INTRODUCTION

In the United States, infection with SARS-CoV-2 caused 380,000 reported deaths from March to December 2020.1 Reported deaths likely underestimate the true number of COVID-19 deaths for a variety of reasons.2 To better reflect the mortality burden of the COVID-19 pandemic, various estimates of excess deaths have been made, where excess deaths are defined as the difference between the observed numbers of deaths in a specific time period and expected numbers of deaths in the same time period based on mortality during prior years. These analyses of all-cause mortality data found large relative increases in mortality during the pandemic among all age groups,3 but particularly people 25 to 44 years of age.4
In the United States and other temperate countries, overall mortality rates are generally highest during the winter months, with peak weekly mortality rates usually coinciding temporally with influenza virus activity.5 Vega and colleagues developed the Moving Epidemic Method (MEM), a useful analytical tool to compare the severity of influenza seasons over time, across countries, and across age groups that may have different baseline levels of morbidity.6The method sets standardized intensity thresholds based on the highest values of a surveillance indicator observed within recent, prior seasons. In this way, even though particular age groups or geographic areas have different baseline levels of mortality, the interpretations of the thresholds would be similar.
We adapted the MEM to all-cause mortality data from the United States to assess the severity of the COVID-19 pandemic across age groups and states. Because case ascertainment, definitions, testing practices, care seeking behavior, and clinical behavior have all changed in response to the pandemic,7-9 many of the available data sources with historical data for estimating severity, such as syndromic surveillance,6 are complicated by such pandemic-influenced changes. Additionally, as SARS-COV-2 is novel, historical cause-specific data are not available for comparison as they would be for influenza. Recent reports have documented that COVID-19 was the third leading cause of death in 2020, accounting for more than two thirds of the increase in all-cause mortality.4,10-12Overall death recording is unlikely to be affected by factors that have changed during the pandemic (e.g., changes in testing, case ascertainment, or care seeking behavior); these data may provide valuable information about the severity of the ongoing COVID-19 pandemic. The purpose of this study was to set intensity thresholds in all-cause death data from past years with the MEM and use those thresholds to describe the severity of the COVID-19 pandemic in the United States over time and by age and by geography.