Two U.S. children were infected with a previously unknown flu virus that apparently formed when a pig influenza virus picked up a gene from the strain that caused the swine flu pandemic in 2009, federal health officials reported Friday.
Both of the children recovered, however, and there is no evidence that the virus is spreading easily among people, meaning that it does not appear to pose a threat of becoming a significant public health concern, officials said.
“We want people to be aware of these things, and we want physicians to be aware,” said Lyn Finelli, chief of surveillance and response at the federal Centers for Disease Control and Prevention’s Influenza Division. “But we don’t think that these cases in themselves are alarming.”
Both children are 2 years old. In one case, a boy in Indiana was apparently infected by a caretaker who had been showing pigs at a county fair a few days before the boy became ill, Finelli said. In the other, a girl in Pennsylvania appears to have been infected when she went to a county fair and petting zoo, Finelli said. No one else, including family members of the two children, appears to have become infected.
“We see four or five of these cases every year. They are commonly reported during times of state fairs and county fairs when there is more contact between people and pigs,” Finelli said. “These infections are similar to those that have been reported before.”
The Indiana boy developed a fever, a cough, shortness of breath and diarrhea July 23. Because he had other chronic health problems, he was hospitalized the next day. But he returned home three days later and completely recovered. The Pennsylvania girl developed a fever, a cough and fatigue Aug. 20. She was treated at a hospital and was sent home and recovered.
Samples from both children were sent to state laboratories, which determined they were unusual and sent them to the CDC for analysis. CDC scientists found the viruses were a strain known as H3N2 but had picked up a so-called M or “matrix” gene from the 2009 H1N1 virus that caused the 2009-2010 swine flu pandemic.
Flu viruses commonly pick up genes from one another when different strains infect the same person or animal. Such “reassortment” events can be a concern when the mix produces a strain that might be more easily spread or cause more serious illness, or a strain to which people have little immunity. Twenty-three such novel flu viruses have been detected since 2004, when state laboratories were beefed up to look for them more aggressively to try to identify the next potential pandemic, Finelli said.
“This does point out how successful the surveillance is. States are recognizing unusual cases,” said Michael Shaw, associate director of laboratory surveillance in the CDC’s Influenza Division.
Although neither last year’s nor this year’s flu vaccines would protect against the new strain, federal health officials are not concerned because similar viruses have circulated among people as recently as the early 1990s, meaning most people would have some immunity against it.
“All the major pandemics of the past were instances where a totally new . . . type of virus had appeared — something the human population had little or no immunity to, which allowed it to spread very quickly,” Shaw said.
This time, that does not appear to be the case. By the time the first H1N1 pandemic cases were reported, the virus had already spread widely in Mexico, Finelli noted.
“There’s no evidence there’s more influenza than normal in these areas, so we think it’s very different,” Finelli said. “When we first heard about the first pandemic case, we had heard about a lot of cases. We had a reasonable suspicion that there was a lot of influenza illness out there. We don’t see that background of influenza illness here. We don’t think this virus is being transmitted from person to person in a very significant way.”