Well I am not sure how big a sample group they had but it seems that the survivors of the Spanish flu of 1918 are immune to this years Swine Flu Pandemic.
University of Wisconsin researcher Yoshihiro Kawaok has discovered that survivors of the 1918 flu pandemic are immune to the current swine flu pandemic, although they are not necessarily immune to seasonal flu (Associated Press).
This seems to confirm further that the 1918 pandemic and the current swine flu are both anomalies; they are more similar in presentation to each other than either one is to seasonal flu.
Kawaok also discovered that in mice, ferrets and monkeys, the current swine flu multiplies more severely in the respiratory tract than in the head and sinuses.
So did the Spanish flu.
Unlike modern flu viruses, the Spanish flu attacked relatively young, healthy adults the hardest. The very old and very young (those segments that are typically immune compromised) were not nearly as affected.
In a cruel sort of irony, the individuals with the healthiest immune systems were the ones who succumbed to the virus, which hit the patient so virulently, that the immune system in response went into overdrive, causing a cytokine storm.
A cytokine storm occurs when the immune system is so overwhelmed that it sends too many antibodies at once to the infected area of the body. The result is that the immune system itself hastens the death of the patient in these instances (New York Times).
In 1918, many of its victims died within twenty four hours. Most of them coughed up blood that resulted from lung hemorrhaging; some of them literally drowned in their own body fluids.
Victims of the mutated flu also lost control of their bowel function, causing them to not only lose massive amounts of blood, but the entire intestinal lining.
There was not time for the patient to recover from the initial illness before being stricken by a secondary infection. Many of the 1918 victims fell sick one day, then died within twenty four hours; a good number of these victims had contracted pneumonia within one day of coming down with the flu.