Make no mistake though that while they have some similarities they are NOT the same virus at all...
This is a very interesting article.
Quote from www.scientificamerican.com
Researchers in both of the new studies found that the 1918 flu (also an H1N1 strain) and the 2009 flu had so-called "bald head" hemagglutinin that weren't covered by sugars, but were instead nearly identical and easily recognized by an immune system that had been exposed to either. In fact, in the Science Translational Medicine study led by Chih-Jen Wei, of the National Institute of Allergy and Infectious Diseases (NIAID) at the National Institutes of Health, a team of researchers vaccinated groups of mice against either the 1918 strain or 2009 strain and then exposed each vaccinated mouse to the other virus. It turned out that exposure to either conferred protection against both, the researchers concluded.
This physical similarity would better explain why the 2009 pandemic flu proved to be more infectious—and deadly—to people under the age of 65. Those who were born in the early 20th century likely came into contact with either the pandemic strains of the 1918 virus or later seasonally circulating versions of it, and thus their immune systems recognized the 2009 H1N1 strain.
The reason H1N1 was able to return to the human population as a pandemic flu—even after it had been circulating several decades ago—has a lot to do with the pattern of virus evolution in animals, explained a team of researchers for Novartis Vaccines and Diagnostics led by Rino Rappuoli in an essay that accompanied the two Science studies. Animals such as pigs and birds, which harbor many diseases that have been known to jump to humans, put little evolutionary pressure on viruses to change because the animals themselves have relatively short lives. Thus each individual animal is unlikely to develop immunity to many viruses—unlike humans, who carry immunity for decades, forcing viruses to mutate in order to continue being infectious. "Birds and pigs harbor an archive of well-preserved antigenic types," the Novartis researchers wrote, which means that as overall human immunity to a strain still circulating in animals decreases, it is more likely to reemerge in humans.
...the 2009 H1N1 pandemic virus is likely to become resistant to the vaccine developed and distributed for this flu by the very difference Wei et al. observed in other strains: by developing sugar coating on the previously bare hemagglutinin sites, and thus becoming a circulating seasonal flu strain.