Lee says that one possible explanation is that the Spanish Flu virus, H1N1, has lain low since its last appearance in 1957, so about a third of those born before 1957 have been infected by H1N1. Thus their immune systems still have some capacity to identify this virus. In contrast, those under 50, who lack the antibody, are at elevated risk.
李秉穎醫師指出，一個可能的解釋是，西班牙流感病毒 H1N1 在 1957 年最後一次出現後即銷聲匿跡，而 1957 年之前出生的人，約有三分之一曾感染 H1N1，其免疫系統仍可依稀「辨識」這株病毒；相對地，50 歲以下不具抗體的人，自然風險較高。