But it did prevent the index patient from becoming seriously ill (i.e. hospitalised). It most likely reduced the initial dose of all that got infected by patien

But it did prevent the index patient from becoming seriously ill (i.e. hospitalised). It most likely reduced the initial dose of all that got infected by patient-0 [0]. And it most likely prevent _any_ of the invected to become hospitalised either. Now you can make the argument (or at least attempt to make this argument), that it would have been _better_ for patient-0 not to be vaccinated, because then, maybe, patient-0 would already have been sick and not at that party. Unfortunately this is the main problem of Covid in the first place: A significant portion of the downstream infections are generated before the onset of symptoms. So yes, the vaccine didn't prevent much of the spread [1], but it still reduces the amount of strain put on the health care system. Somewhere upthread there was the question: "How do the few unvaccinated overwhelm the hospital system?" The answer is: We can't deal with 1% of society being in the hospital. So getting 5% of all unvaccinated there and having 20% unvaccinated is already too much! Last year we managed, because we radically cut down on all interactions and delayed the spread that way. We _can_ do that again. But then this Christmas party just wouldn't have happened at all! [0] Note this is still somewhat a conjecture, as actually getting clean data for this is nigh impossible. [1] I'd still take the 30% reduction, but it's an upper bound and can still decrease. So yeah, let's call this "doesn't help slow down the spread"