Pfizer's vaccine relies on a 'cold chain' that keeps the shots colder than a freezer. Here's how it works.

• Pfizer's new coronavirus vaccine will need to be kept hyper-cold, at -94 degrees Fahrenheit, as it is shipped around the globe. That could make it difficult to get the vaccine distributed, especially in rural areas.
• The company has developed a special dry ice briefcase that can keep its vaccines safe for 10 days.
• But that may not be enough time, especially since the multi-shot course requires two inoculations, spread three weeks apart.
• Here's how cold chains work, and why they're so critical to administering good vaccines.

-94 degrees Fahrenheit. That's how cold Pfizer's new coronavirus vaccine needs to be kept as it journeys around the globe.

It's so chilly that no regular freezer (0°F) fits the bill. So cold that it must be shipped out on dry ice.

The two-shot course, which the company announced was more than 90% effective at preventing COVID-19 in late-stage human trials earlier this week, could get an emergency approval by the US Food and Drug Administration in a matter of weeks. But the vaccine must be kept so cold that the company is set to ship it out in special dry ice briefcases (built to stay cool 10 days) outfitted with GPS trackers that keep track of their temperature.

Pfizer already has "a stretch of land the size of a football field" in Kalamazoo, Michigan as its vaccine shipping hub, "outfitted with 350 large freezers," the Wall Street Journal reports.

That's just the first segment of the vaccine "cold chain," the critical multi-part pipeline that will keep the shots chilled and ready to work, all the way from manufacturer to your veins.

Many places in the US are signaling they aren't quite ready to make this vaccine chain work. Some hospitals are rushing to buy ultra-cold freezers to keep the vaccines at the proper temperature.

Here's what has to go right in the cold chain for you to eventually get a coronavirus vaccine.

Many vaccines need to be kept chilly to work right

A tuna salad sandwich taken out of the fridge a little early isn't a big deal. But if clinicians or transporters accidentally warm up a vaccine too much before sticking it inside a person, that might ruin the shot.

Not all vaccines thrive at the same temperatures. Many of the most common vaccines, including polio, measles, and tetanus vaccines, all do well in the fridge, at temperatures around 35-46 degrees Fahrenheit (2-8° C).

But Pfizer says its new coronavirus vaccine should be kept at -94°F (-70° C), in order to safeguard the shots' effectiveness.

It's possible that the vaccine could be fine at temperatures above that, but scientists were in such a rush to create the vaccine that they don't know for sure.

Other coronavirus shots, if they win approval, wouldn't need to be so cold. Moderna's vaccine, which is still in late-stage trials, is kept at a more moderate -4°F (-20° C), very similar to the temperature required for Russia's coronavirus shot.

If the cold chain breaks, shots don't work as well

It's not uncommon, even at more regular vaccine refrigeration temperatures, for a link in the cold chain to break.

"Each year, storage and handling errors result in revaccination of many patients and significant financial loss due to wasted vaccines," the CDC says in its cold chain toolkit.

Analysts from UBS Bank expect that roughly 5-10% of the Pfizer vaccine may be rendered ineffective, "due to inadequate storage conditions."

Some of the waste will likely go undetected, meaning that people might get ineffective shots, and won't be well-protected from infection.

The last leg of the chain, in clinics, doctors offices, and pharmacies, may be the hardest to keep cold

The trickiest part of the cold chain will likely be the end. Pfizer's special cold boxes are designed to ship vaccines in increments of at least 1,000 doses at a time, which may make getting small batches of shots out to rural areas hard.

The vaccine can survive in the fridge for up to five days, but after that it has to be thrown out, because re-freezing doesn't work, as ProPublica recently reported.

That means in order to keep the vaccines chilled properly for more than the 10 days the briefcase lasts, many places will likely resort to re-packing their shots on dry ice, which is already in short supply during the pandemic. The dry ice stock for the vaccines can be replenished safely three times, according to ProPublica, meaning the vaccine could, in theory, survive outside high-tech freezers for a full 40 days. But only if strict guidelines are followed during that time.

Pfizer reportedly told the CDC that the cold briefcases should not be opened more than twice a day in order to keep the proper chill, and any time they are, they should be "closed within 1 minute," NBC reported.

Given that in order to be fully effective, the Pfizer vaccine requires two doses administered three weeks apart, the 40 day timing is going to be tight, especially in rural areas.

The company is signaling its chilling issue may only be temporary, though.

Pfizer's chief scientific officer told Business Insider earlier this week that the company is looking into a "powder format" vaccine for next year that could likely be kept alongside many other shots in the fridge.