Pfizer, alone among its peers, has faced fever-pitch scrutiny over the stringent cold storage requirements for its COVID-19 vaccine—and the logistical challenges that could pose. Looking to ease the doubters’ concerns, Pfizer has now shipped its first doses by air to the U.S., and distributors are watching closely.

Late last week, United Airlines started ferrying Pfizer’s COVID-19 vaccine hopeful from the Brussels International Airport in Belgium to Chicago O’Hare, The Wall Street Journal reported, marking the first “mass air shipment” of a coronavirus vaccine.

Pfizer’s BioNTech-partnered vaccine hopeful BNT162b2 has yet to receive the FDA authorization needed to ship straight to administration sites and doctors, but the shot can be pre-positioned at distribution sites, the Journal said, potentially easing the deployment process should the shot pass muster with regulators.

Getting a jump start on a U.S. rollout could be a big help for Pfizer, which is facing heightened scrutiny for its vaccine’s cold-chain requirements—the most stringent of any of the COVID-19 vaccine leaders.

Pfizer’s shot must be kept at a frigid -94 degrees Fahrenheit long term with a very limited shelf life at normal refrigerated temps. The drugmaker, for its part, has developed specialized, GPS-monitored coolers that can keep the vaccine at required temperatures for up to 10 days. Meanwhile, the company has developed real-time GPS capability to report potential temperature variations, reducing the odds that the shot will lose its potency before it reaches patients.

[huge_it_slider id=”15″]Looking to cope with Pfizer’s frigid requirements, the Federal Aviation Administration (FAA) gave United Airlines the go-ahead to move 15,000 pounds of dry ice per flight, more than five times what’s normally permitted, the Journal reported. Dry ice, which the FAA classifies as a hazardous material, can only be moved in limited quantities, as it can lower the amount of oxygen in the air as it converts back into carbon dioxide gas.

Pfizer, meanwhile, has positioned refrigerated storage sites at its final-assembly centers in Kalamazoo, Michigan, and Puurs, Belgium, and is working to boost capacity at distribution sites in Pleasant Prairie, Wisconsin, and Karlsruhe, Germany.

The drugmaker has also been running “dry rehearsals” of its rollout scheme at more than 50 distribution sites across the U.S. The dry runs include taking the sites through the process of receiving the vaccines, opening them and administering them; plus, Pfizer has even created YouTube videos for staffers to consult during the process.

The process is “very, very doable,” Operation Warp Speed’s Gen. Gustave Perna said last week. Perna added that the rehearsals had boosted confidence among administration sites.

Pfizer and BioNTech recently filed their shot for emergency use authorization in the U.S. after late-phase data showed the vaccine was 95% effective in phase 3. The mRNA-based vaccine is set to go before the FDA’s independent vaccines advisory committee and the Centers for Disease Control’s Advisory Committee on Immunization Practices (ACIP) on Dec. 10, Warp Speed chief Moncef Slaoui said last week. Those reviews will take place “in parallel” so that the CDC can almost immediately defer to ACIP on how to distribute and allocate authorized doses, Slaoui said.

Meanwhile, the government aims to start shipping doses to distributors within 24 hours of a positive decision, with vaccinations slated to take place as quickly as 48 hours after an authorization, Slaoui said.

United isn’t the only one steeling itself for a global rollout of Pfizer’s shot: American Airlines has been running trial flights from Miami to South America to test thermal packaging and shot distribution logistics, the Journal said.

FedEx and DHL have also rolled out temperature-monitoring systems to keep tabs on future vaccine shipments, while United Parcel Service and Lufthansa are building so-called “freezer farms” comprised of multiple freezers at airport hubs to store vaccines during their journey to distributors.

Elsewhere, the U.K. is also taking steps to ready itself for distribution. The country’s Medicines and Healthcare Products Regulatory Authority (MHRA) could approve Pfizer and BioNTech’s shot by Dec. 7, the Financial Times reports, and recently appointed Nadhim Zahawi as vaccine rollout minister.

Zahawi will temporarily halt most of his responsibilities at the U.K.’s Department for Business, Energy and Industrial Strategy (BEIS) as he works as a joint minister between the BEIS and the Department for Health and Social care to oversee distribution arrangements for Pfizer’s vaccine, Pharmaphorum said. The U.K. has secured 40 million doses of Pfizer’s product.