The spread of an odd new flu virus that has been jumping from pigs to people in parts of the United States has the World Health Organization gearing up its response planning, a senior official of the agency says.

The UN health body is figuring out what needs to be done if the virus continues to spread and a global response is required, Dr. Keiji Fukuda, assistant director-general for health security and environment said in an interview from Geneva.

The WHO wants to be ready to make recommendations and issue guidance to countries if the need arises -- though Fukuda stressed at this point it is far from certain there will be that need.

"We're very aware that we don't want to over-play or under-play. We're trying to get that right," says Fukuda, a leading influenza expert.

"(We're) trying to make sure that we're ready to move quickly, if we have to move quickly, but also trying not to raise alarm bells."

The desire to be prepared without raising alarm is a legacy of the 2009 H1N1 pandemic. The WHO was heavily criticized in Europe for declaring that event a pandemic when the outbreak turned out to be far milder than originally feared.

But what exactly the agency -- and the world -- might need to prepare for now is very unclear. With the public relations problems of the 2009 outbreak fresh in the minds of health officials, no one is using the "p" word these days.

Yet in some respects the parallels to 2009 are striking.

A new swine-origin flu virus is causing sporadic infections in parts of the United States. Since the new virus was first spotted in July, 10 cases have been confirmed in Maine, Indiana, Pennsylvania and Iowa. All have been children under 10, with a lone exception -- a 58-year-old adult. Three of the cases have required hospitalization but most of the infections have been mild, like regular flu.

It is an influenza A virus of the H3N2 subtype, a distant cousin of H3N2 viruses that circulate in humans.

Scientists at the U.S. Centers for Disease Control say the hemagglutinin gene, the H3, looks like that of H3N2 viruses that used to circulate in people in the early 1990s.

It is sufficiently different from contemporary human viruses that the H3N2 component of the seasonal flu shot is not expected to protect against this virus, though it might boost antibody levels in those who were exposed to the earlier H3N2 viruses.

The CDC is still doing serological work -- checking stored blood samples for antibodies that react to this virus -- to try to figure out how much vulnerability there is to the new virus. The current thinking is most people over the age of 21 or so would have had exposure to similar flu viruses and would therefore have some protection against it.

Teenagers and children might not, though even that's not 100 per cent certain. Flu expert Malik Peiris, chair of the department of microbiology at the University of Hong Kong, says he thinks exposure to contemporary H3N2 viruses might provide some protection against these swine viruses.

"It is important to see the serological data to see how much vulnerability or susceptibility there is in the human population," Peiris says.

Dr. Arnold Monto, a flu expert at the University of Michigan, says if a major part of the human population has antibodies that react to the virus, it may not be much of a threat.

"If there's a lot of immunity in the population, there probably will not be any kind of extensive spread except maybe in these little clusters where you have little folks who don't have much immunity to anything," he says.

Fukuda, on the other hand, says further spread cannot be ruled out: "I think that certainly there's no reason why this virus, if it continues to spread human to human couldn't move from country to country among young people."

The first seven infections appeared to have been instances where the virus passed from pigs to people. But the most recent cases, in Iowa, seem pretty clearly to have involved person-to-person spread.

There were three confirmed cases in that cluster, but it was likely larger. Two contacts of the first confirmed case were also ill, but were not tested. And the people in this cluster seemingly had no contact with pigs, suggesting they caught the virus from an unidentified person.

The virus was previously isolated from pigs in the U.S. Midwest, says Dr. Nancy Cox, head of the CDC's influenza division, though she won't specify where.

Canadian authorities say there are no reports of the virus in this country. And the WHO knows of no cases other than those in the United States, Fukuda says.

To some in the flu world, the situation is reminiscent of 1977. That year an H1N1 virus started circling the globe, causing infections mainly in young people. H1N1 viruses hadn't been spotted for 20 years at that point; it is widely believed the virus was accidentally released from a laboratory.

On some lists of pandemics, the 1977 outbreak is named. Most flu experts, though, do not consider it a pandemic. Some, like Monto, refer to it as a pseudo pandemic.

While the flu world doesn't want to over-react to this virus, it doesn't feel safe ignoring it either.

The CDC asked the laboratory that makes seed strains for vaccine companies to produce a vaccine candidate virus for this H3N2. It is already in the hands of manufacturers.

And the WHO is looking at what it needs to do to be ready. One of the tasks it is currently working on is trying to figure out what to call this virus, if it should continue to spread.

Naming the pandemic virus was a nightmare for public health officials in the start of the 2009 outbreak.

Flu experts accustomed to talking about viruses based on the animals they normally infected -- bird flu, swine flu, dog flu, human flu -- were caught in a political vise when powerful agricultural interests objected to references to the virus's swine origins.

But calling the virus simply H1N1 didn't differentiate it from the human H1N1 that was circulating before the pandemic. (It has since disappeared.) Recently the pandemic virus was officially named H1N1 pdm09.

This swine-origin H3N2 virus poses the same naming challenges.

And this time, the WHO wants to be prepared. Fukuda says the WHO has been in discussion with its animal health counterparts, the UN Food and Agriculture Agency and the OIE, the World Organization for Animal Health, to work out a possible name.

"We're pretty aware that we don't want to increase stigma, we're pretty aware that it is always possible for people to get afraid of food or to enact trade embargoes or things like that. So to the extent that naming the virus in a way which minimizes those things can be done, we think it's better," he says.

"It's just one of those lessons that we've learned. Take a look at those things early. So that's what we're doing."

Still, it's all being done with the realization that there may be no need for heightened public health responses, apart from the increased surveillance the U.S. has mounted.

"This is one of the things that we've discussed," Fukuda says.

"This could be the only cluster we see," he says, referring to the Iowa cases. "We could see some sort of stuttering picture for a long time. Or we could see things jump. All of those things are possible."