The freezer temperature required by Moderna’s vaccine makes it difficult to ship; the ultracold temperature required by Pfizer and BioNTech’s vaccine is nearly impossible to maintain outside of a large hospital or academic center with specialized freezers. For this reason, Pfizer has devised “thermal shippers” that, unopened, can keep the vaccines frozen for up to 10 days;once opened for the first time, they have to be replenished with dry ice within 24 hours then every five days. These shippers are supposed to be opened no more than twice a day to take out vials, and must be closed within one minute. The real catch, though, is that these shippers hold, at a minimum, 975 doses of the COVID-19 vaccine.

A large hospital in a city could deal with that volume, but in rural areas, a 975-dose shipment will need to be broken up into smaller ones—all while making sure the vials stay ultracold. “The other potential would be only shipping that vaccine to our more urban areas,” says Molly Howell, North Dakota’s immunization program manager, “but then we’re leaving out a lot of people who are health-care workers in rural areas or at high risk in rural areas.” To get the vaccine out to those places, her department is looking into buying frozen-transport coolers and potentially a dry-ice machine. If North Dakota is allocated, for example, 2,000 doses, the state will have to open the thermal shipper, repackage smaller allotments in dry ice, and physically drive them to rural clinics across the state. The vaccines are too precious to risk shipping conventionally.

The storage and handling requirements for these vaccines are especially stringent, but they’re also especially uncertain. In time, it may turn out that these mRNA vaccines can be stored at higher temperatures or can be reformulated to be stored at higher temperatures, as other vaccines have been. Scientists are actively trying to create more stable lipid nanoparticles, and Pfizer says it is working on a freeze-dried version of its vaccine that can be kept in normal freezers. These incremental improvements in storage are a normal part of the vaccine-development process, but they take time.