SAN FRANCISCO, CA 12 September 2012—With the case count for swine-origin H3N2variant influenza topping 300, the Centers for Disease Control and Prevention (CDC) continues to urge clinicians to treat all infected people with oseltamivir or zanamivir.

Lyn Finelli, who leads CDC's influenza surveillance and outbreak-response team, said Tuesday at the American Society for Microbiology's ICAAC in San Francisco that 302 people in 10 states had been diagnosed with H3N2v infection in the past nine weeks.

All patients who have been in any of those states and visit a physician because of influenza-like illness should receive antiviral therapy with oseltamivir or zanamivir, Finelli said.

The only death so far has been an older adult with "many very severe, underlying conditions," she said. Sixteen people have been hospitalized.

"We're interested in these infections because sustained and efficient transmission of these viruses represent opportunity for a pandemic," Finelli said.

The infections have tended to occur in young people who had prolonged, close contact with pigs in what Finelli called the "swine belt." This is the region where swine production is concentrated and children enter their pigs into competitions at state and county fairs.

Indiana and Ohio accounted for roughly 80% of the cases at the end of last week, according to CDC.

Jeffrey Fox, of Microbe Magazine, a publication by the American Society for Microbiology, characterized the H3N2v outbreaks as "prize pigs" making people sick.

Fox was interviewing Finelli during a press conference and live video stream from ICAAC.

Finelli likened a pig barn to a classroom, in which it takes only one ill youngster to infect everyone in the room. "These children, these exhibitors, spend extraordinary amounts of time with their pigs in extremely close contact, sometimes even sleeping on the site of the fair and spending seven, eight, nine days with their pigs in close proximity. So, the opportunity for exposure [to H3N2v from an infected pig] is tremendous."

Ten of the H3N2v-infected people this year caught their infection from another person, Finelli said, adding that CDC used a "very generous" definition of human-to-human transmission. Also, to date, none of the infections have arisen from the community. All had a connection to a livestock fair.

Thus, she said, CDC believes that the probability of human-to-human transmission of H3N2v is "extremely low."

The agency's hope is that the number of new H3N2v infections decreases as the fair season closes, Finelli said.

But influenza viruses are known to thrive in cool, damp conditions.

"A candidate vaccine virus—in fact, two vaccine candidates—have been sent to drug manufacturers in case [the number of] cases starts to escalate," Finelli said. "It's hard to predict what's going to happen as both fairs wind down and as cooler weather ensues. And it's possible that we'll see local outbreaks, maybe in daycare centers and in schools in the exquisitely sensitive population."

One of the vaccine candidates would be produced in eggs, and the other in a cell culture, she said after the press conference.

ICAAC was previously known as the Interscience Conference on Antimicrobial Agents and Chemotherapy.