Federal and state health officials have said that a new swine flu strain recently sickened two pre-school children in Pennsylvania and Indiana, but they cautioned that the cases appeared to be isolated.

Both the children recovered, however, and there is no evidence that the virus is spreading easily among people, meaning that it does not appear to pose a threat of becoming a significant public health concern, officials said.

“We want people to be aware of these things, and we want physicians to be aware,” said Lyn Finelli, chief of surveillance and response at the Federal Centers for Disease Control and Prevention’s Influenza Division.

Both children are 2 years old. In one case, a boy in Indiana was apparently infected by a caretaker who had been showing pigs at a county fair a few days before the boy became ill. In the other, a girl in Pennsylvania appears to have been infected when she went to a county fair and petting zoo.

The Pennsylvania Department of Health, on Friday, issued an advisory urging the public to take precautions to prevent the spread of respiratory viruses in light of its investigation into the new swine flu strain.

Samples from both children were sent to state laboratories, where it was determined that they were unusual and were sent to the CDC for analysis.

Flu viruses commonly pick up genes from one another when different strains infect the same person or animal. Such re-assortment events can be a concern when the mix produces a strain that might be more easily spread or cause more serious illness, or a strain against which people have little immunity.

Each winter, scientists try to predict which flu strains will circulate so they can develop an effective vaccine for the following season.

Influenza viruses are commonly found in humans, swine, birds and other animals and there are more than 50 strains of human flu virus alone. Seasonal flu season typically runs December through March.