In a recent study published in the International Journal of Infectious Diseases, a team of researchers analyzed the mortality in selected countries during multiple waves of the coronavirus disease 2019 (COVID-19) pandemic in 2020 and 2021 to understand the evolution of COVID-19-related mortality according to age and sex.

Background

The emergence of new variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in successive waves during the COVID-19 pandemic with varied outcomes in different countries. While broad trends such as a decline in mortality rates have been observed during the second wave after a severe first wave in some European countries such as Italy, detailed information on trends in mortality related to factors such as age and sex remains largely unclear.

Understanding the demographic structure of the mortality during various waves of the COVID-19 pandemic in different countries can be used to understand the evolution of COVID-19. Furthermore, this information is also useful in evaluating the efficiency of pharmaceutical and non-pharmaceutical mitigation measures in limiting and preventing the spread of the disease.

About the study

In the present study, the researchers obtained age and sex-related COVID-19 mortality data for six countries — Belgium, France, England and Wales, Sweden, Scotland, and the United States (U.S.) — from the database “The Demography of COVID-19 Deaths”. The choice of the countries was based on the official data sources of the countries using comparable definitions for COVID-19-associated mortality. The data covered the pandemic periods between March 2020 and February 2022, except for the data from France, which was only available until December 2021.

To determine the evolution of COVID-19-associated mortality rates over time, the standardized COVID-19 death rates (SDR) were calculated for each of the countries. The COVID-19 mortality rates were also calculated for specific weeks and seasons for each of the countries based on meteorological definitions of winter (December to February), spring (March to May), summer (June to August), and autumn (September to November).

The male-to-female ratio of age-specific COVID-19 mortality was used to understand the differences in COVID-19 mortality rates based on sex, which were also calculated according to different seasons for each country.

Results

The results indicated that the intensities and timing of COVID-19-associated mortality vary across the six countries. The summer of 2020 was a period of stabilization for the European countries included in the study but not for the U.S. The authors believe this could be attributed to the more uniform enforcement of strict disease mitigation measures such as lockdowns in European countries compared to the U.S. Among the European countries, Sweden experienced stabilization much later, which could be associated with having the least strict COVID-19 mitigation policies and not implementing mandatory lockdowns.

COVID-19-related mortality in European countries was the highest during the first two waves, with the subsequent waves of 2021 and early 2022 being milder. The decrease in COVID-19-associated deaths after the second wave could be due to a combination of large-scale vaccination efforts, natural immunity from infections during the first and the second wave, the emergence of new variants with reduced virulence, and the development of improved therapeutical methods.

In contrast, the U.S. experienced an increase in COVID-19-related mortality towards the end of the summer of 2021 and extending into the autumn and winter seasons. These periods, corresponding to the Delta and Omicron variants, had much lower mortality rates in the European countries. The Omicron variant, thought to have reduced virulence despite being highly contagious, caused higher mortality than the Delta variant in Massachusetts, which had high vaccine coverage.

The cumulative COVID-19-related mortality rates were the highest for the U.S., which also had high COVID-19-associated deaths among the younger age groups. In general, COVID-19-related deaths were the highest during the winter months and among the older age group, but towards the spring of 2021, the COVID-19 mortality rates significantly decreased in Scotland, England and Wales, and the U.S. The authors believe this decrease could be due to increased vaccination coverage in these countries.

Analysis of mortality rates according to sex revealed that men had a higher risk of mortality than women for most seasons and age groups, with the sex-based differences being the largest among individuals between 50 and 75 years of age.

Conclusions

Overall, the results suggested that COVID-19-associated mortality rates varied across countries and seasons, with further differences based on age and sex. The highest mortality rates were during winter among individuals above 75. The European countries experienced periods of stabilization during the summer, which was not observed in the U.S. The U.S. also had high mortality among the younger age groups.