The days of medical masks at airports and widespread panic may be coming back—that's because at least 12 humans are believed to have been infected with a new strain of swine flu that's not covered by this season's vaccine.

The new swine flu strain, H3N2v, has shown at least some potential for human-to-human transmission in those 12 individuals, which makes it especially dangerous. Between 2009 and mid-2010, more than 17,000 people died worldwide from the highly contagious H1N1 swine flu strain, leading the World Health Organization to call the strain a pandemic.

The 12 people with the new swine flu strain live in Indiana, Iowa, Maine, Pennsylvania, and West Virginia. Officials for the Centers for Disease Control say the sample size of H3N2 infections is too small to know whether it will pose a threat to the population at large.

"It's a very small sample and it's geographically spread, which makes it more difficult to get a handle on it," says Jeffrey Dimond, a CDC spokesman. "Most of the cases have come through direct contact with the animals, through the 4H Club and that sort of thing."

In order to have a true threat of causing an epidemic or pandemic, Dimond says the virus needs to spread easily between humans.

"If you're in close contact with someone who's ill, that's one thing," he says. "To make it like the pandemic flu of a few years ago, it has to be highly contagious from human to human."

H3N2v or another new flu strain could disrupt what CDC officials expected to be a relatively quiet flu season. Each year's flu vaccine protects against specific strains of the virus that researchers expect to circulate. In October, Joe Bresee, chief of CDC's influenza epidemiology and prevention branch, said he was confident this year's vaccine would protect against the most dangerous flu strains.

"The flu viruses this year's vaccine will protect against are very well-matched to those flu viruses that [were] circulating [in October]," he said. "We will have a vaccine that provides good protection this season to help keep influenza illness and serious complications down."

While it's too early to tell if the new H3N2 strain (or another unexpected strain) will develop into a larger threat, the CDC admits the current vaccine will do little to help stop the virus.