Why Have Mortality Rates Been Rising for US Working-Age Adults?

Notice that this comparison is not about infant or child mortality rates, nor is it about life expectancy for the elderly. Indeed, life expectancies for US infants and for children under the age of 10, and for adults who have already reached their 80s  are higher than for the peer group of countries. It's the in-between age group where the difference arises. 

As one digs into these patterns more closely, here are some of the details that emerge:

The committee identified three categories of causes of death that were the predominant drivers of trends in working-age mortality over the period: (1) drug poisoning and alcohol-induced causes, a category that also includes mortality due to mental and behavioral disorders, most of which are drug- or alcohol-related; (2) suicide; and (3) cardiometabolic diseases. The first two of these categories comprise causes of death for which mortality increased, while the third encompasses some conditions (e.g., hypertensive disease) for which mortality increased and others (e.g., ischemic heart disease) for which the pace of declining mortality slowed. ...

[I]ncreasing mortality among U.S. working-age adults is not new. The committee’s analyses confirmed that a long-term trend of stagnation and reversal of declining mortality rates that initially was limited to younger White women and men (aged 25–44) living outside of large central metropolitan areas (seen in women in the 1990s and men in the 2000s), subsequently spread to encompass most racial/ethnic groups and most geographic areas of the country. As a result, by the most recent period of the committee’s analysis (2012–2017), mortality rates were either flat or increasing among most working-age populations. Although this increase began among Whites, Blacks consistently experienced much higher mortality. ...

Over the 1990–2017 period, disparities in mortality between large central metropolitan and less-populated areas widened (to the detriment of the latter), and geographic disparities became more pronounced. Mortality rates increased across several regions and states, particularly among younger working-age adults, and most glaringly in central Appalachia, New England, the central United States, and parts of the Southwest and Mountain West. Mortality increases among working-age (particularly younger) women were more widespread across the country, while increases among men were more geographically concentrated.

Regarding socioeconomic status, the committee’s literature review revealed that a large number of studies using different data sources, measures of socioeconomic status, and analytic methods have convincingly documented a substantial widening of disparities in mortality by socioeconomic status among U.S. working-age Whites, particularly women, since the 1990s. Although fewer studies have examined socioeconomic disparities in working-age mortality among non-White populations, those that have done so show a stable but persistent gap in mortality among Black adults that favors those of higher socioeconomic status.

Many of these factors overlap in various ways, and the subject as a whole is not well-understood. A substantial portion of the NAS report is a call for additional research. But if I had to extrapolate from the available data, one pattern that seems common is about parts of the US feeling separated and isolated, either by urban/nonurban status or by socioeconomic status. The specific causes of death contributing to the pattern seem to share the trait that they are potentially worsened by life and economic stress. 

Although the report is about long-term trends, not the pandemic, it does offer the insight that COVID-19 has added to the disparity of mortality rates for working-age adults. Yes,  the elderly accounted for by far the largest share of COVID-19 deaths. But if one looks in percentage terms, the report notes: 

Thus, COVID-19 has reinforced and exacerbated existing mortality disparities within the United States, as well as between the United States and its peer countries. The CDC reported that adults aged 25–44 experienced the largest percentage increases in excess deaths during the pandemic (as of October 2020).