In practice these big trials are run as sloppily as possible to get the desired result. There’s a whole cottage industry of consultants. Think unclear instructions leading to dosing mixups and such. The type of trial you’re proposing is notorious for sloppiness.
More time and money for data that you could get more efficiently? I mean we have some data on the Pfizer vaccine and new variants and it looks like the Pfizer vaccine works fine.
You need huge trials to do this.
The power of your trial comes from the "events", i.e. number of people who got symptoms of covid-19. Have 2 vaccinated groups? Not many events.
You need 50% more subjects goes my calculation.
I am not sure, may be it might show the Moderna vaccines are just as ineffective (against the variants circulating now). I am not sure anyone would want that, particularly things like these are cropping up