Pfizer’s coronavirus vaccine candidate, announced earlier November, comes with a major complication that could delay its distribution in rural areas and developing countries: It must be stored at the ultra-cold temperature of -70°C. That means the vaccine must be kept in specialized freezers that cost as much as $20,000 each and are rare outside of medical research facilities.

But that problem may just be temporary. Experts say that the vaccine's cold storage requirements may become less stringent as more is learned about how the vaccine reacts to warmer temperatures.

Tinglong Dai, a medical logistics specialist at Johns Hopkins University, says he wouldn’t be surprised if Pfizer eventually revises the storage requirements for its vaccine. With the current storage requirements, Dai says, “they’re being more cautious.”

“You can only use [a] vaccine at storage conditions that have been proven and documented,” said Barney Graham, Deputy Director of the Vaccine Research Center at the National Institute of Allergy and Infectious Diseases. Early research during a vaccine's development is often done under more cautious conditions, but according to Graham, later testing can demonstrate a vaccine's durability under a wider range of conditions.

This dynamic has already played out with a vaccine candidate from Moderna, which uses similar technology to Pfizer’s to trigger viral immunity. As recently as August, Moderna was expected to seek approval of its vaccine with storage standards similar to Pfizer’s.

But on November 16, the same day it announced promising large-scale trial results, Moderna revealed that its vaccine candidate was effective for up to six months when stored at standard freezer temperatures of -20° C, and for up to 30 days at standard refrigerator temperatures of 2-8° C.

Pfizer declined to comment about the possibility of revising storage requirements for its current vaccine candidate.

Changing storage requirements, according to Dai, would make it far easier to deliver Pfizer's vaccine to populations further from research hospitals. In its current “vaccine playbook”, the CDC advises against local vaccine managers buying specialized freezers.

To help with distributing its vaccine, Pfizer has developed a shipping container that can maintain the required -70° C for up to 10 days unopened. With regular refills of dry ice, it can maintain that temperature for up to 30 days.

But using shipping containers for storage is risky. “Every time you put new dry ice in is an opportunity for mistakes,” says Dai.

Any change to Pfizer’s storage requirements is unlikely to impact the first wave of vaccine distribution, which may start by early 2021. That's only if the federal government approves the vaccine for distribution.

Pfizer did confirm, however, that it's working on a more durable version of the vaccine that is "lyophilized," or freeze-dried. That vaccine, the company said, would be able to be kept at normal refrigerator temperatures and may be ready by 2022. The Food and Drug Administration would have to approve the updated vaccine following additional trials.

Joshua Michaud, associate director for Global Health Policy at the Kaiser Family Foundation, says Pfizer could also reformulate the protective layer of fatty lipids around the core of its vaccine. Moderna says its specific lipid shell technology is key to its vaccine’s greater durability.

The Pfizer vaccine’s storage requirements are slightly less of a problem now considering that easier-to-store candidates, including AstraZeneca’s, are in the approval pipeline. Those more durable vaccines could be distributed in areas that lack the equipment necessary to safely store Pfizer's.

But according to Dai, the unprecedented scale of the challenge – immunizing essentially the entire global population as quickly as possible – means every vaccine will be needed in the short term. The question is whether sturdier vaccines will be available as soon as Pfizer’s, making it easier to protect far-flung patients.