James Hay
@jameshay.bsky.social
Research Fellow at the Pandemic Sciences Institute, University of Oxford. Using maths and stats to understand infectious disease dynamics, mostly viral kinetics and serology. https://hay-idd.github.io/
Many thanks also to @stevenriley.bsky.social @scauchemez.bsky.social @neher.io
@eales96.bsky.social, and Ben Cowling, Freya Shearer and Juliette Paireau for helpful discussion.
(18/18)
@eales96.bsky.social, and Ben Cowling, Freya Shearer and Juliette Paireau for helpful discussion.
(18/18)
November 25, 2025 at 1:51 PM
Many thanks also to @stevenriley.bsky.social @scauchemez.bsky.social @neher.io
@eales96.bsky.social, and Ben Cowling, Freya Shearer and Juliette Paireau for helpful discussion.
(18/18)
@eales96.bsky.social, and Ben Cowling, Freya Shearer and Juliette Paireau for helpful discussion.
(18/18)
Coauthors: @punya-alahakoon.bsky.social @mishkendall.bsky.social @mghafari.bsky.social @chriswymant.bsky.social @christophraser.bsky.social and Alex Greenshields Watson, Rob Hinch, Luca Ferretti and Jasmina Panovska-Griffiths not on bsky.
(17/18)
(17/18)
November 25, 2025 at 1:51 PM
Coauthors: @punya-alahakoon.bsky.social @mishkendall.bsky.social @mghafari.bsky.social @chriswymant.bsky.social @christophraser.bsky.social and Alex Greenshields Watson, Rob Hinch, Luca Ferretti and Jasmina Panovska-Griffiths not on bsky.
(17/18)
(17/18)
Thanks to the @christophraser.bsky.social team for immense work in a very short time, particularly to @punya-alahakoon.bsky.social and Alex Greenshields-Watson who made great contributions under pressure.
(16/18)
(16/18)
November 25, 2025 at 1:51 PM
Thanks to the @christophraser.bsky.social team for immense work in a very short time, particularly to @punya-alahakoon.bsky.social and Alex Greenshields-Watson who made great contributions under pressure.
(16/18)
(16/18)
Caution is needed – we looked only at data from England, whereas other countries might see quite different trends (e.g., bsky.app/profile/cmye...). Historical experience with early H3N2 seasons is also worth bearing in mind: pmc.ncbi.nlm.nih.gov/articles/PMC... (1989/90 season).
(15/18)
(15/18)
November 25, 2025 at 1:51 PM
Caution is needed – we looked only at data from England, whereas other countries might see quite different trends (e.g., bsky.app/profile/cmye...). Historical experience with early H3N2 seasons is also worth bearing in mind: pmc.ncbi.nlm.nih.gov/articles/PMC... (1989/90 season).
(15/18)
(15/18)
Overall, the K clade seems to have higher growth than dominant strains in prior seasons, but not total immune escape (antibody escape != all immune escape). For England, the earlier season might have interacted with half-term to dampen the peak burden (but not necessarily overall burden).
(14/18)
(14/18)
November 25, 2025 at 1:51 PM
Overall, the K clade seems to have higher growth than dominant strains in prior seasons, but not total immune escape (antibody escape != all immune escape). For England, the earlier season might have interacted with half-term to dampen the peak burden (but not necessarily overall burden).
(14/18)
(14/18)
Our plan is to update this analysis again with cleaner data as things continue to unfold. The most current data are presented by UKHSA here: www.gov.uk/government/s...
(13/18)
(13/18)
National flu and COVID-19 surveillance reports: 2025 to 2026 season
National influenza (flu) and COVID-19 report, monitoring COVID-19 activity, seasonal flu and other seasonal respiratory illnesses.
www.gov.uk
November 25, 2025 at 1:51 PM
Our plan is to update this analysis again with cleaner data as things continue to unfold. The most current data are presented by UKHSA here: www.gov.uk/government/s...
(13/18)
(13/18)
Another key limitation is the omission of vaccination. The K clade seems to have drifted away from this season’s vaccine strains, but preliminary data from the UKHSA suggest that vaccine efficacy is in line with previous seasons. www.eurosurveillance.org/content/10.2...
(12/18)
(12/18)
Early influenza virus characterisation and vaccine effectiveness in England in autumn 2025, a period dominated by influenza A(H3N2) subclade K
Influenza A(H3N2) subclade K (J.2.4.1) has dominated the 2025/26 season start in England. Post-infection ferret antisera raised against northern hemisphere 2025/26 vaccine strains showed reduced reactivity to subclade K viruses in England, aligning with World Health Organization reports. Nevertheless, early post-vaccination, vaccine effectiveness against influenza-related emergency department attendances and hospital admissions remained within typical ranges, at 72–75% in children and adolescents (< 18 years) and 32–39% in adults. Hence, vaccination remains effective against clinical disease caused by influenza A(H3N2) viruses.
www.eurosurveillance.org
November 25, 2025 at 1:51 PM
Another key limitation is the omission of vaccination. The K clade seems to have drifted away from this season’s vaccine strains, but preliminary data from the UKHSA suggest that vaccine efficacy is in line with previous seasons. www.eurosurveillance.org/content/10.2...
(12/18)
(12/18)
This analysis was done quickly with many limitations (see preprint!) – we merged imperfect ILI+ datasets from past seasons, combined H3N2 and other subtypes, made many assumptions for our scenario model, and some growth trends might be masked by smoothing introduced through the models.
(11/18)
(11/18)
November 25, 2025 at 1:51 PM
This analysis was done quickly with many limitations (see preprint!) – we merged imperfect ILI+ datasets from past seasons, combined H3N2 and other subtypes, made many assumptions for our scenario model, and some growth trends might be masked by smoothing introduced through the models.
(11/18)
(11/18)
Also compatible but perhaps less plausible are a 10-20% higher R0, or an earlier seed date. In almost all scenarios, an earlier and faster epidemic growth rate leads to depletion of susceptibles before the Christmas period with a dampening effect due to the half term school holiday.
(10/18)
(10/18)
November 25, 2025 at 1:51 PM
Also compatible but perhaps less plausible are a 10-20% higher R0, or an earlier seed date. In almost all scenarios, an earlier and faster epidemic growth rate leads to depletion of susceptibles before the Christmas period with a dampening effect due to the half term school holiday.
(10/18)
(10/18)
The scenarios suggest substantial immune escape is unlikely given current trends. Only a small drop in population immunity is needed to generate an early peak and faster growth. Current data are compatible with small levels of immune escape in all ages, or greater escape in children.
(9/18)
(9/18)
November 25, 2025 at 1:51 PM
The scenarios suggest substantial immune escape is unlikely given current trends. Only a small drop in population immunity is needed to generate an early peak and faster growth. Current data are compatible with small levels of immune escape in all ages, or greater escape in children.
(9/18)
(9/18)
However, it’s tricky to translate immune escape from antibodies in the lab to overall immune escape in human populations, where other immune mechanisms will play a key role.
(8/18)
(8/18)
November 25, 2025 at 1:51 PM
However, it’s tricky to translate immune escape from antibodies in the lab to overall immune escape in human populations, where other immune mechanisms will play a key role.
(8/18)
(8/18)
A key concern for the season has been immune escape of the new clade. Antibodies generated in response to infection from other recent H3N2 viruses, including the vaccine strain, do not appear to recognise K clade viruses very well: youtu.be/jKlthy3YYNQ?....
(7/18)
(7/18)
WHO Information Meeting - Influenza virus vaccines composition for 2026 Southern Hemisphere
YouTube video by World Health Organization Regional Office for the Western Pacific
youtu.be
November 25, 2025 at 1:51 PM
A key concern for the season has been immune escape of the new clade. Antibodies generated in response to infection from other recent H3N2 viruses, including the vaccine strain, do not appear to recognise K clade viruses very well: youtu.be/jKlthy3YYNQ?....
(7/18)
(7/18)
Alex Greenshields-Watson put the model online as a webtool for others to explore: hay-idd.shinyapps.io/ModelFluUk-H.... *We particularly encourage users to tailor scenarios for Scotland, Wales and NI, as the season timing has been quite different to England so far.*
(6/18)
(6/18)
hay-idd.shinyapps.io
November 25, 2025 at 1:51 PM
Alex Greenshields-Watson put the model online as a webtool for others to explore: hay-idd.shinyapps.io/ModelFluUk-H.... *We particularly encourage users to tailor scenarios for Scotland, Wales and NI, as the season timing has been quite different to England so far.*
(6/18)
(6/18)
We developed an age-stratified SIR model, visually calibrated to 2022/23 data (an H3N2 season). We ran scenarios varying intrinsic transmissibility, loss of immunity overall or in children, and the season start date. We accounted for changes in contacts over school holidays and Christmas.
(5/18)
(5/18)
November 25, 2025 at 1:51 PM
We developed an age-stratified SIR model, visually calibrated to 2022/23 data (an H3N2 season). We ran scenarios varying intrinsic transmissibility, loss of immunity overall or in children, and the season start date. We accounted for changes in contacts over school holidays and Christmas.
(5/18)
(5/18)
Rapid spread in children – the usual culprits driving seasonal flu transmission. Compared to the past 2 seasons, there has been a larger difference in growth rate between <14 yos and adults. Half term caused a drop in growth and may have acted as a circuit break for the early season.
(4/18)
(4/18)
November 25, 2025 at 1:51 PM
Rapid spread in children – the usual culprits driving seasonal flu transmission. Compared to the past 2 seasons, there has been a larger difference in growth rate between <14 yos and adults. Half term caused a drop in growth and may have acted as a circuit break for the early season.
(4/18)
(4/18)
Case numbers and epi dynamics don’t look too unusual when you align the curves by date of peak growth rate (* to date*). The flu season has started very early but is otherwise not completely unprecedented (at least in terms of symptomatic cases).
(3/18)
(3/18)
November 25, 2025 at 1:51 PM
Case numbers and epi dynamics don’t look too unusual when you align the curves by date of peak growth rate (* to date*). The flu season has started very early but is otherwise not completely unprecedented (at least in terms of symptomatic cases).
(3/18)
(3/18)
We analysed public data sources (Resp DataMart, RGCP, WHO FluNet) from England to compare growth rates and reproduction numbers to previous seasons. So far, peak growth rates have been higher than the past 10 seasons, but Rt estimates are in line with the upper end of previous seasons.
(2/18)
(2/18)
November 25, 2025 at 1:51 PM
We analysed public data sources (Resp DataMart, RGCP, WHO FluNet) from England to compare growth rates and reproduction numbers to previous seasons. So far, peak growth rates have been higher than the past 10 seasons, but Rt estimates are in line with the upper end of previous seasons.
(2/18)
(2/18)