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Increased Winter Mortality in Scotland 2008/09

Background

Many studies have shown that mortality levels in Scotland are markedly higher in winter months than summer months. Moreover, there are indications that this increased winter mortality has been relatively high in Scotland (and the rest of the UK) when compared with many countries with more extreme winter climates, though UK levels are comparable to several southern European countries (e.g. Greece, Italy, Spain) and lower than those recorded in Portugal and the Republic of Ireland. Whilst a number of theories have been advanced to explain these observations, there is as yet no consensus on the underlying mechanisms involved.

In 2002, the General Register Office for Scotland (GROS) published an Occasional Paper entitled ‘The Raised Incidence of Winter Deaths’. As well as reviewing the various definitions used to assess the extent of increased winter mortality, this paper showed that additional winter deaths were particularly associated with respiratory and circulatory diseases and that few deaths were caused by hypothermia. It also demonstrated that, though not all increased winter mortality is related to influenza, there was a clear link between the number of additional deaths and the level of influenza activity.

For the purpose of the statistics reported here, increased winter mortality is defined as the difference between the number of deaths in the four-month "winter" period (December – March, inclusive) and the average number of deaths in the two four-month periods which precede winter (August – November) and follow winter (April – July).

Results

Table 1 summarises recent trends for Scotland. It is estimated that there were about 3,510 "additional" deaths in Scotland during the winter of 2008/09, 1,330 more than the number in the previous winter. Table 1 also shows the extent to which the winter increase in mortality affects the elderly, particularly those aged 75 and over.

Table 2 gives the figures for the 58 winters for which these statistics are available. The winter of 2008/09 had the largest number of "additional" deaths since the winter of 1999/2000 (when there were 5,190 "additional" deaths). Only five of the twenty most recent winters had more "additional" deaths than 2008/09: 1989/90 (5,460), 1999/2000 (5,190), 1998/99 (4,750), 1995/96 (3,650) and 1996/97 (3,640). However, winter 2008/09 had the 26th lowest figure of the 58 winters for which there are statistics. The winter of 2005/06, with mild weather and no serious outbreaks of 'flu, had the lowest number of "additional" deaths (1,780) since the series started in 1951/52; the winters of 2001/02, 2007/08 and 2000/01 had the second, fifth and sixth lowest figures (1,840, 2,180 and 2,220, respectively). Therefore, the latest nine winters had four of the six lowest figures since 1951/52.

The number of "additional" deaths in winter 2008/09 was around two-thirds (68%) of the figure for 1999/2000, which was the last time that influenza activity was at a high level. Winter 2008/09 did not have an abnormally high level of influenza: Health Protection Scotland's (HPS) weekly Influenza Updates (which are available on the HPS website) for January 2009 (the period of winter 2008/09 in which influenza activity was clearly at its highest level) stated that the surveillance indicators suggested levels of influenza which were "normal", had "dropped from normal activity into baseline rates", or were "within baseline activity", and that these levels were at or below the lower end of the "Normal season" range. The HPS Updates include a chart comparing recent years' weekly "fluspotter consultation rates", which shows that, although the peak for winter 2008/09 was clearly higher than in 2005/06 and 2007/08, it was markedly lower than in 2006/07.

Chart 1 shows the increased winter mortality figures for the past 50-or-so years individually (the bars) and as a 5-year moving average (which should give a better guide to the overall trend, as it "smoothes out" most - but not all - of the effect of year-to-year fluctuations in the figures). The chart shows that there has been an overall downward trend in the number of "additional" winter deaths over the past 50-or-so years: although there have been unusually high numbers in some years (which generally coincide with periods of high influenza activity), the height of the peaks appears to be falling, and the 5-year moving average is tending to decline. However, there are fluctuations around the overall long-term downward trend, such as the short-term rise in the moving average towards the end of the 1990s.

Table 3 gives a more detailed breakdown by age and NHS Board area.

The numbers of deaths registered each winter, and in the adjacent four-month periods, are provided in Table 4, along with the resulting "seasonal differences" in the numbers of deaths. 

Table 4 shows that there were 20,532 deaths in Scotland in the four months of winter 2008/09 (December to March), compared with 19,900 in winter 2007/08. However, despite the rise, the figure for winter 2008/09 did not differ greatly from the average of 20,429 for the eight previous winters (i.e. starting with winter 2000/01, because winter 1999/2000 was the last one with a high level of influenza activity). In the period which began with winter 2000/01, the number of deaths has varied between 19,651 in winter 2005/06 and 21,058 in winter 2002/03.  

The 20,532 deaths in the four months of winter 2008/09 exceeded both the 17,075 deaths in the preceding 4-month period and the 16,969 deaths in the following 4-month period. The seasonal difference (comparing the four winter months with the average of the 4-month periods before and after the winter) was 3,510 for the winter of 2008/09 – compared with a rounded figure of 2,180 for the winter of 2007/08.

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List of Tables

Statistical Service in Scotland