We read with great interest the recent letter by Li and colleagues reporting the emergence of mammalian-adapted clade 2.3.4.4b H5N1 viruses carrying the PB2-E627K mutation in migratory birds at Qinghai Lake. The identification of this canonical mammalian-adaptation marker in avian reservoirs has further intensified concerns regarding progressive viral adaptation and the potential for increased human-to-human transmission, thereby elevating pandemic risk.
The expanding host range of clade 2.3.4.4b viruses, now including poultry, dairy cattle and other mammals since early 2024, underscores their dynamic evolution and increasing interface with human populations, providing repeated opportunities for cross-species transmission and human exposure. Although most human infections have been mild, severe and fatal cases have been reported,2 underscoring the need for refined, functionally informed risk assessment approaches. In mammalian isolates, adaptive substitutions in the viral polymerase, most prominently PB2-E627K but also alternative changes such as PB2-M631L, have been associated with enhanced replication efficiency and pathogenicity. Experimental infection studies in nonhuman primates further demonstrate that clade 2.3.4.4b viruses can cause severe and sometimes lethal respiratory disease. However, whether currently circulating clade 2.3.4.4b viruses require the acquisition of PB2-E627K to exhibit substantial pathogenicity in human respiratory tissues remains unresolved.