Health officials say a novel strain of swine flu has sickened two children in Pennsylvania and Indiana.
One had contact with pigs. The other is believed to have been infected by a caregiver who had contact with pigs, suggesting the virus can spread person-to-person.
The Centers for Disease Control and Prevention says the new virus contains a gene from the H1N1 swine flu that caused a worldwide scare two years ago, plus parts of other viruses that have infected pigs over the last decade.
The children were infected in July and August and have recovered. Both had received flu shots last year.
Officials are investigating other reports of illness in people who attended an agricultural fair in Pennsylvania. No additional cases have been confirmed so far.