Month to month variation in mortality (adjusted for region and time trend) accounted for 17% of annual all cause mortality, but only 7.8% after adjustment for the effects of low temperatures (fitted as the mean temperature over the index and previous 13 days) in the daily time series model (). It accounted for 12.6% when we adjusted for influenza A counts without adjustment for low temperature, and 5.2% when we adjusted for both. Thus, most of the seasonal fluctuation seems to be related to cold, with smaller components attributable to influenza A and other risk factors.
Fraction of deaths attributable to monthly variation, adjusted for region, time trend, and stated covariates
Overall, there were 4221 deaths in 42 162 person years of follow up in winter months (100.1 deaths per 1000 person years, 95% confidence interval 97.1 to 103.1) and 5902 in 77 227 person years of follow up in other months (76.4 deaths per 1000 person years, 74.5 to 78.4).
All analysed variables showed an association with the absolute risk of death in both winter and non-winter months, though there was little difference with factors listed as home circumstances (living alone, reported difficulty making ends meet, and difficulty keeping the house warm) ().
Rates of death in winter and non-winter months, ratio of rates, and relative change in winter:non-winter ratios with levels of potential modifying factors
The overall winter:non-winter rate ratio was 1.31 (1.26 to 1.36), which is slightly higher than that found in this age group in the country as a whole.2
There was little evidence that this ratio varied by geographical region or age (). Women, however, had a larger winter:non-winter ratio than men for reasons other than their greater age, previous health status, social isolation, or socioeconomic position ().
Rate ratios (95% confidence intervals) for excess winter death, all causes: women relative to men*
There was little evidence of a trend of increasing risk of excess winter death with socioeconomic group, housing tenure (not tabulated), or reported difficulty in making ends meet or in keeping the house warm. Nor was there clear evidence that the combination of low socioeconomic group and reported difficulty in keeping the house warm (a combination expected to identify people least able to heat their home properly) was associated with excess risk (). Those who lived alone seemed no more vulnerable than others.
Rate ratios (95% confidence interval) for excess winter death (all causes) in relation to difficulty keeping house warm and Carstairs deprivation group, adjusted for age, sex, and region
Of the various markers of illness and activity status (), only a history of respiratory illness was associated with winter death; the relative risk adjusted for age, sex, and region being 1.20 (1.08 to 1.34). There was no evidence that excess winter death was associated with current smoking, total pack years of cigarettes smoked (not shown), or alcohol consumption. The more detailed assessment of mental health showed no association of the winter ratio with cognitive impairment (mini-mental state examination < 17) or depression (geriatric depression scale ≥ 6).
For most variables, the confidence intervals provide evidence against a substantial increase in risk (most exclude increases above 10%). The exceptions include: often having difficulty keeping the house warm (1.14 (0.89 to 1.46), the upper confidence limit for which is compatible with an appreciable increase), frailty, shortness of breath, a history of cardiovascular disease, and consumption of ≥ 7 units of alcohol a week.
Pre-existing respiratory disease (asthma, emphysema, or pneumonia diagnosed by a doctor, or a positive response to questions on chronic cough or phlegm) was the single strongest predictor of excess winter death (). It was most clearly associated with death from cardiovascular disease. The ratio of winter:non-winter cardiovascular mortality in those with respiratory disease relative to those without was 1.23 (1.02 to 1.47), and this figure varied only slightly with adjustment for different combinations of potential confounding factors (results not tabulated). History of wheeze or asthma or pneumonia in particular seemed to contribute to this higher relative risk, though history of phlegm for three months a year (a marker of chronic obstructive airways disease) did not. In contrast, there was no evidence that pre-existing respiratory illness increased excess respiratory death, nor that cardiovascular illness increased excess cardiovascular death. The finding that death from non-cardiorespiratory causes was greater in participants with a history of myocardial infarction is noteworthy but may be due to chance.
Rate ratios (95% confidence intervals) for excess winter death associated with markers of pre-existing medical illness. All rate ratios adjusted for region, age, sex, and fifth of Carstairs deprivation score