This is exactly right. Most of the population, particularly in America, have at least partial immunity to the "standard" flu virus from year to year due to immunizations and previous exposures to similar strains. This virus is a completely new strain that has never before been seen by the human population, so essentially no one is immune to it. There was some concern that this strain may, in fact, be more virulent than the standard flu (such as the 1918 and bird flu strains were), but those concerns are, as of yet, unconfirmed. That's not to say the possibility doesn't still exist though. As I understand it, the very fact that the virus jumped from swine to human shows that it has quite the ability to mutate. Meaning that, whatever strain it is now, it could develop a new strain more easily than other influenzas. Meaning you could develop the antibodies for one, only to be attacked by the next in a relatively short time. They don't know if this will actually happen or not, but the behavior of the virus suggests it as a possibility. I think this is the main concern for it becoming a pandemic. At least, that's how my physician friend explained it to me.