I’m a bit surprised by this. The entire logic behind booster doses for the immunocompromised (which several countries are already doing, e.g. US/UK) is based on a lack of (or low) immune response to the vaccine, which can be confirmed by an antibody test (measuring antibodies to the spike protein). There are studies with large numbers of people where you can see the median levels of antibodies in healthy adults and compare results (e.g. here and here). These tests are quantitative and give you a number, not just a positive or negative result, some people can have orders of magnitude more antibodies than others as a result of vaccination.
Also my impression is that antibody levels (post vaccination) are possibly correlated or even predict who can be infected and correlate with vaccine effectiveness. Is that not likely to be the case?
This is a bit anecdotal but I’ve read about people in the UK on immunosuppressive therapy getting 4 vaccine doses (as advised by their doctor) in order to generate enough immune response to have detectable antibodies.
The problem is in the first few days of an infection you won’t know whether you’ll be sufficiently sick to be hospitalised on say day 10. We don’t really know who will deteriorate on day 7 and beyond, and who will recover. And if you take the drug later it won’t be as effective.
I suspect this drug will only be used for high risk groups, due to risk of side-effects and the high price (US is paying $700 per course).