada Sunday night. It's incredibly promising, but despite approval from the Canadian government (for use on people over 16), people have a lot of questions.
The good news is that we do have more information about the Pfizer-BioNTech COVID-19 vaccine than people think. It uses modified messenger RNA (mRNA) which is genetic material our cells use to read proteins. Specifically, the vaccine contains the mRNA of what's known as spike protein, which is located on the surface of the SARS-CoV-2 virus and is what it uses to invade host cells, says Carlos Malvestutto, MD, MPH, who specializes in infectious disease at The Ohio State University Wexner Medical Center. The novel coronavirus uses spike protein like a key to gain entry to our cells; once inside, the virus is free to replicate, making us sick.
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The vaccine works by breaking that key. When the vaccine enters the body, the immune system recognizes the mRNA in it as foreign and begins producing antibodies that target the COVID-19 spike protein. Later, if you're exposed to the SARS-CoV-2 virus, the body will already have antibodies that fight that spike protein on hand, and the virus won't be able to get inside your cells or infect you.
Most existing vaccines use a different approach — for example, the measles vaccine actually contains a weakened live measles virus that stimulates our immune system into fighting the illness. If you're worried about getting infected with COVID-19 from the vaccine, though, Dr. Malvestutto says it is "absolutely not" possible for that to happen because the vaccine is not made of the virus itself — just the spike protein. "It doesn't have the adjuvants [an ingredient that creates a stronger immune response] or other materials that are usually needed in other vaccines," he explains. "Technically you could [create a vaccine using the mRNA of] any other protein that you have the genetic code for," Dr. Malvestutto adds. "But we know that because the spike protein is on the surface of the virus, it makes sense to target that specific structure."
The Pfizer-BioNTech vaccine will be administered in two doses, 21 days apart from one another. "The first dose primes your immune system, and the second dose will give you the maximum antibody level," Dr. Malvestutto explains. The trials showed 95% efficacy after the second dose, which Dr. Malvestutto says is excellent compared to the minimum that was expected at 50%. "We should be able to see an impact earlier as we start to vaccinate people because a much larger proportion of those vaccinated will be protected by this," he explains.
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As for the other ingredients of the Pfizer COVID vaccine, they're pretty straightforward. It's made up of the mRNA wrapped in what Dr. Malvestutto says you can think of as a "fat bubble" — lipid nanoparticles that protect the mRNA so it can do its job. The FDA lists these additional ingredients as: lipids ((4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2- hexyldecanoate), 2-[(polyethylene glycol)-2000]-N,N-ditetradecylacetamide, 1,2-distearoyl-snglycero-3-phosphocholine, and cholesterol), potassium chloride, monobasic potassium phosphate, sodium chloride, dibasic sodium phosphate dihydrate, and sucrose.
Dr. Malvestutto emphasizes that according to the trials — which were conducted with more than 43,000 participants — this vaccine is safe. "There were no shortcuts here," he says. Yes, it was created quickly, but no corners were cut in the process.
Of course, there are expected side effects, as there are with any vaccine. According to Pfizer's data, over half of those between ages 16 and 55 experienced mild symptoms, like fatigue and headaches, after receiving the injections. What's more, after the U.K. granted its EUA, two staff members from the National Health Service reportedly experienced serious reactions to the vaccine. Both people had reported histories of allergic reactions. Because of this, the Medicines and Healthcare products Regulatory Agency (MHRA) has advised that anyone who has a history of “significant” allergic reactions to medicines, food, or vaccines to hold off getting it as a precaution.
Dr. Malvestutto points out that as we start to vaccinate millions of people, it's likely that some side effects will occur. "It's important to keep that in context," he explains. "We have thousands of people every year experience kidney damage and heart damage from taking anti-inflammatories. We have patients who have serious gastrointestinal bleeding from taking aspirin." He says that no medication is 100% safe, but it's important to keep in mind the frequency of side effects. "There may be rare ones, but it's unlikely that they will be frequent or serious," he says.
With results as transparent and promising as these, it's hard to ignore how effective this vaccine is. Now all we have left to do is wait for the rollout, and do our part to continue stopping the spread by wearing masks, staying home, and practising safe social distancing.
COVID-19 has been declared a global pandemic. Go to the Public Health Agency of Canada website for the latest information on symptoms, prevention, and other resources.

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