Past exposure to previous seasonal influenza and prior vaccination increases the probability that an individual will have some cross-reactive antibody to a new flu strain. This does have a detectable but relatively minor effect with the current swine origin H1N1 virus because it has acquired several new genes. While some unpublished Canadian data has suggested that individuals with past seasonal influenza vaccination may have been more likely to have been diagnosed with swine origin H1N1 in the spring of 2009, this observation has not been observed in other countries. It’s currently believed to be a spurious finding that’s being investigated further.