It's not about smoldering or neuraminidase: there were 2 variants of the A(H3N2) pandemic virus differing in internal genes.
Reichert
Thomas A
TA
Christensen
Ronald A
RA
D001315:Australia; D004196:Disease Outbreaks; D005060:Europe; D006801:Humans; D053121:Influenza A Virus, H2N2 Subtype; D053122:Influenza A Virus, H3N2 Subtype / Q000145:classification* / Q000235:genetics*; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000821:virology; D009439:Neuraminidase / Q000145:classification / Q000235:genetics; D009656:North America
D009439:Neuraminidase