Background
The authors write that "in the Czech Republic, mortality associated with heat waves (Kysely, 2004; Kysely and Kriz, 2008) and cold spells (Kysely et al., 2009) has been examined," but they note that "previous studies were based on different definitions and approaches which did not allow for a comparative analysis." Hence, they attempted to atone for this important deficiency.

What was done
Working with a nation-wide database of daily mortality records that cover the 21-year period 1986-2006 -- which period, in their words, "encompasses seasons with the hottest summers on record (1992, 1994, 2003) as well as several very cold winters (1986/87, 1995/96, 2005/06)" -- Kysely et al. (2011) compared the effects of hot and cold periods on cardiovascular mortality using analogous definitions for heat waves and cold spells that were based on quantiles of daily average temperature anomalies and did not incorporate any location-specific threshold, while excluding periods characterized by epidemics of influenza and acute respiratory infections that occur primarily in winter and are also responsible for many deaths.

What was learned
The four Czech scientists found that "both hot and cold spells are associated with significant excess cardiovascular mortality," but they say that "the effects of hot spells are more direct (unlagged) and typically concentrated in a few days of a hot spell, while cold spells are associated with indirect (lagged) mortality impacts persisting after a cold spell ends." And although they report that "the mortality peak is less pronounced for cold spells," they determined that "the cumulative magnitude of excess mortality is larger for cold than hot spells."

With respect to gender differences, the researchers say there was a "much larger excess mortality of females in hot spells and more lagged effects in females than males associated with cold spells." And with respect to age, they report that "effects of hot spells have a similar temporal pattern in all age groups but much larger magnitude in the elderly," while in the case of cold spells, "relative excess mortality is largest in the middle-aged population (25-59 years)."

What it means
Kysely et al. (2011) write that "in the context of climate change, substantial reductions in cold-related mortality are very likely in mid-latitudinal regions, particularly if the increasing adaptability of societies to weather is taken into account (cf. Christidis et al., 2010)," and they say that "it is probable that reductions in cold-related mortality will be more important than possible increases in heat-related mortality [italics added]."

References
Christidis, N., Donaldson, G.C. and Stott, P.A. 2010. Causes for the recent changes in cold- and heat-related mortality in England and Wales. Climatic Change 102: 539-553.

Kysely, J. 2004. Mortality and displaced mortality during heat waves in the Czech Republic. International Journal of Biometeorology 49: 91-97.

Kysely, J. and Kriz, B. 2008. Decreased impacts of the 2003 heat waves on mortality in the Czech Republic: an improved response? International Journal of Biometeorology 52: 733-745.

Kysely, J., Pokorna, L., Kyncl, J. and Kriz, B. 2009. Excess cardiovascular mortality associated with cold spells in the Czech Republic. BMC Public Health 9: 10.1186/1471-2458-9-19.