Why Do COVID-19 Vaccines Have To Be Stored at Different Temperatures?

Within two and a half weeks of each other, two big-name pharmaceutical companies—Pfizer (working with BioNTech) and Moderna produced viable COVID-19 vaccines that potentially signaled the beginning of the end of the pandemic.

The Pfizer and Moderna vaccine candidates received emergency use authorization (EUA) from the Food and Drug Administration (FDA) in December 2020.

They were rolled out about nine months after the World Health Organization (WHO) officially declared COVID-19 a global public health crisis—the fastest turnaround time in world history.

The Pfizer vaccine (whose working name is BNT162b2), has stringent storage requirements. It must be kept unusually cold—about minus 70 degrees Celsius (minus 94 Fahrenheit), or within a range of minus 80 to minus 60 degrees Celsius (minus 112 to minus 76 F).

To put this in perspective, this is chillier than a winter’s day in Antarctica.

And by contrast, the seasonal flu vaccine has to be kept at a comparatively warm 4 degrees Celsius or 39.2 degrees Fahrenheit, according to Sheila Keating, PhD, associate professor of laboratory medicine at the University of California, San Francisco School of Medicine.

How Will These Storage Demands Be Met? 

Keating anticipates that these requirements will significantly complicate the distribution of the Pfizer vaccine. In order to ensure the efficacy of the vaccine, people will need to be vaccinated at “centralized locations with access to minus 80 degrees Celsius freezers” or dry ice containers, she said.

This equipment is high maintenance in and of itself. Dry ice containers need “to be replenished regularly, and dry ice supply may prove to be difficult to maintain,” she said.

Pfizer has tried to preempt criticism by developing and manufacturing storage units specifically tailored to the vaccine. Roughly the size of a suitcase, these units can carry at least 975 doses and are packed with enough dry ice “to recharge it one more time,” said Jessica Atwell, PhD, assistant scientist in the division of global disease epidemiology and control in the department of international health at the Johns Hopkins Bloomberg School of Public Health.

However, it won’t be feasible to ship them worldwide. 

“Doing that in high-income countries like the U.S. is one thing," Atwell said. "Trying to do that in low- and middle-income countries around the world, (with) even a normal 2 to 8 degrees Celsius, refrigerator-like temperature, can be really difficult in many parts of the world. So it's definitely an implementation challenge.” 

Perhaps the biggest barrier to the widespread distribution of a vaccine that needs to be kept as cold as Pfizer's is there's no precedent for it.

“We don’t currently use any [vaccines] that require minus 70-degree storage,” Atwell said. 

How Does Pfizer Stack Up To Its Main Competitor? 

The Moderna vaccine (also known as mRNA-1273) is lower maintenance. It needs to be kept at about minus 20 degrees Celsius (minus 4 Fahrenheit) or a range between minus 25 to minus 15 Celsius (-minus 13 to 5 Fahrenheit).

(By contrast, the storage environment of the Johnson & Johnson vaccine is comparatively warm: It must be stored at between 36 and 46 degrees Fahrenheit).

Why Must These Vaccines Be Stored at Different Temperatures? 

Both the Pfizer and Moderna vaccines are messenger RNA (mRNA) vaccines, meaning that they are made using fragments of viral genetic material.

Essentially, Atwell says, the two vaccines are “lipid nanoparticles that encapsulate the messenger RNA inside” and have a stabilizing effect. She attributes the big temperature variation in their storage temperature requirements to the “sort of lipid nanoparticles that encapsulates the mRNA.”

Simply put, the vaccines must be kept so cold to prevent them from breaking down. If they do, they become useless.

Even scientists would like to know why there is such a disparity between the Pfizer and Moderna storage requirements. But they know the answers lie in the scientific formulations for the vaccines. And drug companies are known to guard these "recipes" zealously.

So in other words, scientists may never know for certain.

"It just comes down to what their data is," vaccine researcher Margaret Liu tells NPR. "If their data shows that it's more stable at a certain temperature, that's it."

While drugmakers are certain to keep their secrets "close to the vest," it's always possible that some researcher, somewhere, will unlock the mystery—and maybe remove one of the impediments of getting "shots in arms" across the country and across the globe.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.