A new swine flu strain put U.S. health officials on alert for more cases after it was found in three children in Iowa this month.

It was reported that ten people in the U.S. have been infected since July by S-OtrH3N2 viruses that picked up a gene from the 2009 H1N1 swine flu pandemic.

The new flu strain (S-OtrH3N2) combines a rare influenza virus (H3N2) circulating in North American pigs and the H1N1 virus from the 2009 outbreak.

New flu strains begin when flu viruses combine in new ways.

They can pose considerable health risks because people have not yet developed immunity to them, ABC News reports.

The Centers for Disease Control and Prevention (CDC) reported the new cases in Wednesday’s Morbidity and Mortality Weekly Report.

CDC also reported that of the other seven cases of the new swine flu, three were in Pennsylvania, two were in Maine and two in Indiana.

In the seven cases all the patients or close contacts had been recently been with pigs.

According to CDC experts, the fact that the three new cases did not go near pigs shows that there may be limited person-to-person contact with the new virus.

The CDC said it has developed a “candidate vaccine virus” that could be used to make a human vaccine against S-OtrH3N2 viruses.

It has already sent it off to vaccine manufacturers.

One of the three Iowa children, a healthy girl referred to as Patient A, became sick during the second week of November.

The girl’s doctor tested her and sent a respiratory sample to the Iowa state laboratory for further analysis.

Patient B, a boy, developed a flu-like illness two days after Patient A became ill.

One day after Patient B became sick, his brother, Patient C, also became ill.

All three children had been at the same gathering on the first day Patient A became sick.

Iowa epidemiologists launched an investigation and decided that the gathering was the only common link among the three children’s illnesses.

None of their families had travelled recently or attended community events.

Eight days after Patient A became ill, Iowa state laboratory testing said that the three might have S-OtrH3N2 influenza.

The CDC went on to confirm it was that strain, which included the matrix (M) gene from the 2009 H1N1 pandemic.

The new flu strain is resistant to rimantidine and amantadine, both commonly used antiviral drugs.

Health experts said the new swine flu strain would most likely respond to osteltamivir (Tamiflu)

Health experts said it would most likely respond to osteltamivir (Tamiflu) and zanamivir (Relenza).

CDC scientists said they believe this year’s seasonal flu vaccine to provide adults with limited protection from the new flu virus, but that it wouldn’t protect children.

Doctors who suspect swine flu infections in their patients should treat them with Tamiflu where appropriate, CDC recommends.

Doctors should get nose and throat specimens and send them to state public health labs, which should report them to CDC.

CDC also encourages anyone who has contact with pigs and develops a flu-like illness to be tested.