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H1N1 is closing schools in places like Texas and Virginia. But other regions are still waiting for the virus to show up again.

"We're not seeing H1N1 influenza in New York City right now," said Dr. Thomas Farley, the New York City health commissioner.

CDC data shows last spring, New York and New Jersey were especially hard hit but have been relatively spared this fall. The opposite is true of the Southeast, which saw fewer cases last spring but is now seeing an H1N1 surge, reports CBS News medical correspondent Dr. Jon LaPook.

"One of the current ideas is that that wave of this virus last spring has left a number of people protected who are immune from the virus," said Dr. Martin Blaser, the chair of medicine at NYU Langone Medical Center.

This protective effect is called herd immunity.

When enough people are immune, either through previous infection or vaccination, virus does not spread easily.

"In places like New York and the Northeast, there's probably a substantial amount of herd immunity right now, which is dampening the spread of the virus," Blaser said. "It doesn't mean it's not going to appear. It just means that it's going to appear later, and maybe not quite as bad as it is in the Southeast right now."

Experts say the best way to increase herd immunity is vaccination. The CDC says that even if you think you had H1N1 last season, you cannot be absolutely sure unless you were specifically tested and most people were not. The CDC recommends getting the vaccine if you're in a subgroup for which it's advised.