CDC reports that between January 2022 and April 2024 there have been 26 cases of HPAI (highly pathogenic avian influenza) H5N1 reported worldwide, 14 severe/critical with 7 deaths.

https://www.cdc.gov/flu/avianflu/spotlights/2023-2024/h5n1-technical-report_april-2024.htm

That’s a case fatality rate of 26%. The US case reported in April was mild in severity. Initial verbal investigation at farms with infected herds have suggested that many more human cases within the US have already occurred, and that they’ve likely been mild to moderate in severity. With widespread testing, we’re likely to see more mild/asymptomatic cases which will bring down the case fatality rate. Case fatality rate estimates for prior influenza pandemics vary, but are typically less than 3% even for the 1918 avian influenza pandemic. More recent estimates vary, with the Ontario health department suggesting an HPAI H5N1 pandemic will have a higher mortality rate of 14-33%.

https://pubmed.ncbi.nlm.nih.gov/18477756/

The WHO’s mortality rate estimate is 60%.

https://www.who.int/news-room/questions-and-answers/item/influenza-h5n1

I think these estimates are too high; like with COVID-19 when initial reports suggested 15-20% mortality rates, but it was ultimately adjusted down to around 0.5% as we learned more about the disease. It may have also been that its mortality decreased a bit from the initial strain that was prevalent in Wuhan. This could also end up being the case with H5N1; that a less lethal but more virulent disease may ultimately be the strain that’s most widely propagated.

Specifically, I am referring to the USA as having a case fatality rate of <5% in the event of an H5N1 pandemic. The case fatality rate is likely to be high initially, but will likely resolve lower as testing increases. This question will resolve no if an H5N1 pandemic does not occur before 2026, or if the case fatality rate is >5% in the USA 12 months after a pandemic has been declared.