CONCLUSIONS

All-cause mortality during the COVID-19 pandemic was unusually high compared to recent, historical all-cause mortality. We attribute the very high severity of mortality during the study period to the COVID-19 pandemic, though not all of the increase in all-cause mortality may be directly attributable to infection with SARS-CoV-2; other causes may have been indirectly affected by the pandemic, including addiction-related deaths and motor-vehicle fatalities.17-19 Cause-specific mortality data are not yet available by age and state, but in the future the MEM could be applied to specific categories of causes of death (e.g., respiratory, circulatory, injury, etc.) to observe which causes were more or less affected by the COVID-19 pandemic and among which age groups and states.
This method is especially useful for comparing across geographies or age groups that have very different baseline levels of mortality. While weekly all-cause mortality was highest among adults at least 65 years of age (Figure S1-S4), when comparing age groups to their own historical peak mortality rates, we noted a lengthy period of sustained very high mortality among younger adults, 18-49 years of age, with 29 consecutive weeks of all-cause mortality in the very high severity level. As cause-specific mortality rates become available, further investigation of cause-specific mortality rates may help explain how this age group was impacted by the pandemic. During the COVID-19 pandemic, or future public health emergencies, this method flags potential trends that would benefit from further exploration.
We also noted substantial geographic heterogeneity. The three periods of very high mortality across the states in the spring, summer, and fall/winter coincide generally with increases in reported COVID-19 cases and deaths nationally.20 Some states experienced very high severity in multiple time periods, although most states did not experience very high severity levels until the fall/winter weeks. On average, jurisdictions experienced 7.5 very high severity weeks. Multiple jurisdictions experienced >10 very high severity weeks (Connecticut, District of Columbia, Florida, Illinois, Maryland, Michigan, Montana, New Jersey, and Texas); and a handful of states experienced ≤1 very high severity week (Alaska, Hawaii, Maine, New Hampshire, Oregon, Vermont, and Washington) (Figure 2). This may be another area where this method can be used to help target a deeper investigation of why some states had relatively more or less burden of the COVID-19 pandemic.
This method does have some limitations. While all-cause mortality among adults at least 50 years of age in the historical seasons had a strong seasonal pattern, mortality among children and adults 18 to 49 years of age did not. Because of the lack of seasonality, the highest mortality weeks may be spaced throughout the annual season instead of close together. However, the interpretation remains valid, in the sense that all-cause mortality during the study period was unlikely to be highly elevated for several weeks, much less for 29 consecutive weeks. Our results rely on extrapolation of the secular trend from March 2020 to December 2020, and interpretation of our results should be more qualitative as the study period progresses. For future use of the MEM to analyze all-cause mortality data, the inclusion of March to December 2020 in the historical data will increase both the mean and the variance of peak weekly mortality, resulting in ITs which are both higher and further spaced. Depending on the desired interpretation, adjustments to the MEM may be helpful.
It is perhaps not surprising, given the high number of reported COVID-19 deaths, that unusually high mortality rates were observed in the United States during the COVID-19 pandemic. By setting standardized statistical thresholds in the mortality data, however, this method allows for easy comparison across age groups, geography, and over time, even when the baseline mortality rates differ, to identify the time periods, subpopulations, and places that experienced the greatest standardized increases in deaths as a result of the pandemic. Furthermore, because this analysis relies on all-cause mortality, a metric that is widely monitored and less likely affected by misclassification or reporting changes during the pandemic, similar comparisons could be made across countries if death registration data during 2020 and prior years were available. This method for standardized comparison of pandemic severity over time across different geographies and demographic groups provides valuable information to better understand the differential impact of the COVID-19 pandemic across locations or subgroups. Results can inform future investigations into the factors that may have contributed to differences in the severity of the pandemic across populations in terms of relative increases in all-cause mortality.