Underestimation of COVID-19 mortality during the pandemic

Discussion

There are a number of international perspectives provided by this analysis of the mortality impact of COVID-19 and its reporting. Firstly, there is a marked variation in the trends in all-cause mortality between countries. Many countries reported a greater than 50% increase in mortality at the peak of the pandemic. However, a few countries that imposed early and stringent lockdown measures, reported none or a reduction.

Secondly, there was marked variation between countries in the proportion of excess deaths from historical rates that were registered as COVID-19-related, ranging between 30% and 197%, in those countries that had an excess of deaths during the period of observation. This variation is likely due to differences in health care systems, case definitions, availability of testing, variations in the historical baseline all-cause death rates from which the number of excess deaths were estimated, and other factors that contributed to deaths from causes other than COVID-19. As a result, country-specific COVID-19 mortality statistics should be interpreted and compared with caution. Both the variability and the magnitude of differences between countries, suggest that excess deaths, calculated as total all-cause mortality minus historical mortality for the same week from the previous 5 years, provides a better estimate of COVID-19 mortality than official reports of deaths registered as due to COVID-19, in those countries with definite increases in mortality. This interpretation does not apply to countries in which definite increases in all-cause mortality did not occur.

Potential explanations for the greater increase in all-cause mortality than reported COVID-19 related deaths include misclassification of deaths due to COVID-19 attributed to another cause, and deaths from causes indirectly related to COVID-19 such as cardiovascular [13] and thrombotic events [14]. Mortality from delayed seeking of medical care, avoidance of care, mental health distress, and exacerbation of chronic illnesses as a result of the response to the pandemic such as lockdowns may also possibly contribute to excess mortality. Furthermore, overwhelmed health systems and stretched capacity could potentially result in substandard management of non-COVID-19 related conditions, amplifying pre-existing deficiencies and inequities contributing to excess mortality. These effects could continue to contribute towards life-years lost and/or excess mortality for months to years to come. However, the full extent of the effects that the COVID-19 pandemic has had on health and mortality is currently unknown.

There is emerging data to support this interpretation of our findings. In England and Wales, there has been a general increase in mortality across the leading causes of death throughout the pandemic. Of note, from late March onwards there have been large increases in deaths attributed to dementia and Alzheimer disease (220% of five year average at maximum) and “symptoms, signs and ill-defined conditions” (242% of five year average at maximum) [15]. The latter category is used to include deaths certified as caused by “old age” or “frailty”. The majority of these deaths occurred in care homes where there was rapid infection and transmission in this vulnerable population. There was also an increase in deaths related to asthma and diabetes, which occurred at significantly higher rates compared to the preceding five-year average, potentially related to a delay in seeking or receiving treatment. Similarly in the United States there were large proportional increases in deaths due to non-respiratory causes such as diabetes, heart disease, cerebrovascular disease and Alzheimer disease, likely in part due to misclassification of COVID-19 related deaths [7].

These findings are consistent with reporting of mortality in previous unexpected epidemics or catastrophic events. During the Chikungunya epidemic in Jamaica, no chikungunya-associated deaths were reported despite an excess of 2499 deaths [16]. The mortality burden of 1139 excess deaths due to Hurricane Maria in Puerto Rico in 2017 was higher than the official death toll of 64 [17].

Thirdly, increases in all-cause mortality preceded the increase in COVID-19 mortality in most countries in which definite spikes in COVID-19 mortality occurred. This temporal pattern was also observed at the onset of the second major wave later in the pandemic. This further suggests that changes in all-cause mortality, compared with historical mortality rates, may be more informative than COVID-19 specific mortality in recognising the onset and extent of the early impact of COVID-19, in those countries in which definite increases in all-cause mortality occurred. This may also apply to other epidemics or pandemics due to a novel virus where there is a lack of knowledge and widespread testing or detection, and for which robust contract tracing may not be in place. Of note, Ecuador, which had the longest delay in reporting of COVID-19 mortality after the onset of the pandemic-related increase in all-cause mortality, had the greatest increase in mortality during the pandemic, recording a 497% increase at its peak. This observation suggests that the lack of recognition of the presence and extent of the pandemic may have contributed to the lack of adequate public health measures to contain the pandemic.

Finally, for a few countries with early and stringent lockdown measures, the reported all-cause mortality was less than historical rates at periods during the pandemic. Countries such as New Zealand, which did not experience a spike in COVID-19 mortality, may have had reduced mortality from other causes e.g. motor vehicle accidents or other infectious respiratory causes such as influenza, due to stringent lockdown measures, and the general improvement in social and public health practices [18, 19]. In Italy, which experienced a marked spike in COVID-19 mortality, a late reduction in all-cause mortality may be due to premature death of older adults, including in residential care facilities. Thus, all-cause mortality may also provide insight into the net outcome of both the impact of COVID-19 and the public health measures implemented during the pandemic.

Limitations of this ecological analysis are that pandemic public health measures may have affected other causes of mortality and there are likely inaccuracies in country mortality databases, and in particular the registration of COVID-19-related deaths on which this analysis was based. The methodology used by countries in reporting COVID-19 mortality vary and some countries such as the United Kingdom, France and Spain have changed methods over the course of the pandemic [20]. Our World in data also publishes the death on the date it is reported while the Financial Times data set uses national registry data, which is reported by date of death. This discrepancy may generate a potential lag depending on the efficiency of each country's systems but even when accounting for it, the difference in the time trends of reported all-cause mortality and reported COVID-19 mortality is evident. Using a five-year average to calculate historical deaths do not account for changes in population over time. High quality data can be difficult to obtain in a pandemic situation given the evolving nature of the disease and its burden on established reporting systems; however, the data used in this analysis is the most accurate that is currently available. Finally, there is no data from many other affected countries of interest such as China, Taiwan, South Korea and Australia.

We conclude that this analysis is consistent with the underestimation of COVID-19 mortality by at least a third. We propose that all-cause mortality may be more informative than the COVID-19-specific mortality when gauging the impact of COVID-19 within countries, particularly early in the pandemic. The time trends reported suggest that the stringent public health measures to contain COVID-19, and the impact of the pandemic itself, may have led to a reduction in deaths from other causes, an observation that requires further investigation.