Lots of vulnerable left, in my opinion the virus just caught a part of those proclaimed "vulnerable".

Further different effects took place. Because of the lockdowns the stress levels of big parts of the population decreased. Less travelling, more home office and so on. The whole world slowed down. This also strengthened the immune system.

Health isn't a permanent state, it needs to be cultivated and preserved. One can be for example exposed to lots of inside and outside stress which makes people vulnerable to any kind of virus. New infections, a pre-existing normal flu or cold or any other new disease one didn't have some weeks ago, lack of sleep, lack of good nutrition paired with sorrows and other psychological stressors and there you go. Just saying.