Science Research Evidence: winter modelling 2025 to 2026 - Part 2: definitions

Admissions and occupancy

Admissions and occupancy refer to daily new admissions to hospital and daily hospital bed occupancy respectively. For admissions, the relevant diagnosis code is filtered in any position (that is not limited to being the primary diagnosis) for the first episode of the spell only (that is the admitting episode). For more information, see NHS Wales data dictionary. Day cases are included in the admissions.

ICD-10 codes

More specific ICD-10 codes for influenza, RSV, and COVID-19 have been used for analysis in this year's 2025 to 2026 winter modelling report (compared to analysis in previous winter modelling reports). These pathogen-specific codes align more closely with the ICNET dataset (which uses multiplex testing for identify admissions due to specific pathogens), enabling more accurate tracking of actuals with the modelled scenarios (‘observed data’) through the winter.

Averages

Unless stated otherwise, averages refer to the mean.

Peaks

Peaks refer to the maximum values recorded during each winter. Peak dates correspond to the date on which the maximum value was observed. Peak dates might some multiple days in some cases.

Dates

Most analysis in this report focusses on the winter period (between 1 September and 31 March inclusive). Unless stated otherwise, analysis carried out is usually only over the winter period. However, there are parts of the report where the whole year is considered, such as the vaccine rollout as it is important to deduce vaccine uptake throughout the year.

Scenarios

In previous years, we referred to Most Likely Scenario (MLS) and Reasonable Worst Case (RWC), but it is more appropriate to use “low”, "moderate" and "severe" scenarios rather than make assumptions early on about how the respiratory flu season may progress.