This year's seasonal flu vaccine does not protect against the new virus, known as H3N2v, which appears for now to spread from pigs to people but not easily from humans to humans.
"What we're recommending is that people avoid exposure to swine in places like fairs, especially if they're under 5, over 65, or have other risks, like pregnancy, diabetes or asthma," said Michael Shaw, a flu expert with the Centers for Disease Control and Prevention in Atlanta.
"Going by what we're seeing in the southern hemisphere, there were some localized outbreaks, but it wasn't an unusual season," he added. "But then, flu always has a way of surprising you."
What most people call "the flu" refers to a set of unpleasant symptoms -- fever, sore throat, cough, stuffy nose and muscle aches -- that are far more severe than the common cold. More than 200,000 people end up in the hospital with flu-related complications each year, according to the CDC. Between about 3,000 and 49,000 die from it.
Even though two cases of flu can seem similar, there are a variety of influenza viruses that infect the respiratory tract and cause the flu.