Has Influenza A(H1N1) mutated (again)?

At first no one was worried when a number people in Montgomery County (Texas, USA) started to get seriously ill with an unidentified flu-like illness, but then they were dying and everything changed. At the moment (December 20th, 2013), six people have died and 14 are critically ill. And these numbers are likely to rise.

Reports suggest that these patients are infected the Influenza A(H1N1) virus. However, there is a problem: the majority of the cases are not in the typical risk groups. They are adults between the ages of 20 to 60 with no other underlying health concerns.

[Influenza A(N1H1) evolution]

One would suspect that this category of patients would have some earlier contact with the virus and - as a result – would have acquired some level of immunity. Even when these previously healthy adults would never had any exposure with the Influenza A(H1N1) virus, it was expected that they wouldn’t become as ill.

The Influenza A(H1N1) virus is prone to mutations and has already changed several times since its first appearance in 2009[1]. Has it mutated again?

[1] Anderson et al: Population dynamics of cocirculating swine influenza A viruses in the United States from 2009 to 2012 in Influenza and Other Respiratory Illnesses – 2013.