2 children sickened by novel swine flu strain

So far, there's no sign that the virus has spread beyond the two children, the Centers for Disease Control and Prevention reported Friday.

"We wanted to provide some information without being alarmist," because people have contact with pigs at fairs this time of year and doctors should watch for possible flu cases, said Lyn Finelli, the CDC's flu surveillance chief. "We're always concerned when we see transmission of animal viruses to humans."

People rarely get flu from pigs - only 21 cases have been documented in the last five years - and it's too soon to know how infective this virus will be, she said.

The new strain is a hybrid of viruses that have infected pigs over the last decade and a gene from the H1N1 strain that caused the pandemic two years ago. It is the first combination virus to turn up in people since the pandemic, said Michael Shaw, a lab chief at the CDC. It's classified as an H3N2 virus.