1 Introduction

Excess mortality, defined as the difference between the observed all-cause mortality and the expected one [1, 2], during the COVID-19 pandemic serves as an index for calculating the global illness burden attributed to the emerging SARS-CoV-2 infection [3]. Estimating excess mortality caused by SARS-CoV-2 variants is crucial, as different countries not only experienced different strains and scales of the epidemic [4, 5] but also affected by inadequate medical capacity during the outbreak period of Omicron, one of SARS-CoV-2 variants, after 2021 [19, 20]. However, compared to non-Baby Boomers, Baby Boomers aged 58–76 years had fewer negative excess deaths in 2020 and more excess deaths in 2021 because the indirect benefit of strict containment measures may be offset by the Baby Boomers’ already low medical utilization for their high need for medical care, which could worsen if strict containment measures were in place during the COVID-19 pandemic. Second, when both Baby Boomers and non-Baby Boomers experienced a community-acquired outbreak of Omicron VOC between May and June 2022, they experienced the same significant excess deaths, supporting the previous finding that excess deaths were more prominent for Omicron VOC outbreaks than Delta VOC outbreaks [21]. Nonetheless, the profiles of Baby Boomers and non-Baby Boomers who are responsible for an excess of mortalities from all causes remain distinct. Non-Baby Boomers may see an increase in mortality, particularly among the elderly aged 80, due to the loss of indirect benefits between 2020 and 2021, as well as the high likelihood of Omicron infection due to the disproportionately high proportion of unvaccinated older individuals. On the other side, an increase in non-COVID-19 deaths due to a scarcity of health-care personnel may be associated with an increase in total Baby Boomer mortality. Although the direct cause of case fatality for Omicron was lower than for other SARS-CoV-2 variant like Delta, the indirect reason of a scarcity of health workers due to the high Omicron infection could have a higher impact during the pandemic era of Omicron. The WHO report [6] promotes direct and indirect influences strongly.

The evaluation of excess mortality also plays a significant role in determining the global illness burden when the issue of underreporting in particular countries is recognized. Wang et al. (COVID-19 Excess Mortality Collaborators, 2022) employed an effective statistical model to estimate the magnitudes of excess mortality during the COVID-19 pandemic period from the beginning of 2020 to the end of 2021 by country’s level of economic development. After fitting the proposed model, which accounted for a multitude of significant factors, they found that the reported number of mortalities from all causes worldwide was underestimated by one-third. Later, the WHO developed a revised form of excess mortality, concentrating only on Germany and Sweden [3]. One of the reasons for the correction is to determine whether or not it is appropriate to utilize mortality rates from before the COVID-19 epidemic. In contrast to these valuable studies that focused on estimating the global disease burden across countries, we evaluated excess mortality at the country level for those affected by aging Baby Boomers with an increasing background mortality trend prior to the COVID-19 pandemic period in order to reflect an unbiased estimate of excess all-cause deaths during the COVID-19 pandemic period. Supplemental Fig. 3 depicts countries with and without a Baby Boom. These results demonstrate conclusively that the Baby Boom had a far bigger impact on excess mortality during the Omicron VOC pandemic phase than prior SARS-CoV-2 strains. The findings also highlight the importance of exercising caution when interpreting excess all-cause deaths based on global comparisons when evaluating excess all-cause mortality at the country level, due to the fact that the background mortality of each country is affected by various factors, such as the Baby Boom effect proposed in this study and also the disparity of health care system across geographic areas and various ethnic groups. However, as the health care system in Taiwan is very homogeneous under a nationwide health insurance system, we still think the underlying disease burden of Baby-Boomers in the face of Omicron, given the reduced medical capacity, would be a strong explanation for excess all-cause mortality on this occasion.

Despite these findings on Baby Boomer-caused excess mortality, our study is limited in elucidating the detailed causes responsible for the disparity of excess all-cause-deaths across different SARS-CoV-2 variants between Baby Boomers and non-Baby Boomers, as the influences of Baby Boomers on excess mortality were intertwined with their diverse characteristics, such as the emerging SARS-CoV-2 subvariants, the compliance with containment measures, susceptibility to COVID-19 infection, the coverage rate of vaccination, the accessibility and the availability of anti-viral therapy, and the resilience to insufficient medical capacity. Additionally, future research should consider how vaccination rates are associated with mortality rates among Baby Boomers and non-Baby Boomers, as this was not addressed in the current study. Therefore, an ongoing research is needed to investigate these factors by performing large retrospective cohort research with the collection of individual-level information in the future. Regarding the emerging SARS-CoV-2 subvariants, it is still important to continuously apply the proposed Poisson time series design with the DAG approach to a series of the emerging SARS-CoV-2 subvariants with higher transmission and evasion of immunity,, such as XBB.1.5 and BQ.1.1, circulating in the USA and Europe as of the writing of this manuscript because monitoring excess all-cause mortality in each period of the emerging subvariant would provide an indicator for the surveillance of the severity of COVID-19 and the provision of medical capacity. In conclusion, the profiles of excess mortality from all causes for Baby Boomers and non-Baby Boomers differed both before and throughout the COVID-19 pandemic era. Prior to the COVID-19 pandemic period, the background mortality rate for Baby Boomers was growing, whereas the trend for non-Baby Boomers was practically stable. In 2020 and 2021, the COVID-19 epidemic caused a rise in the number of Baby Boomer deaths relative to non-Baby Boomer deaths. During the Omicron epidemic period, Baby Boomers aged 58 to 76 had the same excess of all-cause fatalities as non-Baby Boomers aged 77 or older, but they were more likely to die from causes other than COVID-19.

In Taiwan, the fast-aging Baby Boomer generation experienced a high number of deaths from all causes. This could be attributed to a lack of access to medical care when the COVID-19 epidemic is under control from Wuhan D614G, Alpha, and Delta VOC and a lack of medical capacity when dealing with large-scale outbreaks of Omicron VOC.