Post Reply - H5N1/flu (other types)

and often killing,

   DJ, Good info on earlier flu infection and how that may work out if one catches H5N1;https://afludiary.blogspot.com/2024/11/preprint-immune-history-shapes-human.html or https://afludiary.blogspot.com/2024/11/preprint-immune-history-shapes-human.html ;Nearly 18 years ago, in a blog called A Predilection For The Young,  I wrote about H5N1's affinity for infecting, and often killing, younger adults, adolescents, and children.  The WHO chart (above) illustrates that pattern with disturbing clarity.
Six years later we saw the opposite trend with avian H7N9 in China, which skewed heavily toward older adults (see H7N9: The Riddle Of The Ages).
The concern here is that an H5N1 pandemic might be far more impactful for young adults and children than for those, say . . . over 50.  While we think of flu taking its biggest toll on the elderly, we've seen 3 flu pandemics in relatively recent history where the burden shifted to a younger cohort.
The 1918 pandemic showed a unique W-Shaped Curve (see below), where young adults (25-30) were particularly hard hit, while mortality rate actually dropped in those over the age of 60.
In 1978, H1N1 returned after an absence of 20 years (potentially from a lab leak in Russia or China) sparking a pseudo-pandemic (aka `the Russian Flu') that primarily affected those under the age of 20 (see mBio: The Reemergence Of H1N1 in 1977 and The GOF Debate).
While the average (mean) age of a flu-related fatality in a `normal’ flu season here in the United States is about 76 years, the average during the (relatively mild) 2009 H1N1 pandemic was half that; at 37.4 years (see Study: Years Of Life Lost Due To 2009 Pandemic).
Over the past decade we've seen a lot work on the idea that the first flu HA type (Group 1 or 2) you are exposed to can shape your immune response to influenza for the rest of your life through a process called Original Antigenic Sin (OAS) (see PLoS Path.: Childhood Immune Imprinting to Influenza A).
Those born prior to the mid-1960s were almost certainly first exposed to Group 1 flu viruses (H1N1 or H2N2)
Those born after 1968 and before 1977 would have been exposed to Group 2 (H3N2) 
After 1977, both Group 1 and 2 viruses co-circulated, meaning the first exposure could have been to either one. 
All of which brings us to a new preprint, which suggests that younger people may be more heavily impacted by an H5 virus, and that they may benefit more from a pandemic vaccine than adults. 
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Although recent reports of `mild' H5N1 infections in American farm workers are somewhat reassuring, there are no guarantees how long that will persist. 
The virus continues to evolve, and in Cambodia H5N1 has killed roughly 1/3rd of those that it has hospitalized over the past 2 years, and in China, H5N6 has a reported fatality rate of 50%. 
While it is widely assumed that some (perhaps large) number of milder cases go unreported, and the `real world' CFR (Case Fatality Rate) is likely far lower, even a 1%-2% fatality rate would have a devastating impact. 
Any `protection' provided by past exposure to HA Group 1 viruses is likely to be limited, and any advantage to those born before 1968 may be offset by other age-related comorbidities. Which makes the rapid development of a safe and effective H5 vaccine a high priority. 
But, as we saw last week in SCI AM - A Bird Flu Vaccine Might Come Too Late to Save Us from H5N1 - there are many obstacles to overcome - including the growing public distrust of vaccines. 
Given the stakes, it is still worth doing everything we can to keep the H5N1 virus from becoming a pandemic.  
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https://afludiary.blogspot.com/2024/11/review-article-influenza-viruses-in.html or https://afludiary.blogspot.com/2024/11/review-article-influenza-viruses-in.html ;
Given the news last week that a pig in Oregon tested positive for the H5N1 virus, the following review article - recently published in the Journal Viruses on the ecology and distribution of influenza A viruses in swine - is especially timely.   
Swine (and Equines) are the only hosts known to be susceptible to all 4 types of influenza (A, B, C & D), and reassortment in swine has been linked to the generation of previous pandemic viruses. 
DJ, Following human reported US cases of H5N1 is getting harder...https://afludiary.blogspot.com/2024/11/cdc-h5-updates-current-situation.html or https://afludiary.blogspot.com/2024/11/cdc-h5-updates-current-situation.html ;
Late yesterday the CDC published two updates (here and here) on the current situation of H5 in the United States and their ongoing response.  There are some differences between the number of human cases confirmed by the CDC (n=44) and what has been reported by states (at least 48), but both numbers continue to rise. 
DJ, Even in a very optimistic "view"-1 confirmed case may have only one unconfirmed human H5N1 case...the US is moving to 100 human cases of H5N1 human infections...Reality is worse...with increasing chances of all kinds of mixing of flu-types in humans/mammals and co-infections "Flu-Rona"etc...