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Comparison of Estimated Excess Deaths in New York City During the COVID-19 and 1918 Influenza Pandemics

Our take —

The study compared the all-cause mortality in New York City during the H1N1 pandemic in 1918, and the COVID-19 pandemic, primarily in 2020. The study found that the H1N1 pandemic had an increased mortality ratio of 2.80 compared to prior years, and that there was a 4.15 rate ratio for the COVID-19 pandemic compared to prior years. The study was limited in the actual comparability between the H1N1 pandemic, given that the century of medical and technological changes that occurred that may also have impacted the mortality rates. Therefore, this study offers important context to the current pandemic compared to that of the past, demonstrating the COVID-19 pandemic had greater relative mortality than the 1918 H1N1 pandemic, though it remains a crude ecological estimate overall.

Study design

Ecological

Study population and setting

The study objective was to investigate whether the excess deaths in New York City during the 1918 H1N1 pandemic were comparable to those during the initial months of the COVID-19 pandemic. The analysis used public mortality data from the CDC from 1914 – 1918, the NYC Department of Health from 2020, and the US Census from 2017 – 2020. It calculated the all-cause mortality rates per 100,000 person-weeks in these years, and compared them to one another via incident rate ratio. It used mortality from the 61-day period corresponding to the peak of the respective pandemics. In other words, for the 1918 pandemic, it used October and November, 1918 for 61 days total, and calculated the person-month mortality rate for October to November in 1914 to 1917; for the 2020 pandemic, data from March 11 to May 11,2020 for 61 days total as well were used, and calculated the mortality rate based on comparisons between this period and March 11 to May 11 for 2017 to 2019.

Summary of Main Findings

The study found an incidence rate of 287.17 (95% CI: 282.71 – 291.69) deaths per 100,000 person-months during the influenza pandemic, with a 2.80 incident rate ratio (95% CI: 2.74 – 2.86) comparing when the pandemic began in 1918 to the prior period’s average mortality before from 1914 to 1917. For COVID-19, there were 202.08 deaths per 100,000 person-months (95% CI: 199.03 – 205.17), with a 4.15 rate ratio (95% CI: 4.05 – 4.24) comparing the mortality rate in 2020 to the prior period’s average mortality from 2017 to 2019. Comparing the H1N1 pandemic and the COVID-19 pandemic, there was a rate ratio of 0.70 (95% CI: 0.69 – 0.72).

Study Strengths

The study drew on publicly available datasets using an outcome that is often well-measured (all-cause mortality) even in 1918. By also examining the years before each pandemic, the study sets up an important baseline of mortality. Given there is a 100-year difference between each pandemic, using more recent years as a baseline is important to better account for the technological and medical advances that have occurred in the interim. The study used the 61-day period corresponding to the peak of the pandemics in order to focus on deaths likely due to these pandemics, as opposed to taking the entire year which is subject to more fluctuations in mortality due to other causes.

Limitations

The study compares the 1918 pandemic to the 2020 pandemic when there was a number of health and safety measures and technological/scientific limitations during 1918 that may inflate the mortality rate greater than what would have occurred based on the 1918 pandemic’s virulence alone, leading to a reduced rate ratio when comparing 2020 to 1918 due to this larger denominator. The study compared the all-cause mortality rate, which is assumed to be related specifically to the pandemic, and groups individuals who died directly due to the pandemic because of infection with individuals who died from other causes, potentially due to disruptions in receiving hospital care, reduced employment resulting in other disparate and social factors increasing the risk for death, etc. During this 61-day period, also, there may be different degrees of social distancing or other mitigation measures that impact the indirect mortality estimates for the pandemic overall, but are missed by this 61-day window.

Value added

This study directly compares the all-cause mortality changes from the 1918 H1N1 pandemic and the COVID-19 pandemic to better contextualize the changes in death rates seen in New York City.

This review was posted on: 10 September 2020