Winter mortality modifies the heat-mortality association the following summer

¹ Institution of Occupational and Environmental Medicine, Dept of Public Health and Clinical Medicine, Umeå University

^* To whom correspondence should be addressed. E-mail: joacim.rocklov{at}envmed.umu.se.

	Abstract

We aim to study how the heat related increase in deaths in summer and the extent of mortality displacement depend on influenza and other categories of mortality in the previous winter, which when low leaves a greater pool of susceptible individuals.

Mortality data from 1990–2002 was stratified into a summer period and a winter period. A Poisson regression model was established for the daily mortality in the summer with temperature and confounders as explanatory variables. In addition, we incorporated indicators of total, respiratory, cardiovascular and influenza mortality of the winter period as effect modifiers in the summer model, and studied lagged effects in strata defined by indicators.

A high rate of respiratory as well as cardiovascular mortality in winter reduces the heat effect the following summer, and influenza mortality tended to do so as well. Short term mortality displacement was more pronounced when the winter mortality had been low, and when the heat effect was immediate. The heat effect was somewhat more delayed when the previous winter mortality had been high.

This paper shows that high respiratory, cardiovascular and influenza mortality in winter leads to lower temperature effects in the following summer and less mortality displacement. It also suggests that persons for whom influenza may be fatal are often also susceptible to heat and this subgroup might therefore not benefit as much as expected from influenza vaccinations.