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.