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It's been widely reported in the press that Pfizer's vaccine needs storage at -70C but Moderna's only needs -20C. Why is this so, when they are both mRNA vaccines?

There's actually a looong article in a publication called Science News (published by the SSP, no relation to the journal Science or the AAAS) which sorta, kinda tries to explain this, but actually it seems able only to say that there's a proprietary formulation difference... (99% of that article explains the common basis of mRNA vaccines, actually.) So, is there a more insightful explanation than punting to proprietary formulation differences?

MIT News quotes an expert saying that it may have something to do with freeze drying:

Both the Moderna and Pfizer RNA vaccines are carried by lipid nanoparticles with PEG. [...]

One way to make RNA vaccines more stable, [Daniel] Anderson [a professor of chemical engineering at MIT] points out, is to add stabilizers and remove water from the vaccine through a process called lyophilization, which has been shown to allow some mRNA vaccines to be stored in a refrigerator instead of a freezer.

Lyophilization is more or less a fancy for free drying, which has been used for (traditional) vaccines as well. There's mention in a Nature news article that Pfizer is working on a lyophilized formulation, but it's not said there if Moderna is already using one (or not). I'm not sure what the studies of freeze drying for mRNA (alluded by Anderson) are though. Did these some of these show stability at -20C, for instance?

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