Study Notes on the 1918 Pandemic H1N1 Influenza A Virus

Genesis and Pathogenesis of the 1918 Pandemic H1N1 Influenza A Virus

Authors

  • Michael Worobey, Guan-Zhu Han, Andrew Rambaut

  • Affiliations:

    • Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721

    • Institute of Evolutionary Biology and Centre for Infection, Immunity, and Evolution, University of Edinburgh, Edinburgh EH9 3JT, UK

    • Fogarty International Center, National Institutes of Health, Bethesda, MD 20892

Publication Details

  • Source: Proceedings of the National Academy of Sciences of the United States of America

  • Publication Date: June 3, 2014

  • Volume: 111, Issue: 22, Pages: 8107-8112

  • Published by: National Academy of Sciences

Significance of the Study

  • The origin of the 1918 pandemic influenza A virus (IAV) and its unusual fatality rate among young adults remain significant biomedical mysteries.

  • The study utilizes advanced methods to describe the virus's evolutionary history and its severe impact on certain cohorts.

Key Findings of the Research

  • Pandemic Virus Origin:

    • The 1918 pandemic virus likely emerged shortly before 1918 from a reassortment of a preexisting human H1 IAV lineage and an avian virus, possessing avian neuraminidase genes in addition to internal protein genes.

    • Background: A human H1 virus is inferred to have emerged before ~1907.

  • Host-Specific Molecular Clock Approach:

    • This methodological advance provides more accurate inferring of IAV origins, revealing that individual host lines have distinct evolutionary rates.

  • Epidemiological Evidence:

    • The study was able to explain the higher mortality rates observed in age cohorts of 20-40 years, particularly those born from around 1880-1900, who likely had no protective childhood exposure against this virus.

  • Decline in Mortality Post-1918:

    • The paper discusses the rapid return to typical IAV mortality patterns by the early 1920s.

High Mortality Patterns

  • The pandemic distinctly affected adults aged 20-40, peaking in deaths among those aged 25-29 while sparing both older adults and children aged 5-15—an unusual trend when compared to typical influenza cases.

  • Outcomes were frequently due to secondary bacterial pneumonia following initial viral infections.

Childhood Exposure Theory

  • The childhood exposure to different influenza virus strains significantly influenced adulthood immunity and thus affected mortality rates.

  • Those adults in the affected groups had childhood exposure to the heterosubtypic putative H3N8 virus, offering little protection against the pandemic H1N1 strain.

Hypotheses Regarding 1918 H1N1 Virus Origins

  • Introduction of Avian Segments:

    • The first hypothesis suggests that all genome segments of the virus originated directly from an avian source.

    • Alternatively, it was possible that reassortment between pre-existing viruses circulating decades prior contributed to pandemic virulence.

  • Molecular Clock Analysis:

    • The study supports that H1 emerged in humans before ~1907 rather than in 1918, allowing for characterizing the evolution context of the epidemic.

  • A notable point in genetic analysis indicated significant high mortality was linked with immunity patterns based on previous exposure to the H3N8 virus.

Phylogenetic Evidence

  • HA (Hemagglutinin) and NA (Neuraminidase) Subtypes:

    • Re-assessment of H1 HA and NA suggests lineages and variants came from different evolutionary timelines, impacting mortality outcomes.

    • Seasonal H1N1 likely did not derive directly from the pandemic virus thus providing insight into protective immunity.

  • The phylogenetic results indicated that individuals born before or after certain years (specifically 1880-1900) would have had varied immunity against the 1918 virus due to their exposure to historical virus strains.

Methodologies and Analytical Techniques

  • Statistical Approaches:

    • Employed host-specific local clock (HSLC) analysis to derive more accurate timelines and genetic histories of IAV.

    • The methodology included genomic analyses of H1N1 evolutionary rates and diversification patterns across populations.

  • Further surveillance and results were corroborated through available seroepidemiological data connected to different age groups, underpinning the study's hypotheses.

Implications of the Study

  • The findings elevate understanding of how age-related immune exposures could function as crucial predictors in future IAV pathogenicity and seasonal influenza behavior.

  • Emphasizing the connections between historical pandemics and current influenza epidemiology, the research outlines how original exposure influences responses to new strains.

Conclusions Drawn

  • An understanding of cohort immunity effects across decades can better predict and manage future pandemics, particularly regarding how initial exposure patterns can influence disease severity.

  • The study calls for the reconsideration of influenza management strategies, considering prior exposures among adult populations.

Research Contributions

  • Collaboration: The study was supported and developed through joint efforts of different academic institutions, with Michael Worobey notably leading the research.

References

  • Full bibliography of sources and related articles is documented, illustrating the extensive research bases examined for this study; includes classic studies as well as more recent investigations contributing to the insights presented in this analysis.

Key Figures

  • Data visualizations demonstrate the mortality rates across various age cohorts and seroarcheological patterns linking to the pandemic's viral impacts. Charts show correlations between antibody levels and mortality statistics from the outbreak period.