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Oh sure, it's doing that as we speak. So it's obviously possible strains different enough for currently immune people to still get it will develop, see also this quite interesting explanation about the flu (which also explains what all of this H-something-N-something-else is about). But in the end, it's not influenza, and it is a much more 'high profile' type of illness, not sure it will be around and about at all times the same way influenza is.
This will also mutate, no?