Post Reply - A new pandemic

(but curious)

   https://www.thailandmedical.news/news/breaking-child-dies-from-h5n1-bird-flu-infection-in-andhra-pradesh-india-sparking-fears-of-possible-surge-in-human-cases or https://www.thailandmedical.news/news/breaking-child-dies-from-h5n1-bird-flu-infection-in-andhra-pradesh-india-sparking-fears-of-possible-surge-in-human-cases 
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https://afludiary.blogspot.com/2025/04/preprint-population-immunity-to-hpai.html or https://afludiary.blogspot.com/2025/04/preprint-population-immunity-to-hpai.html ;
A little over 18 years ago, in A Predilection For The Young, I wrote about the disturbing (but curious) skewing of H5N1 cases (and deaths) among younger individuals (see WHO Chart above). 

While there are no records of humans ever having dealt with an H5 influenza pandemic (going back 130+ years), those who were born before before 1967 appeared far less susceptible to the virus - and those born before 1958 - even more so. 
 A lot of theories were proposed, but answers were elusive. Then, in 2013 an equally novel avian H7N9 virus emerged in China - sparking 5 years of seasonal infections - which skewed dramatically toward those over 40
DJ, The India 2 y/o girl dying from H5N1 had no immunity...A main reason why H5N1 is underreported in humans is lack of testing and lots of "mild" cases...

In 1977-78, the H1N1 seasonal flu virus - which had been absent for 20 years, suddenly appeared in the Far East, and caused a pseudo-pandemic, primarily affecting those born after 1957. 

And during the  2009 H1N1 pandemic, we saw a similar age shift, where people in their 40's were hardest hit.  Here is what the CDC had to say about the impact of the virus in 2012's First Global Estimates of 2009 H1N1 Pandemic Mortality Released by CDC-Led Collaboration.
2009 H1N1 Pandemic Hits the Young Especially Hard

This study estimated that 80% of 2009 H1N1 deaths were in people younger than 65 years of age which differs from typical seasonal influenza epidemics during which 80-90% of deaths are estimated to occur in people 65 years of age and older.
By early in the last decade many researchers were convinced that the first flu you are exposed to early in life `primes' your immune system to preferentially fight similar influenza infections.  
Over time, this theory was refined to say that the HA Group type (I or II) you are exposed to first could substantially affect your immune response to influenza A (see Science: Protection Against Novel Flu Subtypes Via Childhood HA Imprinting).

The idea is that if your first influenza exposure was to H1N1 or H2N2 (Group 1), you may carry some limited degree of immunity to H5 viruses (H5N1, H5N6, etc.), while if your first exposure was to H3N2 (Group 2), you may carry some degree of protection against H7 viruses instead

DJ, Age can offer a + by being exposed to more health risks so more immunity-however age - is immunity decreasing...

Although the oft-quoted 50% CFR (Case Fatality Rate) of H5N1 is probably greatly exaggerated (see discussions here and here), even a more reasonable 2%-5% CFR would represent a public health crisis unlike anything we've seen in the modern era.
Making any advantage - even a small one - very much worth having. 

DJ, Younger age groups may be most at risk...