Why is flu circulating earlier than usual in England in 2026?

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1. 1

   Influenza A/H3N2 epidemiology in England during the 2025 to 2026 season: a mathematical modelling study.

   Communications health (Hay et al.)Published Aug 5, 2026Checked Oct 11, 2026

   “Weekly epidemic growth rates were estimated using a Gaussian random walk model, and time-varying reproduction numbers using EpiEstim. We also developed an age-stratified transmission model and interactive web tool to explore scenarios varying immune escape, transmissibility, and seed date, using 2022/23 as a baseline season.ResultsPeak A/H3N2 growth rates and time-varying reproduction numbers for the 2025/26 season are of similar magnitude but earlier than previous severe seasons. Scenario analyses suggest early trends are compatible with moderate levels of immune escape, a 10% higher R 0 , or an earlier seed date, though it is not possible to distinguish the relative importance of these mechanisms from these data alone.ConclusionsThe 2025/26 influenza season is characterised by early but not unusually rapid growth. Earlier growth does not systematically lead to especially large epidemics due to earlier susceptible depletion combined with a dampening effect from school holidays. Laboratory evidence for antibody escape does not directly translate to large reductions in population immunity, supporting the need for complementary real-time epidemiological analyses and modelling.”

2. 2

   Epidemiological diversity of seasonal influenza and post-COVID-19 pandemic changes in the UK, 2015-2025: a surveillance-based analysis.

   BMC public health (Yuan et al.)Published Feb 2, 2026Checked Oct 11, 2026

   “Before the COVID-19 pandemic, across five influenza seasons in the UK, seasonal influenza peak positivity, total seasonal positivity, and epidemic timing metrics—including onset weeks, offset weeks, and epidemic duration in weeks—varied by nation, season, and/or virus type. Post the COVID-19 pandemic, changes in the aforementioned parameters also varied by season, region, and/or virus strain. The main patterns that can be summarized are as follows: aggregated peak positivity and total seasonal positivity for influenza A(H3N2) and influenza B were reduced across all three nations with available post-pandemic data; influenza A(H1N1)pdm09 and A(H3N2) exhibited a consistent alternating dominance pattern across the four nations before the COVID-19 pandemic, which was disrupted post-pandemic; and in Wales, the onset week advanced, the offset week delayed… Pre-COVID-19, influenza seasonality in the UK varied by nation, season, and/or virus strain. The pandemic disrupted prior dominant strain patterns with considerable consistency, yet it did not consistently alter the timing and intensity of seasonal influenza epidemics across nations and/or virus types. TRIAL REGISTRATION: Not applicable.”

3. 3

   Mid-season influenza virus genetic characterizations in U.S. Department of War populations, 2025-2026.

   MSMR (Gruner et al.)Published Aug 20, 2026Checked Oct 11, 2026

   “Significant antigenic drift led to the dominance of subclades K, D.3.1/D.3.1.1, and C.3.1, respectively. Testing indicated that the 2025-2026 vaccine components offered reduced protection against these emerging strains. Antigenic cartography confirms that the 2026-2027 vaccine strains provide significantly improved protection against circulating viruses. Antiviral resistance markers for neuraminidase inhibitors such as oseltamivir remain rare, with only 2 identified in A(H1N1)pdm09. These data are critical for maintaining force health protection and assessing vaccine performance within highly mobile DOW populations. Analysis of 2025-2026 influenza season data revealed the predominance of subclades A(H1N1)pdm09 D.3.1/D.3.1.1, A(H3N2) K, and B/Victoria C.3.1. Antigenic cartography employing recent candidate vaccine viruses from the U.S. Centers for Disease Control and Prevention confirmed that the World Health Organization's strain recommendations for the 2026-2027 Northern Hemisphere influenza vaccine provide the best protection against these circulating strains.”

4. 4

   Shifts in seasonal timing of respiratory diseases and causes of death following a natural pandemic event.

   PLOS global public health (Sieber & Traulsen)Published Jul 15, 2026Checked Oct 11, 2026

   “During the COVID-19 pandemic the seasonal surge in respiratory infections occurred substantially earlier, initially driven by the emerging COVID-19 and during later seasons by the resurgence of influenza and RSV. Remarkably, this shift was accompanied by a similar shift in the seasonality of all-cause mortality, and in particular mortality due to cardiovascular disease. The observed shift in epidemic timing is a consistent, but transient, outcome of disrupted epidemic seasonality, predictable from basic epidemiological principles. Our study reinforces that the buildup of susceptibles after the pandemic disruption was responsible for the shift in timing of the seasonal epidemic of respiratory infections. We show that more generally the rate of immune waning is the main determinant of the exact timing of seasonal epidemics of infectious diseases. The corresponding shift in seasonality of cardiovascular mortality suggests a connection between recent respiratory infections and severe cardiovascular events. This highlights the importance of monitoring individual infection history and improving vaccination coverage, in particular against influenza.”

5. 5

   What Different Surveillance Systems Tell Us About Winter Respiratory Illness in England: A Time-Series Analysis Combining Participatory, Syndromic and Laboratory Datasets, 2022-2025.

   Influenza and other respiratory viruses (Munro et al.)Published Sep 1, 2026Checked Oct 11, 2026

   “Multivariable (beta and negative binomial) regression models were used to explore the association between ILI and SARS-CoV-2, influenza and RSV positivity in each dataset.ResultsTrends in ILI align well across participatory surveillance systems, with ILI peaking around Weeks 50-52 each season. Participatory surveillance system trends align well with syndromic healthcare use for ILI, with ILI symptoms peaking about a week before healthcare use. ILI in each dataset is driven by a different combination of respiratory viruses, with ILI in the community driven by a combination of SARS-CoV-2, influenza and RSV, whereas healthcare attendance was driven by influenza only.ConclusionsWhile each surveillance system differs in its purpose and population, trends in ILI align across datasets. Comparing a range of surveillance systems can provide increased confidence in each system. While syndromic healthcare use surveillance systems highlight the burden of severe illness, integration with participatory surveillance systems can demonstrate the wider burden across a spectrum of severity.”

6. 6

   Influenza (Wikipedia)

   WikipediaPublished Oct 10, 2026Checked Oct 11, 2026

   “Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin one to four (typically two) days after exposure to the virus and last for about two to eight days. Diarrhea and vomiting can occur, particularly in children. Influenza may progress to pneumonia from the virus or a subsequent bacterial infection. Other complications include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease. There are four types of influenza virus: types A, B, C, and D. Aquatic birds are the primary source of influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs. Influenza B virus (IBV) and influenza C virus (ICV) primarily infect humans, and influenza D virus (IDV) is found in cattle and pigs. Influenza A virus and influenza B virus circulate in humans and cause seasonal epidemics, and influenza C virus causes a mild infection, primarily in children.”