key: cord-0803011-rhwofd6n
authors: Shanks, G Dennis
title: COVID-19 versus the 1918 influenza pandemic: different virus, different age mortality patterns
date: 2020-06-02
journal: J Travel Med
DOI: 10.1093/jtm/taaa086
sha: db7b862a6e6bdd4b50b3aaae55da85af2f8a357d
doc_id: 803011
cord_uid: rhwofd6n

nan

Pandemics occur occasionally when an infectious agent wins its Darwinian gamble to successfully exploit another species such as ourselves. We are now in the midst of such a lethal pandemic with COVID-19 that has not been seen in living memory now that coronaviruses, likely of bat origin, have switched from high lethality, low transmission as in the case of SARS-CoV 1 to low lethality, high transmissionas in SARS-CoV 2; transmission being determined not only by R 0 but also by when in the clinical course cases become infectious. The likely consequences of this pandemic are unclear even as its enormous economic and human cost are only now becoming apparent. One point, however, has been fairly clear even from the earliest reports from China; COVID-19 mostly kills the elderly. 1

As the elderly are known to be a more vulnerable segment of the population, this should not come as a surprise, but it is distinctly different from the influenza pandemic of 1918-19. In the pandemic at the end of the First World War, young adults particularly those in their late 20s were those with the highest overall mortality rates for reasons that remain obscure a century later. 2 Just as coronaviruses are not the same as influenza viruses, no one would pretend that our epidemiological situation in 2020 is the same as 1918, but there are points of similarity and differences that can perhaps inform our current struggle against COVID-19 particularly in how two different age-mortality patterns resulted from these two distinct respiratory viruses.

The 1918-19 influenza pandemic remains the world's single greatest single mortality event of which we have detailed records. An estimated 50m deaths globally were not evenly spread with young adults, pregnant women and isolated populations bearing disproportional shares of the mortality. Most of those exposed to what we now know to be H1N1 influenza in 1918-19, however, did not become ill; typical attack rates were 1:5 to 1:3. Most of those who became ill had ordinary influenza-like illness and did not experience any severe symptoms.

Pandemics 1918 vs. 2020 Perspective 4 Death was an unusual outcome (1-3%) that despite anecdotal horror stories of sudden death, more typically occurred in the second week of illness. 3 In the vast majority of cases mortality was directly due to secondary bacterial infections (e.g. pneumococcal, streptococcal) that came long after the influenza virus had completed its destruction of the respiratory epithelium. 4 Although it is uncertain why mortality rates were particularly elevated among young adults, it is unlikely that the mortality curve which centered on age 28 years of age happened by chance. Some epidemiologists think it is more likely that the 1890 birth cohort was immunologically primed by exposure to the 1890 influenza pandemic, the last pandemic prior to that in 1918. 5 

variably been referred to as "original antigenic sin" or "antigenic seniority" whose mechanism remains uncertain. 6 Children except the very young were largely spared; the elderly mortality patterns sometimes were high forming the third part of a "W" shaped curve, but that was not always seen. 7 The unique feature of the 1918 influenza pandemic was its The contribution of the immune system to mortality is also uncertain but many patients (e.g.

diabetes mellitus) seem to have a dysregulated reaction sometimes referred to as a "cytokine storm" of inflammatory events leading to respiratory compromise and death. 10 Despite the uncertainties about the mechanism, the outcomes have been frighteningly clear with large 

The author does not claim any conflict of interest.

Funding: GDS is an employee of the Australian Defence Force; no specific funding was given for this epidemiological study. 

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