Background
Although age-standardised mortality is typically reported as the number of deaths per 1000 people at risk per annum, deaths are rarely evenly distributed throughout the year. Typically, more deaths due to cardiovascular disease (CVD) occur in winter compared to summer [
1]. Paradoxically, seasonal variations in cardiovascular-related mortality are not simply explained by exposure to environmental provocations such as cold temperatures, reduced daylight hours, infections, or increased pollution [
2‐
5]. Rather, they appear to reflect a more complex interplay between the environment and an individual’s physical and psychological condition, their behaviours and the culture/society in which they live [
4,
6]. In Scandinavia, for example, an individual-to-societal adaptation to extremely cold temperatures undoubtedly mitigates the cyclic exposure and physiological responses to seasonally driven provocations to cardiovascular health [
7].
Previous studies have sought to link clusters of increased mortality to large earthquakes [
8] and the FIFA World Cup [
9]. Beyond these exceptional events, there is an event that has strong potential to be detrimental to an individual’s cardiovascular health on an annual basis [
10,
11]. At Christmas, people around the world engage in potentially stressful social interactions and provocative behaviours they would not normally expose themselves to. In those already at risk of seasonal patterns of mortality (i.e., where Christmas coincides with winter), these factors may act as additional, short-term triggers for a broad range of cardiovascular-related events [
12]. A number of studies based on administrative data have previously demonstrated increased rates of mortality [
12,
13], hospitalisation [
11] and acute myocardial infarction (AMI) in Sweden during the Christmas holidays [
10]. Beyond these studies, however, this phenomenon remains poorly characterised [
1].
We hypothesised that over and beyond long-term seasonal trends within a population periodically exposed to cold winters, we would find an additional risk of dying over the Christmas holidays. In effect this would represent an increased period of increased mortality within an already high-risk period of the year. We also hypothesised that CVD would be the major contributor to this phenomenon and that we would find sex-specific differences in this regard.
Discussion
We investigated the seasonal pattern of mortality within the HUNT Study cohort living in Central Norway. This population cohort is regarded as representative for the Norwegian population as a whole, except for a lower proportion of non-whites and the absence of large cities. Our analyses revealed a striking long-term difference in mortality occurring in winter compared to summer. CVD accounted for half of this seasonality. Although not the coldest, December proved to be the deadliest month, with 22 more people dying each year compared to June. Overall, the 3-day period of 25th–27th December was revealed to be the deadliest time of the year with CVD the major contributor. Critically, both the frequency and cause of death in men and women appeared to change over the Christmas period. Compared to the same pre-Christmas/wintery period, men were 22 and 17% more likely to die from all-causes and CVD (particularly AMI), respectively. In women, the equivalent risk increases were 17 and 15%, with the contribution of CVD (particularly stroke) even more prominent. Although previous studies have also identified a specific Christmas effect on mortality [
10‐
13,
20], we are unaware of any studies and findings equivalent to those reported here.
There is pre-existing evidence to support the hypothesis that Christmas can be harmful to some individuals. A study of the overall pattern of mortality in the US during 1973–2001 revealed a “holiday effect” during Christmas, with ~ 5% excess deaths, after adjustment for the winter season [
12]. Similarly, data from a nationwide coronary care unit registry in Sweden revealed a 15% increase in AMI cases during the Christmas holidays [
10]. A higher risk of 30-day mortality or readmission among those hospitalised at Christmas in Ontario, Canada has also been found [
11]. From a Southern Hemisphere perspective there is both supportive [
13] and contrary evidence [
21] of an equivalent phenomenon occurring in summer conditions. Overall, our population-based data, suggest that like the US [
12], there is an increased risk of dying at Christmas in Norway. This likely applies to similar regions across Europe. To put this phenomenon into perspective, if the same pattern of excess deaths at Christmas had occurred within the entire population of Norway (a minimum of 3 million adults alive in 1980) on an age- and sex-specific basis, there would have been more than 11,000 excess deaths (around 350 more per annum) over the Christmas holidays alone in the past 30 years.
In the (understandable) absence of prospective studies, it is challenging to delineate between the overall impact of winter and a Christmas-specific effect. As shown by the Tromsø Study [
14], there is evidence of winter peaks in blood pressure, heart rate, body weight, total cholesterol, and overall CVD risk. Seasonal variation in physical activity may also be an important consideration for cardiovascular-related mortality [
22]. Aerobic exercise, especially with high intensity, can acutely lower systolic BP in the hours following exercise [
23].
As in many parts of the world, life in Central Norway during the Christmas holiday period is characterised by festive celebrations, travel away from home/central services, and reduced health services. This typically begins in early December and peaks (regardless of public holidays and weekends) during the week of December 23rd to 31st (New Year’s Eve) with concurrent public holidays on December 25th and 26th. Reduced access to follow-up health care was noted to contribute to 26 excess deaths (and 188 hospital readmissions) per 100,000 patients in Canada during the Christmas holidays [
11]. However, this phenomenon does not fully explain the size of the phenomenon we observed within our cohort and the contributory reasons are likely to be multifactorial. Consuming a high-fat diet for only 3 days exacerbates insulin resistance and glycolipid metabolism disorders in men with obesity [
24]. Even among healthy men, decreasing physical activity for 1–3 weeks decreases insulin sensitivity and attenuates postprandial lipid metabolism [
25]. Vascular stiffness, due to impaired endothelial function of the conduit vessels, is an important factor in the development of hypertension and an independent risk factor for a fatal cardiovascular event [
25]. After a high-fat meal, which is typically consumed during Christmas in Norway, endothelial function decreases substantially postprandially [
26]. The potential negative impact of increased emotional stress associated with dealing with loneliness and family tensions [
27] with the potential for seasonally triggered depression [
28], also cannot be ignored. As suggested by our sex-specific findings, any, or all of these “stressors” may affect men and women differently. For example, it has been demonstrated that diabetes, high-density lipoprotein levels and triglyceride levels have more impact on cardiovascular health of women compared to men [
29]. The emerging literature around Tako-tsubo cardiomyopathy with a predominance of women affected [
30] is notable when considering the small, but intriguing, increase in deaths due to heart failure in women, but not men, at Christmas.
Unfortunately, in the absence of specific interventions, expert clinical guidelines rarely mention or address seasonality. We are currently conducting a randomized trial to address seasonal patterns of hospitalization in 300 vulnerable individuals with chronic heart disease in Melbourne, Australia. Beyond ensuring appropriate vaccination against influenza [
31], there is a strong justification for more proactive screening and management of high-risk patients by general practitioners leading up to Christmas. The identification of educational levels in women and marriage status as modifying mortality risk in both sexes, reinforces the importance of considering health literacy and the emotional well-being of individuals leading up to provocative times of the year. Promotion of a healthy lifestyle should occur all year round [
32], but should perhaps be highlighted and re-emphasized in the lead-up to Christmas: a time of excessive indulgence of all kinds with potentially tragic consequences. The current COVID-19 pandemic both directly (via residual cardio-pulmonary impairment post-infection [
33]) and indirectly (via its negative effects on emotional and psychological well-being, patterns of social interaction, seeking care for pre-existing chronic conditions and reduced exercise levels), has further potential to exacerbate Christmas mortality [
34].
Study limitations
To robustly test our primary hypothesis, we examined patterns of long-term mortality within the HUNT cohort [
15,
17] in Central Norway. Although this is a well-characterised population, the pattern of risk and subsequent health outcomes in this semi-rural population may not be reflective of the broader Norwegian population or that of Western Europe. Nor was the study specifically designed to examine the issue of seasonal patterns of disease. As previously noted, Norway has a distinctive climate and culture, and these specific conditions may have contributed to our specific findings. Hence, there is a need to validate these findings in other population cohorts with equivalent data. To maintain the size of outcome data for analyses, we relied upon baseline profiling of the original cohort and mortality outcomes. For many individuals there may be multiple contributing causes of death, so any findings from cause-specific mortality data should be interpreted with some caution. The administrative timing of reported deaths (particularly over the Christmas period) may also be disrupted during holiday periods. To date, we have yet to examine the association between observed changes in risk profiles over time with seasonal patterns of mortality. Nor have we confirmed if the same pattern of seasonality and increased risk of death at Christmas is reflected in the pattern of hospital admissions. We have plans to address these limitations. However, we will not be able to ascertain the quality of care and extent of outpatient follow-up at key times such as Christmas and the New Year period. However, the timing of death (unless a sudden cardiac death) is not indicative of exactly when a person becomes unwell and/or is admitted to hospital [
11]. Moreover, we do not have specific data on seasonal changes in risk behaviours (e.g. increased alcohol and food intake) to correlate with the subsequent timing and trajectory of illness and death. Finally, we examined the pattern of mortality on a historical basis, during which time, significant changes in the pattern of life-style behaviours and public health measures have occurred.
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