Charts from worldometers.info. Notations added by me. The data certainly doesn't scream "overwhelmingly effective."
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Seems overwhelmingly effective to me.
You need to consider the incubation period of Covid.
Thus, if you start social distancing on Day 0, you would not expect to see any effect to BEGIN until 14 days later.
So notice in the charts -- at 14 days, the peak was reached and the numbers started to drop. Without social distancing, the numbers would continue to INCREASE.
It comes down to the reproduction ratio -- Known at R0. Without social distancing, R0 of coronavirus is over 2.0. -- Imaging that being the equivalent of putting the foot on the gas. Your foot is on the gas AND you are going downhill.
With successful social distancing, you bring down the R0. At R0 = 1, you are coasting, tapping the gas just enough to maintain speed.
At R0 of 0, you are slamming on the brakes. We can never get to a R0 of 0.
Between R0 of 0 and 1, your foot is gently on the brake, gently slowing down.
So:
Day 0: Foot on the gas and going down hill. Numbers are escalating quickly. You start the lock down -- that's a tap on the brake.
But you're still going downhill for another 2 weeks... so the numbers keep increasing.
Day 14 -- you're no longer doing steeply downhill. Your foot is still lightly on the brake. You can start slowing down.
You may coast for a long time yet, but you are gradually slowing down.
So yes, those charts prove conclusively that lockdowns were VERY effective.