Official Statistics

Cold mortality development plan: 2024 to 2025

Published 18 February 2026

Applies to England

Purpose

This development plan sets out the rationale for publishing this analysis output as official statistics, including the reasons for designating the statistics as ‘official statistics in development’ at this stage. It explains how the statistics meet the standards of trustworthiness, quality and value required for publication, while recognising that further development is needed before they can be confirmed as official statistics.

The document outlines the planned development activity to strengthen methods, outputs and user value. This includes actions to improve quality, enhance policy relevance and interoperability, and respond to user feedback. Together, these activities are intended to support the transition from official statistics in development to official statistics for the next publication.

Official statistics in development

These statistics are labelled as ‘official statistics in development’ (previously termed ‘experimental statistics’). ‘Official statistics in development’ are developed under the guidance of the Head of Profession for Statistics. The goal is to develop statistics that can, in due course, be produced to the standards of the Code of Practice for Statistics. This statement provides further detail on the nature of the development and how we are continuing to assess these statistics against the Code of Practice.

The statistics meet the required standards of trustworthiness, quality and value to be published as official statistics; however, they are still subject to further development to strengthen the methodological choices and enhance the value to users over time. Publishing as ‘official statistics in development’ allows us to be transparent about current limitations while supporting early use and feedback.

What the statistics are

The estimates presented in this report show the level of cold-associated mortality for each cold episode in winter 2024 to 2025, based on statistical modelling of how mortality has responded to low temperatures over a recent 5 winters. This differs from the UK Health Security Agency (UKHSA) heat-associated mortality report, which uses observed daily deaths in a single summer. It is not possible to use the same methodology for cold as for heat because cold-associated mortality overlaps with factors such as influenza and other circulating diseases and there is an observed delay between the cold event and rise in mortality.

Why the statistics are needed

Previously published statistics such as the Office for National Statistics (ONS) experimental statistics on climate-related mortality in England and Wales, 1988 to 2022 do not provide episode-specific estimates of cold-associated mortality. Current published outputs do not allow mortality to be estimated for individual cold or adverse weather episodes. They also do not provide breakdowns by demographic groups or report on timing of mortality impacts. This limits understanding of the health impacts of specific events and reduces the ability to assess risk during periods of adverse weather and throughout the winter.

Current approaches rely on long time periods that can mask important relationships. Existing statistics often use long historical periods to characterise the relationship between temperature and mortality. While useful for some purposes, this can lead to inaccurate estimates when applied to individual seasons or events, as the relationship is averaged over time and does not fully reflect changing population vulnerability, exposure or adaptation.

These new statistics provide more timely and relevant evidence for decision-making. By focusing on shorter time periods and specific episodes, this output provides estimates that are more relevant for operational and seasonal decision-making, including during periods of heightened risk.

The output directly supports UKHSA’s strategic objectives. The statistics are vital for delivering the aims set out in the Adverse Weather and Health Plan, strengthening the evidence base on the health impacts of cold and adverse weather.

These estimates inform the Weather Health Alerts system and support wider winter preparedness and response planning across the health and social care sector. They identify who is most at risk, how impacts unfold over time, and enable better decision-making, targeted action, and planning ahead of cold weather.

Early publication supports improvement through use and feedback. Publishing these statistics as ‘official statistics in development’ allows users to access important evidence now, while providing feedback that will help refine methods and improve future releases

We have considered the statistics against the 3 lenses of the Statistics Code of Practice and determined that:

Trustworthiness

• the statistics are produced and released by professional analysts under established governance and quality assurance arrangements

• methods, assumptions and limitations are published openly, including through a Quality and Methodology Information (QMI) document

• the statistics are explained impartially and objectively

Quality

• the statistics are based on the best available data and methods at this stage of development, and are based on peer-reviewed methodologies
• data sources have been assessed for suitability, with known limitations and uncertainty clearly communicated within the QMI report
• methods have been internally quality assured and are robust enough for publication
• further refinement, validation and testing are planned to improve quality over time

Value

• the statistics address a clear user need and fill an important evidence gap as there is currently no other report which provides estimates of cold-associated deaths per cold episodes
• they complement existing UKHSA outputs and support public health understanding and decision-making such as the Weather-Health Alerting system

• clear guidance and proactive engagement support intelligent use of data and can influence preparedness activities

• commentary, charts and supporting material aid appropriate interpretation

• user feedback mechanisms are in place to ensure the statistics meet user needs

Planned developments

The label ‘official statistics in development’ should be used to alert users to statistics that may be affected by a development. In the case of the cold mortality monitoring report 2024 to 2025, this is a new report with potential for future enhancement to the methodology and content of the report as we gather information about user requirements.

Prior to the publication of the 2025 to 2026 report, the following developments are planned:

Short-term actions will focus on proactive communication, user engagement and gathering feedback alongside the first release. This will include understanding how users intend to apply the statistics in operational and policy settings, and identifying priority areas where outputs can better support decision-making, including within the Weather Health Alerts system and winter preparedness and response planning.

Further methodological refinement and testing will be undertaken before the approach is considered final. This will include exploring alternative methodological choices and conducting sensitivity analyses to understand the impact of key assumptions on the results. For example, we will assess whether regional comparisons are better supported by the use of region-specific temperature thresholds, whether the current assumption of a 14-day lag for cold-associated mortality best reflects epidemiological evidence and user needs, and whether additional adjustments are possible and appropriate to account for other circulating respiratory viruses in winter, such as respiratory syncytial virus (RSV) for example. Wider sensitivity analysis will be used to test the robustness of estimates to changes in key parameters and modelling choices. Any resulting methodological changes will be quality assured and clearly documented, with impacts on the results transparently communicated in updated supporting materials.

Specific outputs

In the next report, we will clearly communicate how the statistics have developed since first publication. This will include a summary of user engagement activity, highlighting any important feedback received and explaining how this feedback has informed changes to the statistics or supporting materials. Where feedback cannot be addressed immediately, we will explain why and set out planned next steps.

We will also report on methodological developments undertaken during the development phase. This will include setting out which methodological options have been tested, the outcomes of that testing, and confirming the stable methodological approach that will be used for future publications. Any changes implemented will be clearly explained, alongside their impact on the results, to support transparency and continuity for users.

Office for Statistics Regulation

Our statistical practice is regulated by the Office for Statistics Regulation (OSR). OSR sets the standards of trustworthiness, quality and value in the Code of Practice for Statistics that all producers of official statistics should adhere to. You are welcome to contact us with comments about how we meet these standards by emailing extremeevents@ukhsa.gov.uk

Alternatively, you can contact OSR by emailing regulation@statistics.gov.uk or via the OSR website.