Read on Twitter
woo boy guess I'm doing a fill-finish thread tonight! #sciencefactoftheday 1/? https://twitter.com/lmorello_dc/status/1339338398172127232
First things first! We aren't manually filling every vial once a bulk drug substance is produced - big machines do it super fast. This one is from Emergent Biosolutions, a Baltimore company who's helping make Pfizer's vaccine 2/
Because it's a machine, there's inherent variation and tolerance in any setting. For instance, if you want it to dispense 1 mL every time, sometimes you'll get 0.9 and sometimes 1.1 (I don't know the actual tolerance for Emergent's machines, this is just an example). 3/
So in this example, to make sure you get 1 mL in every vial, you'd set the target higher so that the minimum end of the range meets your criteria. Say we set it for 1.1, then every vial will have at least 1.0. This means that some vials could have up to 1.2 in this scenario 4/
Depending on your dose and the amt that you overfill, you could have enough for an extra dose in a vial. That's what's happening here. But you can't label the vials that way, because not every vial will have an extra dose (so you end up under-promising and over-delivering)
There's also overfill you want to include when you set your target to make sure it's EASY to get the vaccine out with a needle. If you have to extract every last drop, it's annoying and you'll draw bubbles. If you have extra, it makes it so much easier, especially w/ viscosity
And the regulation 21 CFR 201.51(g): https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=201.51
so that's overfill in a nutshell! it's great that we can use the extra vaccine doses, but administrators need to be super careful not to bank on the extras when they account for everyone getting a second dose when they need it.
