On the day that a COVID-19 vaccine is approved, a vast logistics operation will need to awaken. Millions of doses must travel hundreds of miles from manufacturers to hospitals, doctor’s offices, and pharmacies, which in turn must store, track, and eventually get the vaccines to people all across the country. The Centers for Disease Control and Prevention, along with state and local health departments, coordinates this process. These agencies distributed flu vaccines during the 2009 H1N1 pandemic this way, and they manage childhood vaccines every day. But the COVID-19 vaccine will be a whole new challenge.
The leading vaccine candidates both deploy a new, long-promised technology. Their core is a piece of mRNA, genetic material that in this case encodes for the spike protein—the bit of the coronavirus that helps it enter human cells. The vaccine induces cells to take up the mRNA and make the spike protein and, hopefully, stimulates an immune response.
By using mRNA, vaccine makers do not need to produce viral proteins or grow viruses, methods that are used in more traditional vaccines and that add time to the manufacturing process. This is why Moderna and Pfizer/BioNTech have been able to get their vaccines into clinical trials so quickly. Moderna went from a genetic sequence of the coronavirus to the first shot in an arm in a record 63 days.
To get a naked strand of mRNA inside a cell, scientists have learned to encase it in a package called a lipid nanoparticle. mRNA itself is an inherently unstable molecule, but it’s the lipid nanoparticles that are most sensitive to heat. If you get the vaccine cold enough, “there’s a temperature at which lipids and the lipid structure stop moving, essentially. And you have to be below that for it to be stable,” says Drew Weissman, who studies mRNA vaccines at the University of Pennsylvania and whose lab works with BioNTech. Keep the vaccine at too high a temperature for too long, and these lipid nanoparticles simply degrade. Moderna’s and Pfizer/BioNTech’s vaccines have to be shipped frozen at –4 degrees and –94 degrees Fahrenheit, respectively. Once thawed, Moderna’s vaccine can then last for 14 days at normal fridge temperatures; Pfizer’s, for five days.
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.