Mortality Patterns Associated with the 1918 Influenza Pandemic in Mexico: Evidence for a Spring Herald Wave and Lack of Preexisting Immunity in Older Populations

Abstract

Background While the mortality burden of the devastating 1918 influenza pandemic has been carefully quantified in the US, Japan, and European countries, little is known about the pandemic experience elsewhere. Here, we compiled extensive archival records to quantify the pandemic mortality patterns in two Mexican cities, Mexico City and Toluca. Methods We applied seasonal excess mortality models to age-specific respiratory mortality rates for 1915–1920 and quantified the reproduction number from daily data. Results We identified 3 pandemic waves in Mexico City in spring 1918, fall 1918, and winter 1920, characterized by unusual excess mortality in 25–44 years old. Toluca experienced 2-fold higher excess mortality rates than Mexico City, but did not have a substantial 3rd wave. All age groups including those over 65 years experienced excess mortality during 1918–20. Reproduction number estimates were below 2.5 assuming a 3-day generation interval. Conclusion Mexico experienced a herald pandemic wave with elevated young adult mortality in spring 1918, similar to the US and Europe. In contrast to the US and Europe, there was no mortality sparing in Mexican seniors, highlighting potential geographical differences in pre-existing immunity to the 1918 virus. We discuss the relevance of our findings to the 2009 pandemic mortality patterns.

Death Patterns during the 1918 Influenza Pandemic in Chile

“…Further reports from the United States and Europe have shown that influenza-related deaths among seniors ( > 50 years of age) were significantly reduced during the lethal 1918–19 pandemic wave relative to baseline periods. In contrast, this protective effect for seniors did not apply to Mexico, Colombia, and remote island populations, probably because of differences in prior immunity between regions ( 6 , 13 ). In addition, large geographic variations in mortality rates from the pandemic have been documented ( 14 – 16 ).…”

“…Our understanding of the epidemiologic patterns of this pandemic has improved over the past decade as a result of intensive efforts to locate, digitize, and analyze archival disease records ( 3 ). In particular, studies focusing on the United States ( 4 , 5 ), Mexico ( 6 ), Colombia ( 7 ), Brazil ( 8 ), and Peru ( 9 ) have shed light on geographic variation in the patterns of timing, intensity, and patient age during successive pandemic waves across the Americas. Yet there have been no reports from temperate locales of South America; this gap prevents a complete understanding of the epidemiology of the pandemic throughout the continent and across different climatic zones.…”

Phylogeography of the Spring and Fall Waves of the H1N1/09 Pandemic Influenza Virus in the United States

“…The epidemiology of the three influenza pandemics of the 20th century (1918 [H1N1], 1957 [H2N2], and 1968 [H3N2]) has been studied intensively, particularly the variation in mortality and transmissibility among countries and cities and between successive waves (1,6,18). However, the viral gene sequence data acquired from these past pandemics has been insufficient to precisely reveal epidemiological patterns and dynamics, especially at the level of individual cities.…”

Pandemic influenza and socioeconomic disparities: Lessons from 1918 Chicago

“…In 2006, a seminal study revealed more than 30-fold variation in 1918 pandemic excess mortality rates across a sample of 20 countries, with socioeconomic factors explaining a significant fraction of the observed variation (2). In Latin America, pandemic excess mortality rates varied from 0.4 to 2.9% in national and province-level data, a greater than sevenfold variation (4,8,9). In contrast, United States and European populations fared relatively well during the pandemic, despite intense disruption at the end of World War I (excess mortality rates of 0.5-1.1%) (3,5).…”