U.S. health officials remain on the alert for additional cases of a new swine flu strain that infected three Iowa children this month.

Since July, 10 Americans have been sickened by S-OtrH3N2 viruses that picked up a gene from the 2009 H1N1 swine flu pandemic, the federal Centers for Disease Control and Prevention reported. The new flu strain combines a rare influenza virus (H3N2) circulating in North American pigs and the H1N1 virus from the 2009 outbreak. New flu strains develop when flu viruses combine in new ways. They can pose health risks because people haven’t yet developed immunity to them.

Of the other seven cases of the new swine flu, three occurred in Pennsylvania, two in Maine and two in Indiana, the CDC reported in a Wednesday dispatch in its   Morbidity and Mortality Weekly Report. In all of those cases, either the patients or close contacts had been recently exposed to pigs. The lack of pig exposure in the three newest cases suggested that the new virus may involve limited person-to-person contact. As part of routine preparedness to counter pandemic threats from new flu viruses, CDC said it had developed a “candidate vaccine virus” that could be used to make a human vaccine against S-OtrH3N2 viruses and has sent it to vaccine manufacturers.

One of the three Iowa children, a previously healthy girl referred to as Patient A, became sick during the second week of November. Her doctor tested her as part of routine surveillance and sent a respiratory sample to the Iowa state laboratory for further analysis. Patient B, a boy, developed a flu-like illnesses two days after the Patient A became ill. A day after Patient B became sick, his brother, Patient C, also became ill. Both tested positive for swine flu. All three children had attended the same small gathering on the first day Patient A was fell ill.

After a detailed investigation, Iowa epidemiologists determined that the gathering was the only common link among the three children’s illnesses. None of their families had recently traveled or attended community events, and none of the three or their families had been exposed to pigs, according to the CDC.

Eight days after Patient A became ill, Iowa state laboratory testing indicated the three might have S-OtrH3N2 influenza. The CDC subsequently confirmed the three youngsters had that strain, which included the so-called matrix (M) gene from the 2009 H1N1 pandemic. The new flu strain is resistant to two commonly used antiviral drugs, rimantidine and amantadine, but based upon their genetic structure, would likely respond to osteltamivir (Tamiflu) and zanamivir (Relenza).

CDC scientists said they expected this years’ seasonal flu vaccine to provide adults with limited protection from the new flu virus, but that it wouldn’t help children. They recommended that doctors who suspect swine flu infections in their patients  treat them with Tamiflu where appropriate, obtain nose and throat specimens and send them to a state public health labs, which should report them to CDC. CDC also encourages anyone who has contact with pigs and develops a flulike illness to be tested.