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FluView Week 5: Influenza Remains Elevated & Slight Uptick In Antiviral Resistance
CDC FluView Report Week 5
#19,053
For the 9th consecutive week, influenza remains above the National Baseline, and with the recent rise in influenza B cases, has spiked upwards again slightly (see FluView chart above).
While obviously the biggest story, buried deep in the narrative we also see a small increase (see chart below) in viruses showing signs of (mostly) moderate antiviral resistance.
Although the numbers remain reassuringly low, they are worth noting, as we've been following reports of creeping antiviral resistance around the globe for the past 2 years, primarily in seasonal H1N1 (and occasionallyHPAI H5).
This is in contrast to the week 53 FluView report which showed zero resistance (among 193 viruses tested) between Sept 28th and the end of December.
Since then an additional 233 H1N1 viruses have been tested, and 10 (4.29%) have shown varying levels of oseltamivir resistance. The summary from this week's FluView reads:
Three A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring highly reduced inhibition by oseltamivir and peramivir.
Seven A(H1N1)pdm09 viruses had amino acid substitutions NA-I223V and NA-S247N and showed reduced inhibition by oseltamivir.
While none of these numbers are alarming, they are elevated, and they come on the heels of increasing reports of growing resistance being reported around the globe.
A brief recap:
In March of 2024 we saw a worrisome report in The Lancet - Global Emergence of Neuraminidase Inhibitor-Resistant Influenza A(H1N1)pdm09 Viruses with I223V and S247N Mutations - which reported a much higher incidence of oseltamivir resistance among samples tested in Hong Kong in 2023.
Unlike the near total collapse of oseltamivir effectiveness in 2008 - which was due toan H275Y mutation in the NA - this was due to I223V and S247N, and was not as profound of a loss in effectiveness.
Since then we've seen a number of studies on the rise of NAI resistance around the globe, including 2024's Viruses: Increase of Synergistic Secondary Antiviral Mutations in the Evolution of A(H1N1)pdm09 Influenza Virus Neuraminidases, where the authors warned:
It seems likely that the viruses have reached the next stage in the evolution of prerequisite viruses that enable the emergence and spread of stable lineages of resistant viruses. . .
While most of these increases have been modest, just over 4 months ago Taiwan's CDC reported that 6.5% of the H1N1 viruses they have characterized in 2025 have shown signs of oseltamivir resistance.
And China's Influenza Surveillance Weekly Report has been reporting roughly 4% (range 3.8%-4.2%) of H1N1 viruses tested have shown signs of resistance since last summer (translation of most recent report follows).
The National Influenza Center conducted drug resistance analysis on some influenza virus strains collected since March 31 , 2025. The results showed that: 3.9% ( 32/829 ) of the A(H1N1)pdm09 subtype influenza virus strains showed reduced or highly reduced sensitivity to neuraminidase inhibitors.
Many of these reports have been vague, and have not specified the degree of reduced susceptibility to oseltamivir, making their clinical significance difficult to ascertain.
Somewhat more concerning, last December, in Eurosurveillance: Expansion of influenza A(H1N1)pdm09 NA:S247N Viruses with Reduced Susceptibility to Oseltamivir, Catalonia, Spain, and in Europe, July to October 2025, we looked at the abrupt rise in resistance in Catalonia, Spain over the summer and fall.
Based on limited testing, over a 3 month period 45% of H1N1 viruses showed signs of reduced susceptibility to oseltamivir, with 2 weeks coming in at 100%.
For now, these resistance reports have turned up primarily in seasonal H1N1, although we've seen some scattered reports in HPAI H5 (Emerg. Microbes & Inf: Oseltamivir Resistant H5N1 (Genotype D1.1) found On 8 Canadian Poultry Farms).
And eleven months ago St. Jude Researchers warned: Current Antivirals Likely Less Effective Against Severe Infection Caused by Bird Flu in Cows’ Milk.
Although there are alternatives to oseltamivir for the treatment of influenza A (e.g. Baloxavir) - with the exception of Japan - oseltamivir remains the most widely available (and affordable) antiviral stockpiled today.
Twice in the past two decades we've lost our primary influenza antiviral due to rising resistance; Amantadine in 2006 and (temporarily) Oseltamivir in 2008 (note: Oseltamivir was unexpectedly resurrected with the arrival of a susceptible pandemic H1N1 strain in April 2009).
While we are only seeing sporadic hints of increased resistance - and this trend could always fizzle - the reality is most antibiotics, antifungals, and antivirals enjoy a limited lifespan.
Nature eventually figures out a way around them, making it important that we take note of findings like those listed above, and plan accordingly.
CDC FluView Report Week 5
#19,053
For the 9th consecutive week, influenza remains above the National Baseline, and with the recent rise in influenza B cases, has spiked upwards again slightly (see FluView chart above).
While obviously the biggest story, buried deep in the narrative we also see a small increase (see chart below) in viruses showing signs of (mostly) moderate antiviral resistance.
Although the numbers remain reassuringly low, they are worth noting, as we've been following reports of creeping antiviral resistance around the globe for the past 2 years, primarily in seasonal H1N1 (and occasionallyHPAI H5).
This is in contrast to the week 53 FluView report which showed zero resistance (among 193 viruses tested) between Sept 28th and the end of December.
Since then an additional 233 H1N1 viruses have been tested, and 10 (4.29%) have shown varying levels of oseltamivir resistance. The summary from this week's FluView reads:
Three A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring highly reduced inhibition by oseltamivir and peramivir.
Seven A(H1N1)pdm09 viruses had amino acid substitutions NA-I223V and NA-S247N and showed reduced inhibition by oseltamivir.
Two B viruses had amino acid substitution NA- M464T and showed reduced inhibition by peramivir.
High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented.
High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented.
While none of these numbers are alarming, they are elevated, and they come on the heels of increasing reports of growing resistance being reported around the globe.
A brief recap:
In March of 2024 we saw a worrisome report in The Lancet - Global Emergence of Neuraminidase Inhibitor-Resistant Influenza A(H1N1)pdm09 Viruses with I223V and S247N Mutations - which reported a much higher incidence of oseltamivir resistance among samples tested in Hong Kong in 2023.
Unlike the near total collapse of oseltamivir effectiveness in 2008 - which was due toan H275Y mutation in the NA - this was due to I223V and S247N, and was not as profound of a loss in effectiveness.
Since then we've seen a number of studies on the rise of NAI resistance around the globe, including 2024's Viruses: Increase of Synergistic Secondary Antiviral Mutations in the Evolution of A(H1N1)pdm09 Influenza Virus Neuraminidases, where the authors warned:
It seems likely that the viruses have reached the next stage in the evolution of prerequisite viruses that enable the emergence and spread of stable lineages of resistant viruses. . .
While most of these increases have been modest, just over 4 months ago Taiwan's CDC reported that 6.5% of the H1N1 viruses they have characterized in 2025 have shown signs of oseltamivir resistance.
And China's Influenza Surveillance Weekly Report has been reporting roughly 4% (range 3.8%-4.2%) of H1N1 viruses tested have shown signs of resistance since last summer (translation of most recent report follows).
The National Influenza Center conducted drug resistance analysis on some influenza virus strains collected since March 31 , 2025. The results showed that: 3.9% ( 32/829 ) of the A(H1N1)pdm09 subtype influenza virus strains showed reduced or highly reduced sensitivity to neuraminidase inhibitors.
Many of these reports have been vague, and have not specified the degree of reduced susceptibility to oseltamivir, making their clinical significance difficult to ascertain.
Somewhat more concerning, last December, in Eurosurveillance: Expansion of influenza A(H1N1)pdm09 NA:S247N Viruses with Reduced Susceptibility to Oseltamivir, Catalonia, Spain, and in Europe, July to October 2025, we looked at the abrupt rise in resistance in Catalonia, Spain over the summer and fall.
Based on limited testing, over a 3 month period 45% of H1N1 viruses showed signs of reduced susceptibility to oseltamivir, with 2 weeks coming in at 100%.
For now, these resistance reports have turned up primarily in seasonal H1N1, although we've seen some scattered reports in HPAI H5 (Emerg. Microbes & Inf: Oseltamivir Resistant H5N1 (Genotype D1.1) found On 8 Canadian Poultry Farms).
The CDC also reported finding the above mentioned NA-S247N mutation in 3 HPAI H5 infected poultry workers from Washington State, which they stated may slightly reduce the virus's susceptibility to antivirals.
And eleven months ago St. Jude Researchers warned: Current Antivirals Likely Less Effective Against Severe Infection Caused by Bird Flu in Cows’ Milk.
Although there are alternatives to oseltamivir for the treatment of influenza A (e.g. Baloxavir) - with the exception of Japan - oseltamivir remains the most widely available (and affordable) antiviral stockpiled today.
Twice in the past two decades we've lost our primary influenza antiviral due to rising resistance; Amantadine in 2006 and (temporarily) Oseltamivir in 2008 (note: Oseltamivir was unexpectedly resurrected with the arrival of a susceptible pandemic H1N1 strain in April 2009).
While we are only seeing sporadic hints of increased resistance - and this trend could always fizzle - the reality is most antibiotics, antifungals, and antivirals enjoy a limited lifespan.
Nature eventually figures out a way around them, making it important that we take note of findings like those listed above, and plan accordingly.
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