Winter mortality index - WMCA

The winter mortality index (WMI) is a measure expressed as a ratio of the difference in all cause mortality during winter months (December to March) compared to the average in the non winter months (the preceding August to November and following April to July). The terminology used to describe this indicator has changed to provide clearer explanation of what the analysis represents. The measures have been renamed to winter deaths compared to non winter deaths (previously excess winter deaths) and winter mortality index (WMI) (previously excess winter mortality index). There have been no methodology changes. ## Rationale The purpose of the winter mortality measure is to compare the number of deaths that occurred in the winter period (December to March) with the average of the non winter periods (August to November and April to July). Winter mortality is not solely a reflection of temperature, but of other factors as well. These include respiratory diseases and pressure on services, which have been more intense than usual during and following the height of the pandemic (1). It is an important measure as it allows users to assess whether policies are having an impact on mortality risks during the winter period (2). (1) [Office for National Statistics (ONS), released 19 January 2023, ONS website, statistical bulletin, Winter mortality in England and Wales: 2021 to 2022 (provisional) and 2020 to 2021 (final)]. (2) [Office for National Statistics (ONS), released 19 January 2023, ONS website, QMI, Winter mortality in England and Wales QMI: 19 January 2023] ## Definition of numerator Total number of winter deaths for all ages in defined year 20xx/20xx+1 (number of deaths occurring in December in year 20xx and January to March in 20xx plus 1) minus half the number of deaths in the non winter months (preceding August to November in year 20xx and following April to July in year 20xx plus 1) and registered by 31 December 20xx plus 1. ## Definition of denominator The average number of deaths for all ages ( in defined year 20xx/20xx plus 1) occurring in the non winter months, i.e. the total number of deaths occurring in the preceding August to November in year 20xx and the following April to July in year 20xx plus 1 divided by two and registered by 31 December 20xx plus 1. ## Caveats In 2020, the coronavirus (COVID 19) pandemic led to a large increase of deaths mostly in the non-winter months of April to July 2020. This has impacted the WMI for 2019 to 2020. Because we rely on using the difference between deaths occurring in the winter and the average of non winter months; specifically, the scale of COVID 19 deaths during non winter months has fundamentally disturbed the data time series and so data for 2019 to 2020 should be interpreted with caution. The Office for National Statistics (ONS) Annual Births and Mortality Extract is based on registered deaths (Date of registration) and the Winter deaths compared to non winter deaths and WMI calculations are based on the date of death occurrences (Date of death). It is possible that a number of deaths might not have been registered when the data were released and this could vary between areas. This indicator only includes deaths which are registered by the end of the calendar year 20xx plus 1. Data published in the PHOF will differ from published ONS results which uses an extract of mortality data taken approximately five months after the annual ONS mortality extract is taken, in order to give more time for late registrations (for example, deaths that were referred to a coroner) to appear in the data. The WMI will be partly dependent on the proportion of older people in the population as most winter deaths effect older people (there is no standardisation in this calculation by age or any other factor). This winter period was selected as they are the months which over the last 50 years have displayed above average monthly mortality. However, if mortality starts to increase prior to this, for example in November, the number of deaths in the non winter period will increase, which in turn will decrease the estimate of winter deaths compared to non winter deaths. The counts are presented rounded to the nearest 10, in line with how data is presented by the ONS.