Whether a similar difference in continent-wide susceptibility explains why swine flu is spreading in the United States but standing still in Europe will take a while to figure out.

• 人们还需要进一步研究来证实，猪流感在在美国蔓延而在欧洲并未引起流行这一状况是否就是因为与之类似的洲际易感性的差异。