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 occasionally HPAI 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.
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.
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. . .
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.
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%.
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.
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.
