  In temperate climates, mortality is seasonal with a winter-dominant pattern,
due in part to pneumonia and influenza. Cardiac causes, which are the leading
cause of death in the United States, are also winter-seasonal although it is
not clear why. Interactions between circulating respiratory viruses (f.e.,
influenza) and cardiac conditions have been suggested as a cause of
winter-dominant mortality patterns. We propose and implement a way to estimate
an upper bound on mortality attributable to winter-dominant viruses like
influenza. We calculate 'pseudo-seasonal' life expectancy, dividing the year
into two six-month spans, one encompassing winter the other summer. During the
summer when the circulation of respiratory viruses is drastically reduced, life
expectancy is about one year longer. We also quantify the seasonal mortality
difference in terms of seasonal "equivalent ages" (defined herein) and
proportional hazards. We suggest that even if viruses cause excess winter
cardiac mortality, the population-level mortality reduction of a perfect
influenza vaccine would be much more modest than is often recognized.
