Immunity and epidemiology:

The HIN1 variant entered the UK in elite dutch pigs. The pigs had appeared negative by MAFF HI tests because the H1 virus was different to the H1 test virus. The virus recycles on a farm because litters of pigs become susceptible once their maternal antibodies have waned.

Pigs are also the source of new reassorted mammalian subtypes because they occasionally become co-infected with avian and human viruses at the same time. This occurs in China. Pigs are also believed to have started the massive flu pandemic in Nov 1918. Frozen mass graves in Alaska have been used to obtain these 1918 isolates or their RNA to investigate this hypothesis.