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I assume that there are hundreds of possible factors.
One thing I do know is that the reasons behind the deadly second wave of the Spanish Flu are still up for debate. Some research shows that it was likely caused by a mutation, other research shows that it was likely caused by simple seasonal weather changes. There is even research out there claiming that the second wave of Spanish Flu deaths was actually exacerbated by the over-prescribing of Aspirin.
I suppose that the uncertainty doesn't change much in that the authorities will still have to be ready for the possibility. I just wanted to emphasise that a 2nd wave isn't necessarily a nailed on thing.
Stonehedge-
I guess a second wave in this case (China or anywhere post lockdown) is really just a re-animated first wave rather than an actual second wave (mutated version). The first wave was stopped / slowed and now it can get going again as restrictions are lifted.
I have no idea what I'm talking about though.
villa-ru
Surely it would depend a lot on 1) what the 'first wave' is like and 2) what restrictions on the public are put in place and how they are relaxed exactly?