This is impossible to state with any certainty because:
- The exact path of the "Spanish" Flu virus in its early stages is unknown; and
- Early deaths from the virus were not recognised at the time and received little attention.
Available evidence suggests that the Spanish Flu was "seeded" around the world in a number of localised outbreaks well before the mass deaths that occurred in 1918.
Reports of influenza deaths in countries as widely spread as Norway, Sweden, Finland, Canada, Spain, Britain, France, Germany, Senegal, Tanzania, Nigeria, Ghana, Zimbabwe, South Africa, India and Indonesia were recorded. The very wide geographic spread of these deaths in such a short period, in the absence of air travel at the time, suggests that the disease had spread around the globe prior to this time and that 'seeding' had occurred.
Oxford, J. S., et al. "Early herald wave outbreaks of influenza in 1916 prior to the pandemic of 1918." International Congress Series. Vol. 1219. Elsevier, 2001.
Finland recorded unusually high levels of pneumonia deaths in 1917; in Britain the Bochum Administration of Sick Insurance reported a 40% increase in influenza cases in 1916; and a major epidemic of respiratory diseases swept through the United States at the end of 1915.
Although in 1918 influenza was not a nationally reportable disease and diagnostic criteria for influenza and pneumonia were vague, death rates from influenza and pneumonia in the United States had risen sharply in 1915 and 1916 because of a major respiratory disease epidemic beginning in December 1915
Taubenberger JK, Morens DM. 1918 Influenza: the Mother of All Pandemics. Emerging Infectious Diseases. 2006;12(1):15-22.
Therefore, it is possible that the first major outbreak of deaths occurred no later than December 1915. The literal first cases of deaths from when the virus emerged is impossible to determine with any precision, but might have occurred when the virus first made the jump to humans from birds or pigs, possibly from 1913 to 1915.