There's one big, important difference between Pfizer and Moderna's coronavirus vaccines

Both Pfizer and Moderna have announced promising vaccine candidates in recent days; is one better or worse?

By Matthew Rozsa
November 17, 2020 11:32PM (UTC)
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A health care worker holds an injection syringe of the phase 3 vaccine trial, developed against the novel coronavirus (COVID-19) pandemic by the U.S. Pfizer and German BioNTech company, at the Ankara University Ibni Sina Hospital in Ankara, Turkey on October 27, 2020. (Dogukan Keskinkilic/Anadolu Agency via Getty Images)

In the span of two weeks, two competing pharmaceutical companies have announced separate, promising coronavirus vaccine candidates, both of which have passed the second-to-last phase of clinical trials. Biotech giant Moderna announced earlier this week that their vaccine displayed "94.5% vaccine efficacy", while Pfizer and BioNTech SE announced similarly positive results for their vaccine candidate last week. 

The dual announcements hints that there may be a light at the end of the tunnel that is the pandemic. But there's a lot of work still to be done from here — and the differences between the two vaccines could have big repercussions for when we may see them, and under what circumstances.


First, the way that both vaccines work, biologically speaking, is the same — they're both mRNA (synthetic messenger RNA) vaccines. mRNA vaccines are a relatively new technology, and have yet to be produced on a mass scale. As STAT News explains, mRNA is the part of a living creature's DNA that tells cells which proteins to make so that they can remain healthy. The concept behind mRNA-based vaccines is that synthetic messenger RNA — whose composition is bespoke, based on the virus in question — is injected, which the body's cells then take in, and in turn, being to produce precise proteins that are akin to those already found in the virus.

Dr. Russell Medford, Chairman of the Center for Global Health Innovation and Global Health Crisis Coordination Center, explained to Salon how these work in greater detail. "The current COVID-19 mRNA vaccines introduce the genetic information that enable human cells to produce a single, important SARS-Cov-2 protein, known as Spike," he said. In illustrative drawings of the novel coronavirus, the spike proteins are the little points that stick out around the sphere of the virus, like points on a sea urchin. Training human cells to produce Spike thus "trains the immune system to recognize Spike and thus protect the human body from SARS-Cov-2 viral infection," Medford continued.

Yet the biggest difference between the two vaccines, and the one that may affect which one ultimately sees the light of distribution, lies in the temperature at which they need to be stored. That's important because it could affect how easy it is to distribute either vaccine. According to Science Magazine, the Pfizer/BioNTech vaccine candidate needs to be kept at –70°C. By contrast, Moderna's vaccine candidate can be kept at –20°C and, the company claims, can remain stable for up to one month at consumer refrigerator temperatures of 2°C to 8°C.


"More 'end user' locations will be equipped to use the Moderna vaccine, with its less stringent cold chain requirements," Dr. Justin Lessler, an associate professor of epidemiology at Johns Hopkins Bloomberg School of Public Health, wrote to Salon. "Likewise, if that vaccine is more thermostable it should be less sensitive to accidental breaks in the cold chain (e.g., a batch being left out on the table for longer than it should) which may increase practical effectiveness."

"That being said, Pfizer has shown commitment to overcoming the issues with distribution and they certainly can be overcome," Lessler added.

Dr. Alfred Sommer, dean emeritus and professor of epidemiology at Johns Hopkins Bloomberg School of Public Health, expressed a similar view.


"The Pfizer vaccine will have greater logistical challenges, especially in low and middle income countries," Sommer wrote to Salon. "Not so much in wealthy countries but still some. I've been told that the because of its need for very cold temperature it will be distributed in special containers holding more than 1,000 doses which will pose a problem for sites servicing smaller numbers of people."

As Benjamin told Salon, "The current vaccine distribution system is well positioned to deal with vaccinating people that don't require unusual storage or handling requirements. A freezer or a refrigerator certainly is okay. And apparently the Moderna product can stay out even at room temperature for a longer period of time. So that means that places like CVS and other retail clinics like Walgreens or your regular pharmacy will be able to handle it a lot easier because they have certainly refrigerators and getting something that's like a small freezer would not be a problem for them."


He added, "The Pfizer product requires ultra-cold storage and boxes using dry ice. Managing dry ice can be pretty tricky. You can get severe frostbite from it." Benjamin said that it would be inconvenient to keep that ultra-cold supply chain going across the country during distribution.

The final, biggest question is when these vaccines will be available for the market. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told Science Magazine that he believes doses of one or both vaccines could start to be offered to people at the highest risk from SARS-CoV-2, the virus that causes COVID-19, by late next month.

Matthew Rozsa

Matthew Rozsa is a staff writer for Salon. He holds an MA in History from Rutgers University-Newark and is ABD in his PhD program in History at Lehigh University. His work has appeared in Mic, Quartz and MSNBC.

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