Mortality attributable to influenza in England and Wales prior to, during and after the 2009 pandemic

Abstract

Very different influenza seasons have been observed from 2008/09-2011/12 in England and Wales, with the reported burden varying overall and by age group. The objective of this study was to estimate the impact of influenza on all-cause and cause-specific mortality during this period. Age-specific generalised linear regression models fitted with an identity link were developed, modelling weekly influenza activity through multiplying clinical influenza-like illness consultation rates with proportion of samples positive for influenza A or B. To adjust for confounding factors, a similar activity indicator was calculated for Respiratory Syncytial Virus. Extreme temperature and seasonal trend were controlled for. Following a severe influenza season in 2008/09 in 65+yr olds (estimated excess of 13,058 influenza A all-cause deaths), attributed all-cause mortality was not significant during the 2009 pandemic in this age group and comparatively low levels of influenza A mortality were seen in post-pandemic seasons. The age shift of the burden of seasonal influenza from the elderly to young adults during the pandemic continued into 2010/11; a comparatively larger impact was seen with the same circulating A(H1N1)pdm09 strain, with the burden of influenza A all-cause excess mortality in 15-64 yr olds the largest reported during 2008/09-2011/12 (436 deaths in 15-44 yr olds and 1,274 in 45-64 yr olds). On average, 76% of seasonal influenza A all-age attributable deaths had a cardiovascular or respiratory cause recorded (average of 5,849 influenza A deaths per season), with nearly a quarter reported for other causes (average of 1,770 influenza A deaths per season), highlighting the importance of all-cause as well as cause-specific estimates. No significant influenza B attributable mortality was detected by season, cause or age group. This analysis forms part of the preparatory work to establish a routine mortality monitoring system ahead of introduction of the UK universal childhood seasonal influenza vaccination programme in 2013/14.

Figure 1. Weekly number of all-cause deaths by age group.

The weekly numbers of all-cause deaths are blue and the proportion of those deaths classified as pneumonia and influenza (ICD-10 J9-J18) are red. Weeks shaded grey correspond to significant influenza activity, defined as an influenza incidence proxy (influenza-like illness consultation rates multiplied by proportion of samples positive for influenza) greater than 0 for three consecutive weeks. Horizontal black lines correspond to weeks with significant RSV activity, defined as a RSV incidence proxy (acute bronchitis consultation rates multiplied by proportion of samples positive for RSV) greater than 0 for three consecutive weeks. Orange circles correspond to weeks in which mean CET was below 0°C.

Figure 2. Average influenza A attributable deaths by cause of death.

Blue boxes correspond to average all-age significant influenza A attributable deaths by ICD-10 chapter. White and grey boxes correspond to more specific causes of death within ICD-10 chapters and their average all-age significant influenza A attributable deaths. The sizes of the blue, white and grey boxes are proportional to the corresponding number of influenza A deaths attributed. Average values were derived from seasons in which significant influenza attribution was reported overall.

Figure 3. Proportion of all-cause deaths attributable to influenza A by age group and season when assessing coded causes of death.

Proportion of all-cause deaths attributable to influenza A by age group and season where significant (corresponding confidence intervals not crossing 0). The number of influenza A-attributable deaths correspond to the following datasets analysed: all-cause deaths (Figure 3a), cardiorespiratory deaths (Figure 3b), respiratory deaths (Figure 3c) and pneumonia and influenza deaths (Figure 3d). Dominant influenza A subtype by season is indicated.