Despite a century of medical advances, more Americans have now died from Covid-19 than the number who succumbed to the 1918 flu pandemic, according to new data.

The latest grim milestone comes as the country is experiencing a fourth-wave driven by the highly contagious Delta variant, with low vaccination uptake in many regions the main cause of death.

Johns Hopkins University tracker showed 675,722 US coronavirus deaths as of Friday, which surpasses the 675,000 US deaths during the influenza outbreak that began in the last year of World War I.

All told, some 50 million died worldwide in the flu pandemic -- sometimes inaccurately referred to as the "Spanish flu" -- making it the deadliest event in human history, according to epidemiologists.

That far exceeds global Covid deaths so far -- around 4.7 million. 

But the United States has borne a disproportionate 14 percent of those fatalities, despite making up only five percent of the world's population.

The American population in 1918 was less than a third of what it is now, meaning the flu deaths would be equivalent to some 2.2 million in today's terms.

Unlike today's influenzas, which impact children and the elderly the most, the 1918 flu caused unusually high mortality among young adults.

According to the Centers for Disease Control and Prevention, with no vaccines and no antibiotics for secondary bacterial complications, control efforts were limited in 1918-19 to non-pharmaceutical measures.

These included "isolation, quarantine, good personal hygiene, use of disinfectants, and limitations of public gatherings," it said.

Many of the same measures, including face masks, were recommended when the Covid pandemic began.

Now, however, there are also multiple safe and highly effective vaccines that were developed and tested in record-time -- but 24 percent of US adults, or almost 60 million, haven't yet gotten their first dose.

Uptake has been hit by a polarized political climate and what experts call an epistemological crisis, with misinformation supercharging vaccine hesitancy to historically new heights.

Beyond vaccines, effective treatments have been developed such as monoclonal antibodies, corticosteroids to dial down hyperactive immune responses in patients with severe Covid, and advanced ventilators.

As for the 1918 flu, descendants of the H1N1 strain that continue to make up the seasonal influenza viruses we fight today, with far less severity.