Reducing infant mortality, and other preventable mortality as well, does serve to increase population in the short term but over a generation or so birthrates plummet as a result. A couple more generations are then needed to begin realizing declining populations. It is a long term strategy. The only things that can reduce population quickly are high CAR/CFR pandemics, all out war, or civilization collapse. CAR/CFR is case acquisition rate (what % of the population gets infected) and case fatality rate (what % of those infected die). Like with many other terms the word pandemic is used loosely these days. To make a dent in world population you’d need something on the order of the black plague on a worldwide basis. What happened in 1918 was trivial comparatively and not anything that actually reduced population. By all out war, I mean war beyond what was seen with WWII. As horrific as WWII was, it did not reduce world population. There were more people on earth when the war ended than there were when it started. It would require something far worse than WWII to reduce population. The last option is not a minor event like the Depression of the 1930’s or a run of the mill currency collapse or some such. To reduce population you would need a long term grid failure or something of that magnitude that causes civilization itself to fail.