Age-specific excess mortality patterns and transmissibility during the 1889-1890 influenza pandemic in Madrid, Spain

Although the 1889-1890 influenza pandemic was one of the most important epidemic events of the 19th century, little is known about the mortality impact of this pandemic based on detailed respiratory mortality data sets.

We estimated excess mortality rates for the 1889-1890 pandemic in Madrid from high-resolution respiratory and all-cause individual-level mortality data retrieved from the Gazeta de Madrid, the Official Bulletin of the Spanish government. We also generated estimates of the reproduction number from the early growth phase of the pandemic.

The main pandemic wave in Madrid was evident from respiratory and all-cause mortality rates during the winter of 1889-1890. Our estimates of excess mortality for this pandemic were 58.3 per 10,000 for all-cause mortality and 44.5 per 10,000 for respiratory mortality. Age-specific excess mortality rates displayed a J-shape pattern, with school children aged 5-14 years experiencing the lowest respiratory excess death rates (8.8 excess respiratory deaths per 10,000), whereas older populations aged greater than or equal to 70 years had the highest rates (367.9 per 10,000). Although seniors experienced the highest absolute excess death rates, the standardized mortality ratio was highest among young adults aged 15-24 years. The early growth phase of the pandemic displayed dynamics consistent with an exponentially growing transmission process. Using the generalized-growth method, we estimated the reproduction number in the range of 1.2-1.3 assuming a 3-day mean generation interval and of 1.3-1.5 assuming a 4-day mean generation interval.

Our study adds to our understanding of the mortality impact and transmissibility of the 1889-1890 influenza pandemic using detailed individual-level mortality data sets. More quantitative studies are needed to quantify the variability of the mortality impact of this understudied pandemic at regional and global scales.