[azepoi]

Exponential growth takes some time at the start before it is noticeable. And then it's unmanageable. Look at the time between the first and the second wave in Italy or France. It took a few months to become uncontrolled again after the lockdown was lifted. So before it was exploding in northern Italy around February it likely needed a few months of spreading throughout society too.

[m0llusk]

But this coronavirus is not capable of exponential growth. It does spread quickly, but not that quickly. The most well known exponential growth model of SARS-CoV2 infections is the Imperial College model which has proven to be at least one order of magnitude off in its predictions. The most accurate model of SARS-CoV2 spread currently available is that published by Michael Levitt which predicts SARS-CoV2 comes well short of exponential spread even in worst case scenarios. The only reason we thought there was exponential growth is because the beginning of the epidemic was so poorly understood.

[azepoi]

Of course it's not really exponential. And even the simplest SIR toy model does not output an exponential solution, except as an aproximation at the start. My point was simply that it took several months between very low numbers at the end of the first lockdown and the decision to put a second albeit lighter lockdown in France and (at least parts of) Italy. So one should not be surprised that the virus was circulating for several months before the problem became really noticeable in february.