A new variant of an influenza virus that circulates in pigs has been jumping occasionally into people, providing a surprisingly early opportunity for public health officials to test out some of the lessons learned from the 2009 H1N1 pandemic.

Since the virus was first spotted in July, there have been 10 cases, all but one of which were children under the age of 10. (The exception was a 58-year-old.) All the cases have been in the U.S.; there have been no reports of this virus in people or pigs anywhere else

The most recent infections, in three young children in Iowa, almost certainly involved person-to-person spread. The Iowa cluster is likely larger—no one in the first child’s family had exposure to pigs, suggesting an unidentified person was the source of virus.

The cases leave public health authorities in the U.S. and elsewhere wondering if a new swine-origin flu virus is circulating at low levels among humans—and what needs to be done if that is indeed happening. (Read “Flu Factories” in the January 2011 Scientific American (preview) to learn why health authorities fear the next pandemic virus may emerge as a result of industrial farming practices.)

Given the mercurial nature of flu viruses—which can easily mutate into lethal pathogens—ignoring the new virus is not an option, even though to date there have been no deaths and most of the infections have produced only mild symptoms. But the widespread perception that the 2009 swine flu pandemic was much ado about nothing means health authorities risk further damage to their already battered credibility if they sound an alarm and this virus turns out to be a dud. And they know it.

The World Health Organization is working to be ready to react if needed, but wants to make sure it neither underplays or overplays its response to this potential new threat, which is technically known as a swine origin influenza A virus of the H3N2 subtype. The new virus has acquired the M gene of the 2009 pandemic H1N1 virus; studies suggest that this gene may enhance transmissibility of the virus.

"We are closely following the information coming out of the U.S. And we just are making sure that if we need to be more active, we will be more active, or if we need to stand down we can do so,” says Keiji Fukuda, the UN health agency’s assistant director-general for health security and environment.

Among the things the WHO is working on is finding a scientifically correct yet politically sensitive name to call this virus. Sales of pork plunged in 2009 when the new H1N1 was identified as swine flu, a reference to the fact it was comprised mainly of genes from flu viruses that circulate in pigs.

This H3N2 poses similar naming challenges. There is already a human H3N2 — a distant cousin of this pig virus — so some way to differentiate the viruses is needed. Pork producers are concerned about how communications about the virus will be handled, admits Paul Sundberg, vice-president for science and technology for the National Pork Board, who says his group has already met with officials at the Centers for Disease Prevention and Control in Atlanta to discuss the naming challenge.

Fukuda says the WHO is trying to draw on the experience of 2009 as it maps out its response to the new virus. “It frequently comes up as a question: ‘What did we learn from that pandemic that we ought to be thinking about in terms of this situation?’”

One of the lessons was that there may be a fair amount of cross-protective immunity in the human population to a number of the viruses currently circulating in swine, some of which were introduced into pigs from people in the past. In 2009 most people 60 or older had some degree of protection: it came from previous exposure to past human H1N1s that were genetically closer to the pandemic virus than are more recent strains.

In this case, there’s likely more protection still. Testing at the CDC shows that the new H3N2 is similar to human H3N2 that circulated in the early 1990s. That suggests many people over the age of 21 probably have some antibodies that could recognize this virus if it starts to spread.

The age cut-off is reminiscent of the immunological situation in 1977, the year of the so-called Russian flu. That year H1N1 viruses unexpectedly reappeared after a 20-year absence. More surprising still was the fact the viruses appeared to have been almost frozen in time; they looked like H1N1 viruses that circulated in the early 1950s. It’s widely believed the virus re-emerged as the result of a laboratory accident.

In 1977, the newly re-emerged H1N1 viruses initially infected younger people as they began to circle the globe. But the strain managed to re-establish itself, eventually infecting more broadly across age groups. While a few scientific papers list 1977 as a pandemic, most flu experts do not consider it one.

If this new H3N2 continues to circulate, it’s conceivable it could do what H1N1 did in 1977 and re-establish itself, says Arnold S. Monto, a flu researcher at the University of Michigan.

But Malik Peiris, a virologist at the University of Hong Kong, points to an important difference between 1977 and now. Back then, people younger than 20 were completely naive to H1N1 viruses. This time many people under 20 will have had exposure to human H3N2 viruses. Though the genetic distance between the human and the swine viruses is significant, Peiris says there may be some cross-protective antibodies that will come into play.

There may also be a lesson to be learned from another outbreak of the 1970s, the original swine flu incident. Human infections with a swine H1N1 virus at Fort Dix, N.J. in the spring of that year triggered fears that the world might be in for a replay of the 1918 Spanish flu pandemic. The U.S. mounted an emergency vaccination program, but the virus didn’t take off. Officials know this one might not as well.

In case it takes a different course, the CDC asked the laboratory that makes candidate vaccine viruses to make one for the swine H3N2. That work is completed and the seed strain is in the hands of vaccine manufacturers. Here’s hoping that they won’t need to use it.