(H1N1) is relatively slow since the structure of the 2009 H1N1 virus is similar to the strain of H1N1 implicated in the 1918 flu pandemic. A study from Hokkaido University found a sequence homology between the Hemagglutinin antigen amino acid residues found in the earlier 1918 strain and the 2009 H1N1 strain. This may have played a role in individuals who had been infected with the 1918 strain and its early descendants in showing stronger specific immunity to the 2009 H1N1 virus. This finding provided insight into future monitoring of the H1N1 virus and its evolution within the human population. Mutation On November 20, 2013, the Norwegian Institute of Public Health released a statement saying they had discovered a potentially significant mutation in the H1N1 influenza strain that could be responsible for causing the severest symptoms among those infected. In the statement they said "The mutation could be affecting the virus' ability to go deeper into the respiratory system, thus causing more serious illness". The World Health Organization said the mutation did not appear to be widespread in Norway and the virus in its mutated form remained sensitive to antivirals and pandemic vaccines. A similar mutation had been detected in H1N1 viruses circulating in several other countries, including China and the United States, in severe as well as in some mild cases. "Although further investigation is under way, no evidence currently suggests that these mutations are leading to an unusual increase in the number of H1N1 infections or a greater number of severe or fatal cases." On December 2, 2009, the WHO announced that they had been informed of two recent clusters of patients infected with oseltamivir-resistant H1N1 viruses. Both clusters, detected in the UK (including Wales) and in North Carolina, occurred in a single ward and involved patients whose immune systems were severely compromised or suppressed. Transmission of resistant virus from one patient to another is suspected in both outbreaks. Resistance , the WHO reported 314 samples of 2009 pandemic H1N1 flu tested worldwide have shown resistance to oseltamivir (Tamiflu). This was not totally unexpected as 99.6% of the seasonal H1N1 flu strains tested have developed resistance to amantadine and rimantadine. no circulating flu had yet shown any resistance to zanamivir (Relenza), the other available anti-viral. Species affected Swine Before being transmitted to humans, a H1N1 type virus is known to have circulated in swine. In August 2007, about