Interesting, it sounds like they took antibodies from patients who were vaccinated and test them against a virus with some given mutation. Theoretically, if the immune system detects and responds to the mutated virus in the same way as with the non-mutant, the immunity should provide good protection.
Testing a drug that is meant to act at a specific target to elicit response is a little different when considering in vitro studies. Usually there will be some tissue sample, such as lung tissue, which is infected with a virus. These infected samples are then tested with varying concentrations of the drug in question, but what makes the usual difference when moving to in vivo is primarily Absorption, Distribution, Metabolism, and Excretion (ADME). This changes how much of the drug and in what form gets to the target tissue, for example it could be run through first pass metabolism which may add or remove functional groups or break the compound down in some other way. This can change how much drug reaches the target, on top of the usual distribution mechanics that may prevent the drug from reaching some tissues at all.
Immunology isn't quite my field, so my understanding of the mechanics there are a bit lacking. The immune system is quite a bit different mechanically than drug distribution.
