In at least two cases, however, volunteers had severe reactions.

One developed a grade three fever, which is classified as 102.2 degrees Fahrenheit (39°C) or above, after receiving the lower vaccine dose. Another developed fatigue so severe it temporarily prevented daily activities, Anderson said.

Drugmakers on the hunt for a COVID-19 vaccine have promised massive supply rollouts in the coming months if their candidates pass regulators' scrutiny. But promises and results are very different things: for two highly touted mRNA-based shots, storage concerns could derail those plans.

A pair of frontrunner mRNA-based COVID-19 vaccines from Moderna and Pfizer could run into supply logistics issues over the ultra-cold storage needs for both shots, SVB Leerink analysts said in a pair of client notes Thursday.

Citing discussion at a CDC advisory committee meeting Wednesday, analysts pointed out that experts expressed concern that the temperatures required to store mRNA vaccines were "severely limiting" to distributors' ability to ship the shots and to clinics' ability to administer them to a wide swath of patients.Pfizer and BioNTech's mRNA hopeful, dubbed BNT162b2, specifically raised eyebrows: The vaccine reportedly needs to be held in storage at -94° Fahrenheit, and will last for only 24 hours at refrigerated temps between 35.6° and 46.4°. Meanwhile, most protein subunit vaccines—the type being developed for COVID-19 by Sanofi and Novavax, among others—can be held at refrigerated temps for months, analysts said.

Given those constraints, the analysts argued that Pfizer's shot could only be used at certain hospitals and clinics with the proper equipment, and would require "intensive one-day vaccination events at such sites ... (that would) cover a fraction of the healthy population."

In a statement, Pfizer argued that its vaccine could be stored at refrigerated temps for up to 2 days, instead of the 24 hours cited by analysts. The drugmaker has also developed shipping containers using dry ice that it believes will solve some shipping worries.

"We are confident of our capability to deliver and store doses to the destination's governments designate, according to product shipping and storage requirements," a spokeswoman said.

Despite similar storage issues, Moderna is pursuing stability data showing its vaccine, mRNA-1273, can be stored at -4° Fahrenheit—which would be a big leg up over Pfizer's candidate. On a Wednesday call with investors, Moderna CTO Juan Andres said the -4° mark was a "very normal storage condition" comparable to home freezers.

"Of course, industrial, well-monitored freezers for pharmaceuticals will be used for storage and shipment, Andres said. "The point here is that the infrastructure is widely available, and we do not need special equipment to use our vaccine."

In the meantime, though, Moderna is likely to pursue an emergency use approval with cold-storage requirements similar to Pfizer's—and analysts said Moderna's eventual advantage over its rival might not be much of an advantage given the range of competitors in the hunt.

"We note that storage/distribution temperature may be less of an advantage–and may in fact still be substantially inferior to non-mRNA competitors utilizing a protein sub-unit approach with much more permissive storage conditions," analysts wrote.

The ability to successfully distribute their vaccines will come as a big challenge for both companies, which have promised hundreds of millions of doses within months if an approval comes through.

"November 25 is the time we will have enough safety data to be able to put into an emergency use authorization file that we would send to the FDA," Bancel said, adding that an approval wouldn't be expected until late first quarter or early second quarter of 2021.

The timeline matches up with what public health experts have previously laid out, which is that a vaccine could be found to be safe and effective by the end of the year at the earliest but widespread distribution wouldn't occur until further into 2021.

One of the frontrunners to develop a safe vaccine first, Moderna earlier in the month said it was slowing its trial enrollment to ensure it was representing disproportionately affected minority groups but later added it could have enough data by November to know if its vaccine candidate works.