A new swine flu strain has infected 10 Americans since the summer, and health authorities, both here and abroad, are on the alert for more cases.

The new flu strain combines parts of a rare influenza virus — H3N2 – circulating in North American pigs, and the H1N1 virus from the 2009 worldwide flu 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.

 Since July, nine U.S. children and a 58-year-old U.S. man  have been sickened by the new swine  flu strain – S-OtrH3N2 — which picked up a gene from the 2009 H1N1 swine flu pandemic, according to the CDC.

“Everybody is watching,” Jeff Dimond, a spokesman for the U.S. Centers for Disease Control and Prevention in Atlanta, said Tuesday.

Hong Kong’s Center for Health Protection will follow ongoing U.S. surveillance and heed any advice from the World Health Organization, according to a statement issued Tuesday. WHO  is currently working on a public health response should the virus continue spreading.

The new swine flu strain has drawn particular interest because none of the Iowa children sickened last month — all of whom have recovered and are doing fine — nor their families, had known contact with pigs, suggesting person-to-person transmission.

“That’s the mystery of it,” said Dimond. “Flu, by its definition, is unpredictable. That’s one of the vexing characteristics of the virus.”

But so far, he said, “the virus has not shown any sustained human-to-human transference. We’re keeping an eye on it” as the Iowa Health Department leads the investigation.

Dr. Keiji Fukuda, WHO assistant director-general for health security and environment, said in an interview with the Canadian Press that WHO wants to be prepared, but doesn’t want to cause undue alarm when global spread isn’t a certainty. “We’re very aware that we don’t want to overplay or underlay,” Fukuda told the Canadian Press.

International health officials need to strike a delicate balance: If they warn of pandemics that don’t pan out, as when the 2009 H1N1 pandemic barely affected Europe, they risk criticism for inciting panic and look ineffectual.

Of the other seven cases last summer, three occurred in Pennsylvania, two in Maine and two in Indiana, the CDC reported in  its Morbidity and Mortality Weekly Report of Nov. 23.  In all of those cases, either the patients or close contacts had been recently exposed to pigs. As part of routine preparedness to counter pandemic threats from new flu viruses, the 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 flulike 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  became 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 children 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, rimantadine and amantadine, but based upon their genetic structure, would likely respond to oseltamivir (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  state public health labs, which should report them to CDC. The CDC also encourages anyone who has contact with pigs and develops  flulike symptoms  to get tested.

“In the meantime, the most important things people can do are wash their hands with warm soap and water,” Dimond said. “If not, use hand sanitizer.” And, he said, avoid touching your eyes or mouth with your hands, as that can spread germs.