Officials from the U.S. Centers for Disease Control in Atlanta say an additional 52 cases have been spotted, bringing the count since July to 276 human infections.
Those cases have been recorded in 10 states; so far no infections with this virus have been seen outside the United States.
The vast majority of the cases have been in young children who had contact with pigs; many of the infections seem to have taken place at state and county fairs.
But the CDC says it now knows of three recent cases where a person was likely infected by another person, not a pig.
Still, the agency's experts say there is no need at this time to call for a closure of the swine exhibits at fairs.
Earlier this week Michael Osterholm, a flu expert from the University of Minnesota's Center for Infectious Diseases Research and Policy, said the pig barns should be closed to minimize the risk of future spread of the virus.
The virus is a swine H3N2, a distant cousin of the human H3N2 viruses. The human virus entered the pig population in the late 1990s and has circulated among swine ever since. This particular H3N2 is identified as H3N2v, with the "v" being short for variant, a signal that it's an animal virus.
Nancy Cox, who heads the CDC's Influenza Division, said Friday that the agency and its partners are weighing a lot of factors as they monitor the situation. For now, they feel the risk doesn't warrant closing the barns.
"When you're looking at shutting down all of this activity where there may not be a problem, you know you are trying to balance many, many things," Cox said in an interview.
"If we saw that there were really high numbers of cases or we saw that there was an increase in severity or an increase in transmission ... there could be a lot of different kinds of signals that would change what we recommended."
To date most of the illnesses triggered by the virus have been mild. There have been 13 people hospitalized with H3N2v infections, but no deaths.
One of the signals officials are watching for is spread of the virus among people. But they aren't looking for just one generation of spread — a sick child infects a family member, but the chain of transmission dies there. They are looking for sustained spread.
"The presence of human-to-human spread is interesting. The presence of community transmission, several generations for instance of community spread is critical," said Dr. Joe Bresee, chief of influenza epidemiology and prevention at the CDC.
Bresee wouldn't say in which states the cases of human spread were spotted. It's up to individual states to release that information, he said.
But he did say these three cases don't come as a surprise.
"We expected that. And we've been looking hard for it and to find it, because we know that this virus at least has that potential to spread in a very limited way between people."
In fact, it has been seen before. Late last year a child in West Virginia was infected with the virus in what was believed to have been person-to-person spread.
The CDC has been urging people with health problems that put them at high risk of becoming severely ill if they contract the flu to avoid pig barns at fairs. And they are recommending that people who do visit the barns not eat or drink while in the facilities, and wash their hands after their visit.
Meanwhile, agricultural authorities are testing pigs as they arrive at fairs in a bid to weed out sick pigs so they don't infect other animals. But a study published last week — a study on which Cox was an author — showed that pigs that don't have symptoms can still be infected with flu viruses.Suggest a correction