U.S. health officials have spotted three more children infected with a new swine-origin flu virus, this time in Iowa.

And while earlier cases of infection with this virus are believed to have been cause by exposure to pigs, this time the evidence points to person-to-person spread.

"There's pretty good indications that one child gave it to the other two children," Iowa state epidemiologist Dr. Patricia Quinlisk said in an interview from Des Moines.

"Right now we have every indication that there's person-to-person spread and absolutely no indication whatsoever that there's animal-to-human spread."

The U.S. Centers for Disease Control has confirmed seven previous cases of infection with this virus, a swine-origin influenza A virus of the H3N2 subtype. The earlier cases have been spotted in Maine, Indiana and Pennsylvania. The first case was spotted in July.

Canada has been on the lookout for this new virus. But so far there have been no findings in this country, said Jirina Vlk, communications executive with the Public Health Agency of Canada.

In fact, so far no other country has reported seeing human infections with this particular virus, said Dr. Nancy Cox, head of the CDC's Influenza Division.

Most of the infections have been mild, like regular flu. But three of the cases have required hospitalization. With the exception of one inflection in an adult aged 58, all have been in children under the age of 10.

At this stage it's impossible to know what's going to happen with this virus. It could peter out or continue to cause sporadic infections. Or these cases could be a sign that a new influenza A virus is establishing itself in the human population.

"It possible that the virus is being seeded and is being transmitted in short transmission chains," Cox admitted.

Given what's known about the virus to this point, that may not be cause for major alarm. The virus is a distant cousin of human H3N2 viruses, meaning a significant portion of people probably has some antibodies that would protect against it.

Testing at the CDC shows the swine H3N2 most closely resembles human H3N2 viruses that circulated in the early 1990s. That means many people over the age of 21 or so likely have some protection against this virus, but people under that age probably would not.

Testing suggests the viruses are susceptible to the flu drugs oseltamivir (Tamiflu) and zanamivir (Relenza), Cox said.

Pharmaceutical companies have already been given a seed strain needed to make a vaccine against this virus, should the need be identified.

But with the current flu vaccine already containing three components -- it protects against two influenza A viruses and one influenza B virus -- adding another is a step that would not be taken lightly.

Iowa has increased flu surveillance state-wide. And Cox said the CDC has asked bordering states to enhance their surveillance efforts as well.

The three children attended a small day-care together. They live in adjacent counties, Webster and Hamilton, in the centre of the state.

One became sick first and appears to have infected the other two. Quinlisk said it's not clear how the first child got infected.

Another child who is a contact of the first child was ill with what may have been influenza prior to the first child's infection, she said. But by the time laboratories had confirmed the cases, that other child had recovered.

Even if that child was the source of the infections, there is no easy explanation for how he or she got infected. That child did not have exposure to pigs, Quinlisk said.

In fact, none of the three confirmed cases contacts had exposure to swine or other animals that could have been a source of the virus, Quinlisk said. And nor did family members or other close contacts.

"It's not outside the realm of possibility that an animal connection would be determined at some point in the future but right now it doesn't look that way," Cox said.

The infections occurred a couple of weeks ago. The children were seen at outpatient facilities and did not require hospitalization. All three have recovered.

H3N2 viruses have been spreading in pigs for years. But this one is slightly different. It has picked up one of the genes from the H1N1 flu virus that caused the 2009 pandemic.

It's not clear whether having that gene, the M gene, is helping this virus transmit to humans. But a study done in guinea pigs -- one of the animal models for influenza -- showed that the M gene from the 2009 H1N1 virus is critical to the pandemic virus's ability to transmit well.