key: cord-1019831-189a8p6p
authors: Shanks, G Dennis; Brundage, John F
title: Did coronaviruses cause ‘influenza epidemics’ prior to 1918?
date: 2020-11-04
journal: J Travel Med
DOI: 10.1093/jtm/taaa206
sha: 91e78521484857ebf5fca3a61cc1a1d2299143a1
doc_id: 1019831
cord_uid: 189a8p6p

There were epidemics of influenza-like illness during the winters of 1915–6 and 1916–7 when mortality was low among infants and children but high among older adults (like SARS-CoV-2 but not influenza). Records suggest that ‘influenza’ epidemics preceding the 1918–9 pandemic were caused by multiple respiratory viruses possibly an undocumented coronavirus.

Coronaviruses prior to 1918? 3 The SARS-CoV-2 (COVID-19) pandemic is often referred to as "unprecedented." However, coronaviruses likely coexisted as commensals with various animal hosts for millennia. They were first isolated from animals in the 1930s and are well known pathogens of the respiratory For this report, we reviewed characteristics of "influenza epidemics" that occurred before coronaviruses were discovered to identify any that may have been caused by a coronavirus.

To distinguish possible coronavirus epidemics from influenza epidemics, we used three characteristics of the ongoing SARS-CoV-2 pandemic as epidemiologic indicators of coronavirus epidemics in general. First, we assumed that during coronavirus epidemics, mortality rates are low among infants and young children. In contrast, consider that during influenza epidemics, age-mortality relationships are typically "U-shaped" with the highest mortality rates among the youngest and oldest. [2] [3] [4] Second, we assumed that during coronavirus epidemics, mortality rates increase slowly with increasing age after childhood through approximately 60 years. In contrast, consider that a hallmark of the 1918-9 influenza pandemic was the spike in mortality rates among young adults in their late 20s ("W-shaped" age mortality curve). 5 Third, we assumed that during coronavirus epidemics, mortality rates increase sharply after approximately 60 years of age. In contrast, consider that during the highly lethal second wave of the 1918-9 influenza pandemic, mortality rates among the elderly were lower than during corresponding months of recent prior years. 2,3,6-8

Coronaviruses prior to 1918? 5 Vital statistics reports from 35 large USA cities documented sharp peaks of "pneumonia and influenza" mortality during the successive winters of 1915-6 and 1916-7. In some locations, the epidemics were severe; overall, they accounted for estimated 22.8 and 14.0 excess deaths / 100,000 population, respectively. 4 Figure 1A such as obesity, diabetes, immune deficiencies, and heart, lung, and kidney diseases. In turn, the age-mortality relationships during SARS-CoV-2 epidemics are "reverse ski slope"

shaped. Figure 1 shows the composite age mortality figures for both the USA (1C) and

England/Wales (1D) emphasizing elderly pneumonia/influenza mortality in 1916 and 1915 respectively even though there is little to differentiate infant mortality at this country-wide level of resolution.

In a review of monthly age-stratified mortality rates in New York City between 1910 and 1920, Olsen and colleagues documented that, during the 1915-6 and 1916-7 winter seasons, the U-shaped age-mortality relationship that typifies influenza epidemics was distorted because mortality rates were unusually low among the youngest (<5 years) and markedly elevated among the oldest (>65 years) aged residents; that is, the age-mortality relationship was SARS-CoV-2-like. 10

Coronaviruses prior to 1918? 7 Similarly, official vital statistics of England and Wales between 1890 and 1919 documented that among females (males were not included due to war time commitments) older than 75, influenza-related mortality rates were higher in 1916 than in any year since 1890, while among those younger than 15 and between 15 and 35, influenza-related mortality rates were lower in 1916 than in any of the previous 25 years. 11 Thus, in 1916 and 1917, influenzarelated mortality rates among females in England and Wales were low among the youngest and unusually high among the oldest aged. See Figure 2B .

In summary, during 1915-6 and 1916-7, age-mortality relationships, at least in some affected populations, were more SARS-than influenza-like. In the U.S., "influenza and pneumonia" mortality rates among the elderly were higher in 1916-7 than during the 1918 pandemic year.

The findings suggest that a SARS-like coronavirus, perhaps with one or more influenza viruses, caused epidemics of influenza-like illnesses during the winters of 1915-6 and 1916-7. 

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