Influenza viruses are constantly mutating. When large numbers of the hosts become immune to a particular strain either through exposure or vaccination, selection favors the spread of a new strain. This new strain may or may not be more lethal than the one it replaces.
There is some evidence suggesting that a new strain of pandemic H1N1 may be spreading. There is limited evidence suggesting that it may be more lethal than the current dominant strain.
From Barr et al. (2010) [hat-tip, Pixie]:
… a genetically distinct variant containing several signature amino acid changes in both the HA and NA genes has emerged in Singapore, Australia and New Zealand, coinciding with the winter influenza season in the latter two countries.
It remains to be seen whether this variant will continue to predominate for the rest of the influenza season in Oceania and in other parts of the southern hemisphere and then spread to the northern hemisphere or merely die out. Already this variant virus has been associated with several vaccine breakthroughs in teenagers and adults vaccinated in 2010 with monovalent pandemic influenza vaccine as well as a number of fatal cases from whom the variant virus was isolated.
There is enough data presented in this paper to warrent steps to produce a vaccine against this new strain. Public health authorities would be ill-advised to wait for guidance from the WHO on this issue.
Barr et al. (2010) A new pandemic influenza A(H1N1) genetic variant predominated in the winter 2010 influenza season in Australia, New Zealand and Singapore. Eurosurveillance, 15: 21 October 2010