This free, full commentary in the journal explains that there are different strains of this influenza.

Correctly defined, what we have is,  “One group of viruses, described by Shinde et al.,¹ are triple reassortants of viruses from pigs, humans, and birds, called triple-reassortant swine influenza A (H1) viruses, which have circulated in pigs for more than a decade. The other group, described by the Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team,² is a recent reassortant of the triple-reassortant swine influenza A (H1) viruses and a Eurasian swine influenza virus, resulting in the swine-origin influenza A (H1N1) virus (S-OIV), currently being transmitted among humans. (Emphasis added)

New England Journal of Medicine

…“The current situation is not “1918 again,” it is “1918 continued,” in that we are still being infected with remnants of the 1918 pandemic influenza virus. Most adults have substantial immunity to H1 variants that have circulated among humans from 1918 through 1957 and then again from 1977 through the present.”

…”The emergence and spread of S-OIV brings out the best and worst of contemporary society. Within days after the first case identification of S-OIV infection in the United States, the scientific community had the complete genetic sequence of the hemagglutinin, and Internet dissemination of the information made it available to everyone for further analysis. Public health and surveillance activities were heightened. Drug susceptibility was determined: S-OIV is susceptible to oseltamivir and zanamivir but resistant to amantadine and rimantadine.

“In contrast, inflammatory political posturing has occurred that illustrates the need for effective communication by physicians and scientists to the public. Some have asked, Why didn’t you close the borders? (Answer: It doesn’t work.) Misguided culling of pigs has also been carried out in one country, but S-OIV is not epidemic in pigs; people are spreading the virus.

“Finally, funding for surveillance, public health efforts, and vaccine development needs to be enhanced. The current S-OIV epidemic is only the latest influenza virus, not the last.” (Emphasis added)

Citation: New England Journal of Medicine, June 18, 2009.Volume 360:2667-2668. Number 25.

Volume 360:2667-2668 June 18, 2009