Excess winter mortality: influenza or cold stress? Observational study

Abstract
A daily record was kept of deaths that occurred in south east England from 1970 to 1999 for all causes and for influenza. We obtained daily estimates of population by linear regression from mid-year values (17.2×106 in 1971 and 18.4×106 in 1998) and used them to calculate mortalities. We used the maximum and minimum temperature at Heathrow Airport each day to obtain the mean. Temperature was lagged three days to give the steepest relation between temperature and mortality.2 Total mortality each year related to cold was obtained as the sum of excess daily mortalities (per million). Excess daily mortalities were mortalities that occurred below the temperature—in a 3°C band—at which mortality was lowest, compared with mortality in that band. The mean temperature of the lower limit of the band over the 30 years was 19.0°C (95% confidence interval 18.2°C to 19.8°C).