Influenza epidemics, seasonality, and the effects of cold weather on cardiac mortality

Area of Science:

• Environmental epidemiology
• Public health surveillance
• Cardiovascular disease research

Background:

• Winter seasonality is linked to increased cardiac disease mortality.
• Competing hypotheses exist for this seasonal pattern.
• The study investigates the role of influenza epidemics in seasonal cardiac deaths.

Purpose of the Study:

• To quantify the contribution of influenza epidemics to seasonal cardiac mortality.
• To assess if influenza data allows for more parsimonious seasonal adjustment than traditional methods.
• To determine if accounting for influenza affects the association between temperature and cardiac events.

Main Methods:

• Utilized daily cardiac death counts and elderly influenza hospital admission data (1992-2000) across 48 US cities.
• Employed quasi-Poisson regression models to analyze associations between daily cardiac mortality and temperature.
• Compared models with and without influenza epidemic data for seasonal adjustment.

Main Results:

• Controlling for influenza admissions resulted in more parsimonious models with improved statistical fit (better GCV, lower serial correlation).
• Influenza epidemics accounted for approximately 2.3% of cardiac deaths during the study period.
• The association between cold temperature (0 to -5°C) and cardiac mortality risk (1.6% increase) was not significantly altered by adjusting for influenza.

Conclusions:

• Influenza epidemic data explained a substantial portion (18%) of the irregular seasonal variation in cardiac deaths.
• Incorporating epidemic data enabled more parsimonious modeling compared to using time-based smooth functions alone.
• The impact of cold ambient temperature on cardiac mortality is not confounded by influenza epidemics.