Its just the increase year over year of excess deaths from the preceding years. 1917 to 1918 and 2019 to 2020 year over year 12% vs 16%. Still a pretty valid statistic comparing 100 years ago.
Adjustments must be made for the differences in population size of the United States in 1918 compared to 2020. Additionally, as health care and public health measures have improved, the population’s lifespan has risen. As a result, the per capita death rates for the two periods are substantially different, which adds complexity to these comparisons. Nevertheless, the 16 percent increase in the death rate in 2020 from preceding year, compared to the 12 percent jump during the 1918 Spanish flu pandemic, is staggering.