Influenza and its relationship to circulatory disorders

Three sources of data (general practice episode data from the Weekly Returns Service of the Royal College of General Practitioners, national hospital admission data for England and national mortality data by date of death) were examined separately in each winter (1994/1995 to 1999/2000) to investigate the impact of influenza on circulatory disease. Weekly data on incidence (clinical new episodes) hospital emergency admissions and deaths certified to circulatory disorders and to respiratory diseases (chapters VII and VIII of ICD9) during influenza epidemic periods (defined from combined clinical/virological surveillance) were examined in age groups 45-64, 65-74 and > or =75 years. Data collected in the four winters in which there were substantial influenza A epidemics were consolidated for the period 6 weeks before to 6 weeks after each peak of the epidemic, and associations between the variables at different time lags examined by calculating cross-correlation coefficients. We also examined deaths due to ischaemic heart disease (IHD) as a proportion of all circulatory deaths and deaths due to influenza/pneumonia as a proportion of all respiratory deaths. There were no increases of GP episodes nor of emergency admissions for circulatory disorders in any of the three age groups during epidemic periods. Increased circulatory deaths occurred in all age groups and particularly in the oldest group. The large cross-correlation coefficients of deaths (circulatory and respiratory) with GP respiratory episodes at weekly lags of 0, -1 and 1 were evidence that the deaths and episode distributions were contemporaneous. The ratios of excess circulatory deaths relative to excess respiratory deaths during epidemic periods were 0.74 (age 45-64), 0.72 (65-74) and 0.57 (> or =75 years). Increased circulatory deaths contemporary with new incident cases of respiratory episodes but with no concomitant increase in admissions suggests rapid death during the acute phase of illness. Influenza contingency planning needs to take account of these deaths in determining policy for prophylaxis and in providing facilities for cardio-respiratory resuscitation.