The official estimate of influenza mortality is produced by the
Health Protection Agency. It is derived from excess all-cause death
registrations in the winter. When the number of all-cause death
registrations rises above an ‘expected’ level in a given week, this excess
is counted. The estimates for the last five years in England & Wales
are: 1965 (2004-05 winter season), 0 (2005-06), 0 (2006-07), 426 (2007-
08), and 10351 (2008-09) . The highest estimate in recent years was for
the 1999-2000 ‘flu season, at 21,497.
This method has its limitations. It captures the numbers of deaths
‘above expected’, but does not examine causation directly. An observed
excess may therefore reflect deaths due to cold weather or other
circulating viruses. When the number of deaths is small, there may be no
significant rise observable above the baseline of ‘expected deaths’. This
leads to a zero estimate, as seen in 2005-6 and 2006-7.
The Health Protection Agency is currently reporting the excess deaths
measure on a weekly basis. At December 17 2009, no excess in all-cause
death registrations had been observed since February 2009 . In other
words, had we relied solely on this measure, we would not have been aware
of any deaths due to A/H1N1 influenza so far. Our study has value in
filling this gap.
A second estimate of ‘flu deaths is found in the annual mortality
statistics produced by the Office for National Statistics. These
statistics record the underlying cause of death. They are based on all
registered deaths, based on the information on death certificates. The
number of deaths for England & Wales with an underlying cause of
influenza (ICD-10 code J10-J11) for the four recent calendar years are: 39
(2008), 31 (2007), 17 (2006) and 44 (2005). Many more deaths are
attributed to pneumonia, some of which will be secondary to influenza.
Coding of death certificates takes place through out the year. Summary
mortality statistics for the year are normally published in the following
Our study includes any death with a mention of pandemic influenza
anywhere (or synonym) on the death certificate. It also includes any death
with a laboratory positive swab for pandemic influenza, irrespective of
the cause of death reported on the death certificate. Our method has also
rapidly captured information on underlying illness patterns. While
mortality measured in absolute number of deaths may not be out of the
ordinary, a relatively large number of deaths in children and young adults
has been observed.
No additional competing interests declared
Competing interests: No competing interests