Seasonal variation of overall and cardiovascular mortality: a study in 19 countries from different geographic locations

Abstract

Background: Cardiovascular diseases (CVD) mortality has been shown to follow a seasonal pattern. Several studies suggested several possible determinants of this pattern, including misclassification of causes of deaths. We aimed at assessing seasonality in overall, CVD, cancer and non-CVD/non-cancer mortality using data from 19 countries from different latitudes.

Methods and findings: Monthly mortality data were compiled from 19 countries, amounting to over 54 million deaths. We calculated ratios of the observed to the expected numbers of deaths in the absence of a seasonal pattern. Seasonal variation (peak to nadir difference) for overall and cause-specific (CVD, cancer or non-CVD/non-cancer) mortality was analyzed using the cosinor function model. Mortality from overall, CVD and non-CVD/non-cancer showed a consistent seasonal pattern. In both hemispheres, the number of deaths was higher than expected in winter. In countries close to the Equator the seasonal pattern was considerably lower for mortality from any cause. For CVD mortality, the peak to nadir differences ranged from 0.185 to 0.466 in the Northern Hemisphere, from 0.087 to 0.108 near the Equator, and from 0.219 to 0.409 in the Southern Hemisphere. For cancer mortality, the seasonal variation was nonexistent in most countries.

Conclusions: In countries with seasonal variation, mortality from overall, CVD and non-CVD/non-cancer show a seasonal pattern with mortality being higher in winter than in summer. Conversely, cancer mortality shows no substantial seasonality.

Figure 1. Distribution of the proportion of cardiovascular deaths (y-axis) per month (from January to December, x-axis) by country.

The numbers close to the country names refer to the mean latitude of a country. The countries are positioned according to their latitude with the northernmost countries on the top. Countries from the Northern Hemisphere close to the Equator and Southern Hemisphere, respectively, in green, red and blue. No data available for France and New Zealand.

Figure 2. Seasonal variation of the ratio of observed to expected deaths for overall, cardiovascular, cancer and non-CVD/non-cancer.

Countries located in the Northern Hemisphere.

Figure 3. Seasonal variation of the ratio of observed to expected deaths for overall, cardiovascular, cancer and non-CVD/non-cancer.

Countries located close to the Equator.

Figure 4. Seasonal variation of the ratio of observed to expected deaths for overall, cardiovascular, cancer and non-CVD/non-cancer.

Countries located in the Southern Hemisphere.