TORONTO - Two new cases of human infection with a flu virus that has been sporadically jumping to people from pigs have been spotted in the United States, the Centers for Disease Control reported Friday.
The new cases, in Maine and Indiana, bring to seven the number seen in the U.S. since July. To date Canada has seen no infections with this virus, the National Microbiology Laboratory said.
While the infections have been mainly mild, the CDC is keeping a close eye on the situation. The head of the Atlanta-based agency's influenza division said a seed strain for a vaccine that would protect against this virus has already been developed and has been given to vaccine manufacturers.
"We're really trying to be in front of events in terms of preparedness," Dr. Nancy Cox said in an interview.
"We're being very vigilant because we realize that there's a portion of the population ... that really has very little cross-reactive antibodies to the swine reassortant virus."
The group Cox was referring to is children. Most of the infections -- two from Maine, three from Pennsylvania and two from Indiana -- have been in young children, though Cox noted one of the cases was a 59-year-old person.
Most of the people who have been infected had close contact with pigs and the two latest cases are no exception.
Dr. Stephen Sears, state epidemiologist for Maine, said the two children from his state who were infected both had a lot of contact with pigs. While they live in the same region of Maine, the children did not have contact with each other.
Sears said there is no thought at this point that the virus is spreading from person to person and there were no reports of flu among members of either child's family.
The virus is a strain of influenza A known as H3N2 that has picked up one of the genes -- the M gene -- of the H1N1 flu strain that caused the 2009 pandemic.
The CDC says the swine H3N2 virus probably picked up the M gene from the pandemic H1N1 virus when a pig was co-infected with swine H3N2 and the H1N1 strain.
It's not clear whether having that gene is helping this virus transmit to humans. But a study done in guinea pigs -- one of the animal models for influenza -- showed that the M gene from the 2009 H1N1 virus is critical to the pandemic virus's ability to transmit well.
Dr. Adolfo Garcia-Sastre, who was involved in that study, said that finding is suggestive, but guinea pigs aren't people.
"Whether these cases now reflect that this particular H3N2 with the M gene of the new H1N1 transmits better into humans, I think it's too early to say," said Garcia-Sastre, a researcher at New York's Mount Sinai Hospital.
The swine-origin H3N2 is related -- though not closely -- to H3N2 viruses that circulate each winter among people. The CDC said the H3N2 component in the annual flu shot would not be expected to protect against the swine-origin variety.
Cox said the virus seems most closely related to human H3N2s that circulated in the early 1990s. That suggests a lot of people would probably have antibodies against this virus, which in turn might make it hard for the virus to be able to spread person to person. But people born after the early 1990s probably would not be protected against the virus.
Cox said it's not possible to be sure, but based on lab work her team believes fewer people would be susceptible to this virus than were vulnerable to the 2009 pandemic virus when it emerged.
Pigs are highly susceptible to influenza and a number of flu strains circulate among swine herds. Occasionally these viruses trigger human infections, generally among people who work with pigs. There have been 28 such cases reported since 2005, the CDC said.
In the last few years the rate of such detections appears to have increased. But no one is sure whether that means there have been more cases, or whether there have always been these numbers of cases but improvements to surveillance means they are now being seen.
The United States and other countries have built up flu surveillance efforts in the past seven or eight years, first due to concern over H5N1 avian flu and later as a result of the 2009 pandemic.
After a spate of human cases of swine H3N2 infection last year, the CDC asked the laboratory that makes candidate viruses for vaccines to make one for a swine H3N2 vaccine. That lab is at New York Medical College in Valhalla.
That virus had the same hemagglutinin and neuraminidase genes -- the H and N in a virus's name -- as the new virus does. And flu vaccines target those two genes.
