I certainly wouldn't give up hope. I've heard that pre-med students have been known to apply (and accept) the UBC MPH as a back-up plan, and then release their seats when they've been accepted to UBC Med in early summer. Can't quite remember where I was told this, but I wonder if someone asked a similar question to the program rep at the UBC Population and Public Health open house I attended last winter.
Also UBC is one of the earliest schools in Canada to offer acceptances to their MPH program, and as a result require a response before many schools have sent out their offers. I'd imagine this would occasionally account for a few more seats being released, as they later receive offers to a preferred program.
I'm not sure the policies on accepting offers then declining at a later date, but I guess it happens. I can see both sides of the coin, but would hope at least there is open communication with the school on the matter if an offer is accepted "tentatively".
Anyway, don't give up hope PacificPH! All the best!