It is known to circulate among pig populations since then.The H1N1 swine flu viruses are antigenically different from human H1N1 viruses and swine flu viruses do not normally infect humans.

• 猪只的甲型流感H1N1型跟人类的甲型流感H1N1型在抗原上是非常不同的，因此在一般情况下猪型流感是不会感染人类。