WHO Recommendations for Influenza Vaccine Composition for the 2026-2027 Northern Hemisphere Influenza Season

Credit ACIP
 

#19,068

Twice each year international influenza experts meet to discuss recent developments in human and animal influenza viruses around the world, and to decide on the composition of the next influenza season’s flu vaccine.

Due to the time it takes to manufacture and distribute a vaccine, decisions on which strains to include must be made at least six months in advance. 

As occasionally happens, that can provide a window of opportunity for `drifted' or mutated flu strains to emerge, and spread globally (see last Nov's Increasing Concerns Over A `Drifted' H3N2 Virus This Flu Season).

Earlier this week, in Preprint: Near Real-time Data on the Human Neutralizing Antibody Landscape to Influenza Virus as of Early 2026 to Inform Vaccine-strain Selection, we looked at a `near real-time' assessment of the human neutralizing antibody landscape against currently circulating influenza A viruses.

A review which found that most people have low antibody defenses against recently emerged H3N2 subclade K and H1N1 D.3.1.1 viruses.

Today the WHO released their recommendations for the composition of this fall's 2026-2027 Northern Hemisphere flu vaccine, which recommends updates for all 3 flu strains; H1N1, H3N2, and Influenza B.

Overview

For vaccines for use in the 2026-2027 northern hemisphere influenza season, WHO recommends the following:

Egg-based vaccines

• an A/Missouri/11/2025 (H1N1)pdm09-like virus;

• an A/Darwin/1454/2025 (H3N2)-like virus; and

• a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus.


Cell culture-, recombinant protein- or nucleic acid-based vaccines

• an A/Missouri/11/2025 (H1N1)pdm09-like virus;

• an A/Darwin/1415/2025 (H3N2)-like virus; and

• a B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus.

The 2026 Southern Hemisphere vaccine already has the recommended updated H1N1 component, but retains older strains against H3N2 and Influenza B. Despite this apparent mismatch, it will still likely provide important protection against severe disease.

Sadly, seasonal flu vaccine uptake in the United States peaked in 2019, and has begun a slow, but steady decline since.

Pandemic fatigue, plus growing anti-vaccination sentiment, and admittedly `hit-or-miss' protection against influenza infection have all had an impact.  

But the more we learn about influenzas' extrapulmonary impacts on the body (see Risk of Cardiovascular Events After Influenza: A Population-based Self Controlled Case Series Study), the more important protecting yourself becomes.

Which is why I consider the flu vaccine to be an important part of my `flu prevention' strategy each year (along with masking, handwashing, and avoiding indoor crowds).    

While none of this guarantees I'll continue to go unscathed, my last bout with `flu' was in the summer of 2009 (after attending a pandemic flu conference!), and I've only contracted COVID once, before the first vaccine was released.

Since my strategy seems to work, I see no reason to `fix' it. 

South Korea: MAFRA Extends Special Quarantine Period for 3rd Month (to March 31st)

 

#19,067

On January 5th, South Korea ordered Special Quarantine Measures Implemented for one Month to Prevent the Spread of HPAI, which was extended in early February due to concurrent outbreaks of FMD and ASF along with ongoing HPAI.

Over the past 3 weeks the number of ASF outbreaks has more than tripled (from 6 to 20), and more than a dozen HPAI outbreaks have been reported in poultry.  

As noted on Wednesday, a new subtype (H5N9) has been increasingly reported in South Korea, and MAFRA has already ordered their third 24-hour halt in the transport of livestock (poultry, ducks, and pigs) this week.

Today, MAFRA published an extraordinary (14.5 mb) collection of 132 maps and aerial photos (see sample below), of migratory bird arrival areas where livestock-related vehicles and workers are restricted from entering.

Given that the northbound phase of Korea's migratory bird season begins in early March - and often extends into May - this may not be the last extension to this special quarantine period we see. 

Some (translated) excerpts from today's announcement follow:

The special quarantine period for livestock infectious diseases has been extended until March, and all-out measures are being taken to prevent further outbreaks.

February 27, 2026, 3:44 PM, Quarantine Policy Bureau 

The Central Disaster and Safety Countermeasures Headquarters ( Director Song Mei-ryeong, Minister of Agriculture, Food and Rural Affairs , hereinafter referred to as the Central Disaster and Safety Countermeasures Headquarters ) for Livestock Infectious Diseases ( Highly Pathogenic Avian Influenza , Foot - and-Mouth Disease, African Swine Fever ) announced that considering the current risk of additional outbreaks of livestock infectious diseases, the existing special quarantine period (October 1, 2025 - February 28, 2026) will be extended by one month until March 31 ( Friday ) to strengthen quarantine management .

1. Highly pathogenic avian influenza (AI)

< Occurrence situation and situation diagnosis >

This winter ('25/'26 season ) , there have been 50 cases of highly pathogenic avian influenza in poultry farms and 59 cases in wild birds .

* Poultry farm occurrence status ( total 50 cases ): 11 cases in Gyeonggi , 9 in North Chungcheong , 9 in South Chungcheong , 4 in North Jeolla , 10 in South Jeolla , 4 in North Gyeongsang , 1 in South Gyeongsang , 1 in Gwangju , 1 in Sejong

** Status of wild bird detection ( total 59 cases ): Gyeonggi 6 , Gangwon 6, Chungbuk 1, Chungnam 14, Jeollabuk-do 6, Jeollanam-do 7 , Gyeongbuk 3, Gyeongnam 5, Jeju 4, Seoul 3, Busan 2, Incheon 1, Gwangju 1

In our country, highly pathogenic avian influenza first occurred in a poultry farm (September 12, 2025) 47 days earlier than the previous 24/25 season (October 29, 2024) during this 25/26 winter season . The outbreak area has also expanded significantly ( from 26 cities and counties to 29 ) compared to last season . In addition , the number of detections in wild birds has increased from 43 to 59 , and the detection area has expanded nationwide (from 30 cities and counties to 38 ) . Therefore, it is necessary to strengthen quarantine management, such as thorough disinfection and access control, more than ever before .

The Central Disaster and Safety Countermeasure Headquarters assessed that there is a risk of additional outbreaks considering that the results of the February migratory bird population survey showed that there were 1.33 million birds * , a large number of birds inhabiting the area , and that highly pathogenic avian influenza continued to occur in domestic poultry farms and wild birds in February as well as sporadic outbreaks during the migratory bird migration period since March . Therefore , the main quarantine measures implemented during the special quarantine period will be extended until Tuesday, March 31 to continue strengthened quarantine management of poultry farms , etc.

* According to the February migratory bird habitat survey ( cooperation with the Ministry of Climate and Environment ), 1.33 million birds were confirmed , a 1.2% decrease in the number of birds compared to the previous month (1.35 million ) , but many migratory birds still live across the country.

* * Farm (50 cases ) : (September ) 1 case → (October ) 1 case → (November ) 4 cases → (December ) 22 cases → (January ) 10 cases → (February ) 12 cases

Migratory birds (59 cases ): (September ) 0 cases → (October ) 2 cases → (November ) 11 cases → (December ) 10 cases → ( January ) 19 cases → ( February ) 17 cases

*** Occurrences in the past March to May : ('23 ) 2 cases in March , 4 cases in April , ( '24 ) 1 case in May , ( '25 ) 8 cases in March , 4 cases in April

< Strengthened quarantine measures >

To prevent further sporadic outbreaks due to migratory birds moving north, the Central Disaster and Safety Countermeasure Headquarters will strengthen quarantine measures as follows until Tuesday , March 31 .

First , we will continue to operate the Central Disaster and Safety Countermeasures Headquarters for Highly Pathogenic Avian Influenza system , cooperate with relevant ministries such as the Ministry of the Interior and Safety and the Ministry of Climate Change to respond with all-out efforts across the government , and all local governments will continue to operate response headquarters and situation rooms .

Second , a special quarantine inspection will be conducted jointly by the Ministry of Agriculture, Food and Rural Affairs, the Ministry of the Interior and Safety, and other relevant organizations in 32 cities and counties at risk during the northern migration period of migratory birds (March 3 to March 17) , and a ' Quarantine Strengthening Public Relations Campaign ' will be promoted together with producer groups to improve quarantine rules for poultry farms . *

* Major farm initiatives for the month of March : ① Changing boots when entering and exiting the barn , ② General cleaning and disinfection , ③ General rat extermination work

Third , to prevent further outbreaks in laying hens, one-on-one dedicated managers will be assigned to laying hens in farms with more than 50,000 hens nationwide by March to manage vehicles and people entering and exiting the farms . In particular, control posts installed in densely populated areas and large-scale laying hens with more than 200,000 hens will be intensively inspected to ensure compliance with quarantine measures such as vehicle disinfection .

* In addition, random environmental inspections will be conducted on hazardous livestock vehicles ( egg , manure , feed transport ) and goods entering and exiting laying hen farms nationwide during March .

Fourth , the existing administrative orders (11 cases) and public notices (7 cases) that have been issued and are in effect will be extended until March , and additional administrative orders ( 2 cases ) and public notices (3 cases ) will be issued and operated to prevent outbreaks at poultry farms .

* Additional measures : ① Restriction on partial shipment of laying hens nationwide for 2 weeks (March 1 to March 14), ② Restriction on the entry of laying hens into outbreak cities and counties for 2 weeks (March 1 to March 14), ③ Restriction on entry of livestock disposal personnel to other farms for purposes other than livestock disposal for 1 month (March 1 to March 31)

- However , shipment of all laying hens from a farm to a slaughterhouse is possible , and in cases where unavoidable circumstances are recognized , movement ( shipment ) is permitted under limited circumstances under the supervision of public officials. Administrative orders and public notices may be adjusted depending on the situation.

        (Continue . . .) 

This announcement also contains special instructions for dealing with ASF and FMD.   

In an update from last week's report on pig feed additives which tested positive for ASF DNA, we have the following update which also found ASF genes in on-farm stockpiled feed.

Once again, this appears to be via PCR, meaning that the infectiousness of this sample hasn't been ascertained. 

African swine fever (ASF) genes detected in compound feed made from plasma proteins, leading to strengthened quarantine measures to prevent its spread.

2026.02.24 23:00:00 Foot-and-Mouth Disease Prevention Division, Quarantine Policy Bureau

The African Swine Fever Central Disaster and Safety Countermeasures Headquarters ( Director Song Mei-ryeong, Minister of Agriculture, Food and Rural Affairs , hereinafter referred to as the Central Disaster and Safety Countermeasures Headquarters ) announced on Tuesday , February 24 that it will strengthen quarantine measures to prevent the spread of African swine fever (ASF) after the gene was detected in pig feed .

1. Detection circumstances

The Korea Centers for Disease Control and Prevention (KCDC) is conducting a comprehensive inspection ( carcasses and environmental samples ) on pig farms nationwide . During this process , a detailed inspection was conducted on carcasses and environmental samples such as feed from a pig farm (No. 1 ) in Hongseong, South Chungcheong Province , and on February 24 , it was announced that the African swine fever (ASF) gene was detected * in two cases ( same product ) of compound feed made from plasma protein .

* Results of testing by the Chungcheongnam-do Animal Hygiene Testing Laboratory and the Animal and Plant Quarantine Agency , positive result due to detection of ASF gene

2. Key quarantine measures

The central government will first order local governments across the country to order owners of feed* in which African swine fever (ASF) genes have been detected at pig farms to dispose of the relevant items in accordance with the Livestock Infectious Disease Prevention Act ** .

* It is assumed to have been manufactured using pig-derived plasma protein supplied by OOO Lab as the raw material.

** Article 23 of the Livestock Infectious Disease Prevention Act ( Incineration of Contaminated Materials, etc. )

In addition , for feed in which African swine fever (ASF) genes have been detected, the relevant companies ( some names ), production dates , and products will be disclosed on the Animal and Plant Quarantine Agency website (www.qia.go.kr) * , and local governments plan to recommend that pig farms nationwide stop using the relevant feed as a preventive measure .

 (Continue . . . )

While it may appear that South Korea's livestock disease woes are far greater than most other countries, it is worth mentioning that few countries are as open about their agricultural disease challenges and responses. 

So how much of an outlier South Korea really is, is difficult to say. 

But given the way that livestock diseases propagate around the globe, countries that are currently under less pressure may find their current status quos to be fleeting.  

California: Background on the Outbreak of H5N1 in Elephant Seals at Año Nuevo Natural Reserve

 
California Parks

#19,066

Yesterday, California Parks and UC Santa Cruz announced the first U.S. detection of H5N1 in Elephant Seals at the Año Nuevo Natural Reserve (see FluTracker's thread) after dozens of seals  were observed either sick or dying  and the virus was confirmed (on Tuesday night) by the USDA’s NVSL lab. 

(Excerpt)

On Feb. 19 and 20, Beltran’s team noticed seals at Año Nuevo Reserve with abnormal respiratory and neurological signs, including weakness and tremors. They collected samples from sick and dead elephant seals and took them to UC Davis for testing at the California Animal Health and Food Safety Laboratory System. Initial screening revealed the samples were positive for avian influenza, which the USDA’s NVSL lab now confirms is HPAI H5N1.

While the details of this outbreak have already been well covered by the media, this is just the latest in a series of concerning HPAI spillovers into marine mammals around the globe.

We've repeatedly looked at the devastation of colonies of pinnipeds (seals, sea lions, walruses) in South America, and elsewhere, due to this virus. 

Although exact numbers are impossible to ascertain, a little over a year ago in Nature Reviews: The Threat of Avian Influenza H5N1 Looms Over Global Biodiversity, we saw the following proffered butcher's bill.

An estimated half a billion domestic fowl have died of H5N1 (ref. 4), and this number is rising; mortality in wildlife is more difficult to quantify than in poultry, but the effects are substantial. By mid-2024, H5N1 infections were documented in at least 406 bird and 51 mammal wild species (according to reports on the World Animal Health Information System (WAHIS)), and available reports suggest that multiple millions of wild animal individuals may have died (Fig. 1a).

The most noteworthy mass-mortality events include more than 200,000 wild birds in coastal areas of Peru6; 24,000 sea lions in South America7; 20,500 wild birds in Scotland8; 6,500 Cape cormorants in Namibia9; and 17,400 elephant seals, including >95% of the pups in Argentina10. These figures, however, largely underestimate actual mortalities, owing to a pervasive lack of monitoring, testing and reporting — particularly in inaccessible areas and in disadvantaged countries4,7.

While North America has seen occasional spillovers of HPAI H5 into marine mammals (primarily seals & dolphins see here, here, and here), they pale in comparison to what has been reported from South America, and have never included elephant seals until now.

(Note: Those will long memories will recall that in 2013 - in The 2009 H1N1 Virus Expands Its Host Range (Again) - we looked at a US Davis study which found that the 2009 H1N1 pandemic influenza virus had managed to jump to elephant seals as early as 2010.)

All of this is a concern - because while H5N1 hasn't managed to figure out how to transmit efficiently from human-to-human - there is growing evidence to suggest it may be close to doing so in marine mammals.  

• Two years ago, in a research letter (see EID Journal: HPAI A(H5N1) Viruses from Multispecies Outbreak, Argentina, August 2023), researchers wrote: ". . . it seems likely that pinniped-to-pinniped transmission played a role in the spread of the mammal-adapted HPAI H5N1 viruses in the region . . ."

• In the summer of 2024, we looked at a Preprint: Massive outbreak of Influenza A H5N1 in elephant seals at Peninsula Valdes, Argentina: increased evidence for mammal-to-mammal transmission

• A year ago, we looked at Preprint: Pathology of Influenza A (H5N1) Infection in Pinnipeds Reveals Novel Tissue Tropism and Vertical Transmission.

• And last March, in Nature Comms: Cross-species and mammal-to-mammal transmission of clade 2.3.4.4b HPAI A/H5N1 with PB2 adaptations, the authors wrote:

• Several mutations were detected months later in sea lions in the Atlantic coast, indicating that the pinniped outbreaks on the west and east coasts of South America are genetically linked. These data support sustained mammal-to-mammal transmission of HPAIV in marine mammals over thousands of kilometers of Chile’s Pacific coastline, which subsequently continued through the Atlantic coastline.

Although H5N1 remains predominantly an avian adapted virus, over the past 5 years is has become increasingly `mammalian friendly'.  Dairy cows, domestic cats, and peridomestic animals like mice, skunk, and foxes have all been greatly affected, and each spillover is another opportunity for the virus to test out new combinations of mammalian adaptations. 

Last September, in ISIRV: Update on H5N1 Panzootic: Infected Mammal Species Increase by Almost 50% in Just Over a Year we looked yet at another concerning milestone for the H5 virus. 

And just last month, in Adv. Genetics (Review Article) : Evolution of H5N1 Cross‐Species Transmission - Adaptive Mutations Driving Avian‐to‐Human Infection, we looked at another cautionary report that warned of the dangers of mammalian spread.

 

These types of reports have become so common, that events that would have inspired screaming headlines 3 or 4 years ago are scarcely noticed today (see Avian Flu's New Normal: When the Extraordinary Becomes Ordinary).

While hopefully this spillover to elephant seals in California won't lead to the kind of carnage we've seen in South America, this should be a sobering reminder that while we continue to dither in our response to H5N1, it continues to explore new ways to expand its geographic and host ranges. 

And, for now at least, H5Nx seems to be on a roll. 

South Korea: H5N9 Rising

 

#19,065

Last last year, South Korea's MAFRA Reported Increased Infectivity & Pathogenicity of This Year's Avian Flu Strains, which - for the first time - included 3 HPAI strains (H5N1, H5N6, and H5N9). 

Of particular concern, their Animal and Plant Quarantine Agency conducted an evaluation of the infectivity and pathogenicity of the highly pathogenic avian influenza virus ( serotype H5N1) and found that the infectivity was more than 10 times higher than in previous years .

While we've been watching subtypes other than H5N1 (H5N6 & H5N5 in particular) around the world, HPAI H5N9 has been comparatively rare. 

• A little over a year ago, California Reported a New H5N9 Reassortant In Ducks, but that appears to have been a one-off event. 

• Last May, the Philippines DOA Reports Their 1st Detection of HPAI H5N9

But as the following WOAH WAHIS report illustrates, global reports have been limited (4 HPAI, 1 LPAI) since 2017:

In early February, however, South Korea reported 3 poultry farm outbreaks (starting in Dec 2025) to WOAH resulting in the culling/loss of 150K birds. 

• ob_181082 - Donggok-ri, Dasi-myeon, Naju-si, Jeollanam-do, Republic of Korea
• ob_181085 - Omani-gil, Cheonbuk-myeon, Boryeong-si, Chungcheongnam-do, 33403, Republic of Korea
• ob_181080 - Jinjuk-ri, Cheongso-myeon, Boryeong-si, Chungcheongnam-do, Republic of Korea

While we haven't seen any new updates posted by WOAH, on February 6th MAFRA reported it would `. . . strengthen quarantine management following the confirmation of a case of highly pathogenic avian influenza (H5N9 type ) at a laying hen farm ( approximately 653,000 birds ) in Yesan, South Chungcheong Province.'

On February 11th , MAFRA warned:

The virus that occurred in domestic poultry farms this winter has a higher infectious power than in previous years, and in particular , in the case of the H5N9 type, symptoms such as death are not obvious in ducks , so the risk of transmission is higher than ever , so local governments, related organizations , and poultry farms across the country must thoroughly implement quarantine measures, 

Yesterday Hong Kong announced the suspension of imports of poultry meat and products (including poultry eggs) from Boryeong-si of Chungcheongnam-do Province in Korea due to a WOAH notification of an H5N9 outbreak, although it isn't clear if this is a new outbreak, or refers to the two from Dec/Jan.

Either way, this appears to be the first sustained spread of HPAI H5N9 in poultry reported anywhere in the world. 

Today, and for the 2nd time in 3 days, South Korea has ordered a 24-hour halt in the movement of live poultry. as new outbreaks of (unspecified) HPAI H5 continue to mount.

South Korea is also dealing with FMD, and a rapidly escalating outbreak of ASF (African Swine Fever), which appears to be due to the sale and use of contaminated pig feed. 

It's a lot to contend with, even for a country like South Korea. 

The spring northbound bird migration begins in early March in South Korea, and runs through May, meaning avian flu activity could easily persist for several more months on the Korean peninsula.

The mild (or subclinical) spread of H5N9 in ducks is particularly problematic, as detection and eradication will be difficult. Long term infection could produce even more reassortants. 

While there are no known human infections with HPAI H5N9, we've seen human infection by other H5 subtypes; including H5N1, H5N2, H5N5, and H5N6 HPAI viruses.   

Since we've also seen novel flu viruses with an N9 neuraminidase spillover to humans (China's H7N9 virus 2013-2019), there is little reason to believe it would present much of a barrier for H5 viruses. 

The $64 question is whether this outbreak of H5N9 in South Korea virus is a flash in the pan, or if it has `legs'.

In 2016's Sci Repts.: Southward Autumn Migration Of Waterfowl Facilitates Transmission Of HPAI H5N1, we looked at a study which found that the flu viruses migratory birds encounter as they fly north in the spring often thrive - or reassort - in their summer high latitude roosting areas, and then are redistributed via the migratory flyways the following fall. 

This is likely how South Korea's disastrous January 2014 H5N8 outbreak made it to the United States and Europe in less than a year, and became globally dominant by 2016. 

A reminder that whatever avian viruses that may emerge South Korea - or any other country - assuming they are biologically `fit' enough, are unlikely to stay put for very long.  

Preprint: Near Real-time Data on the Human Neutralizing Antibody Landscape to Influenza Virus as of Early 2026 to Inform Vaccine-strain Selection

Credit NIAID

#19,064

Later this week the WHO is expected to convene their semi-annual vaccine selection committee to consider the makeup of this fall's 2026-2027 trivalent influenza shot. A decision that must be made at least six months in advance to allow time to manufacture and deliver hundreds of millions of doses of vaccine. 

We reviewed their WHO Recommendations for Influenza Vaccine Composition for the 2026 Southern Hemisphere Influenza Season last September. 

While this year's flu season hasn't ended yet, it has been an overwhelmingly H3N2 dominant flu season so far, with H1N1 making up less than 15% of the influenza A viruses reported (see next 2 FluView Charts). 

Each subtype is made of of multiple subclades, and this year the late (and unexpected) arrival of H3N2 subclade K (92.1% share) helped to lower the effectiveness of this year's vaccine. 

At roughly the same time, an emerging H1N1 clade (D3.1.1) began to turn up in Europe, before spreading globally (see Eurosurviellance Reduced neutralising antibody responses against emerging 2025/26 influenza A(H1N1)pdm09 subclade D.3.1 and A(H3N2) subclade K viruses among healthcare workers, Finland, August to October 2025)).

By week 6, FluView has this new subclade making up nearly 60% of the H1N1 viruses this season.  While a direct descendent of clade 5a.2a.1, this subclade carries several new mutations and is outperforming its ancestors. 

Of note, we've also been seeing small, but significant increases in reduced susceptibility to oseltamivir in some of these emerging H1N1 viruses (see Eurosurveillance: Expansion of influenza A(H1N1)pdm09 NA:S247N Viruses with Reduced Susceptibility to Oseltamivir, Catalonia, Spain, and in Europe, July to October 2025. 

Today we've a preprint, authored by some of the best known flu and vaccine researchers in the world, which provides a `near real-time' assessment of the human neutralizing antibody landscape against currently circulating influenza A viruses. 

They find that many humans have low antibody defenses against both H3N2 subclade K and H1N1 D.3.1.1, and that both strains are only likely to continue to thrive in the near term.   

While both subtypes show continued evolution changes, the authors make specific note of  recent H3N2 subclade K viruses with mutations in antigenic regions D and E, which could further increase its transmissibility. 

I've only reproduced the abstract and the discussion. Follow the link to read it in its entirety.  I'll have a postscript after the break. 

Near real-time data on the human neutralizing antibody landscape to influenza virus as of early 2026 to inform vaccine-strain selection
Caroline Kikawa, John Huddleston, Sam A. Turner, Andrea N. Loes, Jiaojiao Liu, Sydney Gang, Tachianna Griffiths, Elizabeth M. Drapeau, Benjamin J. Cowling, Faith Ho, Nancy H. L. Leung, Janet A. Englund, Kristen Lacombe, Shinji Watanabe, Hideki Hasegawa, Michael Busch, Marion Lanteri, Mars Stone, Bryan Spencer,Richard A. Neher, Derek J. Smith, Trevor Bedford, Scott E. Hensley, Jesse D. Bloom
doi: https://doi.org/10.64898/2026.02.18.706711
This article is a preprint and has not been certified by peer review 

Preview PDF

Abstract

Twice each year, a decision is made on whether to update the strains included in the seasonal influenza vaccine to better match the most recent circulating viral strains.

To characterize the antigenic properties of current seasonal influenza A strains to inform the upcoming decision about which strains to include in the 2026-2027 Northern Hemisphere vaccine, here we perform high-throughput sequencing-based neutralization assays using a library of 57 H3N2 and 34 H1N1 influenza hemagglutinins reflecting the circulating diversity of strains in late 2025 to early 2026. We assay this library against 302 human sera collected in late 2025.

The resulting data set encompasses 27,409 titers, and provides a near real-time portrait of the human neutralizing antibody landscape against influenza virus.

We find that many human sera have lower titers against the K subclade of H3N2 and the D.3.1.1 subclade of H1N1; these subclades have recently become dominant among their respective subtypes. Our measurements also reveal variability in titers to different subvariants within the K subclade of H3N2, with titers especially low to subclade K strains with additional mutations in antigenic regions D and E.

We make all our data and accompanying visualizations publicly available to enable their use in vaccine-strain selection and analyses of influenza evolution and immunity.

       (SNIP)

Discussion

Here we have used high-throughput sequencing-based neutralization assays to measure the titers of a diverse set of human sera against a large set of HAs representing H3N2 and H1N1 influenza strains circulating as of early 2026. For both of these subtypes, new subclades (K for H3N2 and D.3.1.1 for H1N1) have become dominant in the six months since the 2026 Southern Hemisphere vaccine strains were selected.

Our measurements show that these new subclades are generally more poorly neutralized by human serum antibodies than other strains, likely explaining their rise. Importantly, we also find that among subclade K H3N2 strains, those with mutations in antigenic regions D and E have especially low titers, raising the possibility that such strains could increase in frequency over the coming year. 

In addition to these major trends, our data also shows additional fine-grain variation in titers to strains within the same subclades, and extensive heterogeneity in titers across sera, some of which partially stratifies with age group. How to best account for this additional heterogeneity in forecasting evolution and choosing vaccine strains remains an open question. 

By making this large dataset immediately available for analysis, we therefore hope both to inform vaccine-strain selection for the 2026-2027 season as well as spur further studies of how to leverage large and near-real-time neutralization data15 to advance public health by improving understanding and forecasting influenza evolution.

        (Continue . . . )

 
We'll get the WHO's decision late this week, or early next, on which strains to include in next fall's shot.  The FDA will make their own recommendations for U.S. manufacturers, likely in early March. 

Historically, H3N2 has been much harder to predict than H1N1 (see The Enigmatic, Problematic H3N2 Influenza Virus). It has been around since 1968, is more mutable, and it tends to maintain a far more diverse group of co-circulating strains. 

But even when flu vaccines are a mismatch, they can often provide modest protection against the mutated strain; at least in reducing severe illness and hospitalization (see UKHSA Preprint: Early Influenza Virus Characterisation and Vaccine Effectiveness in England in Autumn 2025, A Period Dominated by Influenza A(H3N2) Subclade K).

But of course, that only helps if you get one. 

Preprint: Hematogenous Neuroinvasion and Genotype-dependent Transmission of Influenza A H5N1 Viruses in the Cat Host

 

Based on USDA Data

#19,063

Based on limited - and admittedly opportunistic - surveillance, felines (domestic cats, mountain lions, bobcat, etc.) are (excluding dairy cattle) the non-human mammalian species which are most often reported to be infected with HPAI H5N1.

How many fox, skunk, or mice that are actually infected in the wild is unknown. But cats have a known history of susceptibility to the virus (see 2023's A Brief History Of Avian Influenza In Cats).

Over the years we've seen plenty of evidence of Differences In Virulence Between Closely Related H5N1 Strains, with stark differences in mortality rates reported by countries around the world (see chart below).

While there are scores of H5N1 genotypes currently in circulation (and different subclades in India and Cambodia), among clade 2.3.4.4b viruses, two genotypes currently stand out in North America; `Bovine' B3.13 and avian D1.1.

Both have been reported in poultry, dairy cattle, and humans, but the B3.13 appears to be more at home in dairy cattle, while D1.1 is particularly prevalent in wild birds and poultry. 

B3.13 appears to be milder in humans than D1.1, although that is based on a relatively small sample. Neither genotype approach the virulence, however, of many older H5N1 strains or the current clade 2.3.2.1e in Cambodia.

But H5Nx viruses continues to evolve, and the threats we may face tomorrow may be far different.

Today we've a preprint from researchers at Cornell University in NY state, which looks at the course of infection, virulence, and transmissibility of SPF (specific pathogen-free) lab cats when infected with B3.13 and D1.1 genotypes of  H5N1. 

Notably, while we've seen prior evidence suggesting that genotype D1.1 is more problematic in humans (see J.I.D.: Avian influenza virus A(H5N1) genotype D1.1 is better adapted to human nasal and airway organoids than genotype B3.13), this report finds the opposite seems to be true in cats. 

B3.13 infection produced faster disease progression, greater shedding, higher mortality, and efficient (75%) direct contact transmission. Whereas D1.1 infected cats saw slower progression, lower mortality, and no evidence of direct contact transmission. 

Of note: only two isolates were tested; B3.13 from bovine (TX224) with PB2 M631L mammalian adaptation, and D1.1 from a strictly avian source (NY314925).

Given the diversity of viruses within each genotype, there are limits to how much we can assume based on this study.  A D1.1 virus isolated from a mammalian source might well have performed differently. 

Nevertheless, this is a fascinating, and highly detailed, look at the pathogenesis of two frontrunning H5N1 genotypes in cats, which suggests that B3.13 is better adapted to the feline host. 

Due to its length, and complexity, I've only posted the abstract and a brief excerpt from the preprint. Follow the link to read it in its entirety. 

Hematogenous neuroinvasion and genotype-dependent transmission of influenza A H5N1 viruses in the cat host

Salman L. Butt, Ruchi Rani, Mohammed Nooruzzaman, Elena A. Demeter, Pablo S.B. de Oliveira, Gavin R. Hitchener, Diego G. Diel
doi: https://doi.org/10.64898/2026.02.21.707182
This article is a preprint and has not been certified by peer review 

Preview PDF

Abstract

The spillover of highly pathogenic avian influenza (HPAI) A H5N1 virus to mammalian hosts raises major concerns due to its pandemic potential. Cats are frequently affected mammals, often succumbing to systemic and neurological disease. 

Here, we characterized the pathogenesis and transmissibility of two H5N1 genotypes, B3.13 and D1.1, in cats. Infected cats exhibited high-level viremia and virus shedding in nasal, oral, and fecal secretions were consistently detected.

The virus replicated initially in the upper respiratory tract and lungs, followed by systemic dissemination and neuroinvasion. Notably, the virus crossed the blood-brain-barrier by infecting endothelial cells, spreading to astrocytes and neurons, causing multifocal encephalitis.

D1.1-virus infection caused protracted disease with lower shedding and no transmissibility, whereas B3.13 virus caused rapid onset with efficient shedding and transmission. 

These findings reveal critical H5N1 neuropathogenesis mechanisms and highlight mammalian transmission potential in a species with close human contact.

(SNIP)

The primary concern regarding the rapid genetic diversification observed in H5N viruses is the potential emergence of variants with enhanced transmissibility or pathogenicity. Epidemiological evidence indicates that both H5N1 genotypes B3.13 and D1.1 exhibit improved fitness when compared to their ancestral strains.

The B3.13 genotype successfully spilled over into dairy cattle, establishing efficient transmission chains within this novel host species and subsequently spreading to other mammalian- (including humans and cats) and avian hosts.

Similarly, the D1.1 genotype emerged in wild birds, causing the largest infection wave since the introduction of H5N1 in North America. This genotype also crossed the species barrier, jumping from wild birds to dairy cows in three independent spillover events across Nevada, Arizona, and Wisconsin.

       (SNIP)

Collectively, our results suggest that differences in the polymerase activities between B3.13 (TX2/24) and D1.1 (NY3149/25) viruses may contribute to their distinct abilities to  replicate and transmit in the cat host. 

Further investigations are needed to determine whether both viruses replicate to similar levels in avian hosts in vivo. Our study revealed critical aspects of H5N1 virus pathogenesis in a naturally susceptible animal species, emphasizing the risk posed by B3.13 viruses currently circulating in dairy cattle in the U.S. and frequently transmitted to cats. 

       (Continue . . . )