Gwyddoniaeth Ymchwil Tystiolaeth: modelu'r gaeaf 2025 i 2026 - Part 9: UK and international comparisons and extreme weather conditions

RSV

Overall RSV activity across all UK nations started around week 42 2024 (week starting 14 October 2024), and peaked across nations around weeks 47 to 49 2024 (18 November to 8 December 2024). The timing of peak RSV activity was similar to the 2023 to 2024 season in England and Wales, and slightly later than in previous seasons in Scotland and Northern Ireland.

Flu

In 2024 to 2025, The influenza-like illness (ILI) GP consultations rate for each UK nation peaked December to early January (with a temporary dip over the Christmas to New Year period) reaching medium levels (according to ‘moving epidemic method’, ‘MEM’, thresholds). Wales, Scotland and Northern Ireland had peak ILI GP consultation rates of around 35 to 40 GP consultations per 100,000) in the winter of 2024 to 2025. In both Wales and Northern Ireland, the 2024 to 2025 peak exceeded 2023 to 2024 and was similar to 2022 to 2023. In Scotland the 2024 to 2025 peak exceeded those of the previous two years (2022 to 2023 and 2023 to 2024). In England the 2024 to 2025 peak levels (around 25 per 100,000) did not reach those observed in the winter of 2022 to 2023. Direct comparisons can not be made between the nations due to the differing methodologies in data collections.

Hospital admissions due to flu reached high levels (according to the MEM thresholds) in England in 2024 to 2025 with a peak of 16.18 weekly admissions per 100,000 (in week 52, 2024). Scotland and Northern Ireland reached peaks of 29 and 21.3 weekly hospital admissions per 100,000 population respectively in weeks 52 and 51 2024 respectively. Wales peaked at 306 weekly admissions in week 52 equating to around 9.6 per 100,000 population. Different data collection methods were used for each nation, so these admission rates are not directly comparable between countries. The peak hospital admissions in each UK nation is much higher than the peak of the winter prior (2023 to 2024), around double in England Wales and Northern Ireland and an even higher multiple in Scotland. For England, Scotland and Northern Ireland, the 2024 to 2025 peak is similar to that of 2022 to 2023.

Flu test positivity peaked at around 28% (Respiratory Datamart) for England, 30% for Wales, 35% for Northern Ireland and 36% for Scotland. For Scotland and Northern Ireland, their peak positivity percentage exceeded the peaks of the previous two years (in 2022 to 2023 and 2023 to 2024). However, in England and Wales, although the peak for test positivity was higher last winter in 2024 to 2025 compared with the year prior (2023 to 2024), it was lower than the test positivity peak in 2022 to 2023. Different testing methods were used in each UK nation, so the flu test positivity percentages are not directly comparable between the UK countries.

A literature review of epidemiological data from the southern hemisphere was conducted to inform projections for the upcoming winter season in Wales. 

In Australia, from mid-March to mid-June 2025, the weekly proportion of FluTracking survey participants with new fever and cough symptoms was lower than those seen at the same time in 2022, 2023, 2024, and the five-year average; however, since early July the weekly proportion has been similar to the same time last year and the five-year average. Influenza cases are following a similar trend seen at the same time last year.

From January to mid-June 2025, influenza-like illness (ILI) GP consultation rates were consistent with rates seen in 2024. Since early June 2025, GP consultations for influenza-like illness consultation rates increased and now exceed the five-year average but remain below last year's figures. 

Patients admitted to sentinel hospitals with influenza were mostly admitted with influenza A (predominantly H1N1). There was a comparatively higher proportion of influenza B cases this season than was seen in 2024 and 10.7% more deaths due to influenza than was observed in the first 6 months of 2024. Over 98% of influenza isolates in 2025 match this year's vaccine components.

Between 2023 and 2025, COVID-19 was the leading cause of acute respiratory infection deaths, mainly among older adults although the number of deaths in the first 6 months of 2025 were substantially lower than those observed in 2024 and 2023.

In Hong Kong, the ILI consultation rate from 3 to 9 August was at baseline level, running similar to, or as of this week, lower than last year. In terms of hospital admissions, the baseline threshold was exceeded and peaked in January then decreased to a low level in late March, with a peak rate lower than that recorded last year. In Singapore, the polyclinic attendances for acute respiratory infection are similar to the attendances seen in 2024.

WHO is monitoring 6 variants of COVID-19 at present. NB.1.8.1 (Nimbus) is currently dominant however not associated with more severe disease and risk evaluation by the WHO is low. The XFG (Stratus) variant is currently increasing in prevalence and is associated with rapid spread but similarly, not with severity of symptoms.

Using the international picture to estimate what we may see in Wales, the flu season may see similar figures to 2024 although there could be increased proportion of influenza B and a potential shift in the dominant form of COVID-19. Vaccine effectiveness is expected to be maintained against symptomatic and severe disease.

The UKHSA published the fourth Health Effects of Climate Change (HECC) in the UK: state of the evidence report in December 2023. The HECC report summarises current evidence on the health impacts of climate change and effective interventions. Without adaptation, heat and cold related mortality in the UK is projected to rise due to climate change and demographic shifts. Increasing exposure to extreme weather will affect more people, with cold related deaths also rising due to an ageing population. While these impacts are widespread, their severity will vary by geography and sociodemographic factors.