Moderna's coronavirus vaccine doesn't need to be as icy cold as Pfizer's, and that could make it easier to distribute

• Moderna and Pfizer have both announced promising trial results this week, suggesting their coronavirus vaccines perform very well at preventing COVID-19 infections.
• But Moderna's shots can be kept in the fridge for a month, while Pfizer's can only last five days that way.
• Pfizer has developed special vaccine briefcases that can keep its shots frozen properly for 15 days, but that may not be enough time to distribute the shots in rural areas, or places without special freezers.

If all goes well, by mid-2021 people in the US may have two — or more — safe, highly effective coronavirus vaccines to use.

On Wednesday, Pfizer announced it had completed its final coronavirus vaccine trial, and shown its 2-shot course to be 95% effective at preventing infections in the more than 40,000 people who tested it out around the world. Earlier in the week, Moderna also announced its own coronavirus vaccine trial was nearing its end, with a very similar 94.5% efficacy rate.

Both of the new vaccines must still be green-lit by the US Food and Drug Administration (FDA) with an Emergency Use Authorization before they hit shelves in clinics and pharmacies around the country.

Though the two vaccines are extremely similar, both in stated efficacy, and in vaccine design, there is one key difference between them, that may be an important distinction as decisions are made about how they are shipped around the country, and around the world: temperature.

Moderna's shots can be stored in a fridge for up to a month, according to the company, while Pfizer's shots must be shipped out on dry ice, in special GPS-tracked briefcases which can be kept properly chilled at temperatures much lower than your freezer (-94° F) for up to 15 days. Once Pfizer's vaccines come out of their briefcases, and into vaccine fridges, they must be used up within five days, or tossed in the trash.

"From a medical standpoint, the vaccines are not very different," Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told Insider. "It's about logistics."

Moderna has more practice handling this kind of vaccine

Both Moderna and Pfizer are using a high-tech, new age nucleic acid messenger RNA vaccine platform, which (unlike old-school annual flu shots) doesn't require growing vaccines in any chicken eggs.

No mRNA vaccine has ever been approved for use in humans, yet. But Moderna has had a bit more practice developing these kinds of mRNA vaccines in the lab, while Pfizer is taking a more cautious approach to keeping its initial coronavirus vaccine (developed in partnership with the German biotech firm BioNTech) chilled.

The reason that vaccines are so often kept cold is to preserve their potency, and it's possible that the Pfizer vaccine could survive and work well when kept at higher temperatures, but the company isn't taking any chances, for now. The risk, if vaccines get too warm before they're put into people, is that they won't work as effectively, and people won't be as well protected from infection.

Already, so-called "freezer farms" are popping up in spots around the US, and the world, with ultra-cold freezers that can keep the Pfizer vaccine preserved for up to six months. The federal government's Operation Warp Speed is going to be leading the charge deciding how to ship out vaccines (which are already being manufactured) to states, when (and if) they clear the FDA's emergency authorization.

Pfizer's vaccine could be best used in dense cities, rather than across rural areas

Because Pfizer's vaccine can only safely last five days in the fridge (as far as we know), it might become more widely available in densely-packed cities, and other places where many vaccines can be distributed at once, as each dry ice briefcase comes with 1,000 shots packed inside, or more.

"I think you'll have to make these allocation decisions based upon what type of capacity each state has to distribute either vaccine," Adalja said. "When people are more densely packed, and you can have central points of distribution, it's easier to distribute a vaccine like the Pfizer vaccine, versus a place where things may be more spread apart."

Either way, it's already clear there won't be any single winner-take-all vaccine distributed across the US.

Getting a sizeable enough majority of the country vaccinated to achieve proper herd immunity protection against this virus could require roughly 80% of the population, or more, to get vaccinated, which would mean the US needs nearly 600 million shots (since Moderna and Pfizer's vaccines both require two doses per person).

"Even if the Moderna vaccine is easier to distribute, because it has a less stringent cold chain ... it's still going to be the case that we're going to need the Pfizer vaccine and Pfizer's manufacturing capacity as well," Adalja said.