The reason for the Spanish flu having had such a disastrous impact was that the two enemy coalitions fighting in WWI kept the information about the spreading flu secret. They wanted to sustain morale among the young soldiers, and hid the information.
The reason this episode is called the “Spanish flu” is that Spain was neutral, and was publishing the alarming data within Spain.
It appears then that the main implication from this particular historical event is: Don’t hide the data, so as to prepare for prevention.
The particular implication for the problems the world is facing now is that (if the media can be trusted) China has not been disclosing what was going on within its borders—and while doing so, has been buying up medical supplies around the world (again, if media reports on this can be trusted). It did so with the help of an incompetent World Health (dis)Organization, which did not go to check what was happening on the ground (though this is the institution’s obligation), and the world sleepwalked into the present crisis.
The main lesson of the disastrous 1918-1919 Spanish flu is then prevention and the existence of institutions held accountable to enforce the publishing of information (at least in peaceful times)—and not post-crisis aggregate macroeconomic gobbledygook.