Most in U.S. immune to H1N1 influenza

WASHINGTON, Sept. 28 (UPI) -- Most people are immune to the H1N1 virus that caused last season's pandemic, federal health officials say.

"While human influenza viruses have often surprised us, available evidence leads to the hope that the current pandemic virus will continue to cause low or moderate mortality rates if it does not become extinct," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and co-authors, Jeffery Taubenberger and David Morens, say in a statement.

Approximately 59 percent of the U.S. population has some level of immunity to H1N1 influenza -- either by exposure, exposure to a closely related influenza virus or vaccination, the report says.

This number will increase via immunization with this seasonal influenza vaccines, which contains the pH1N1 strain, Fauci and colleagues write in the journal mBio.

"In order to continue to survive in a population with such a high immunity, the pH1N1 virus must undergo either an abrupt or a gradual change," the article says.

Fauci and colleagues looked at the last six influenza pandemics, going back 163 years to see how they adapted. While some viruses died out for reasons not entirely understood, others, like the 1889 and 1918 pandemics, experienced an explosive recurrence.

However, explosive recurrence of pH1N1 is not very likely because of the already high and increasing population immunity.