Published Date: 2011-11-05 23:16:17
Subject: PRO/AH/EDR> Influenza (66): USA swine-origin H3N2 reassortant, update
Archive Number: 20111105.3298
INFLUENZA (66): UNITED STATES OF AMERICA SWINE-ORIGIN H3N2
A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Fri 4 Nov 2011
Source: The Record.com and the Canadian Press [edited]
More human cases of flu with new swine virus
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 and Prevention (CDC)
reported Friday [4 Nov 2011]. The new cases, in Maine and Indiana,
bring to 7 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 --
2 from Maine, 3 from Pennsylvania and 2 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 2 latest cases are no exception.
Dr. Stephen Sears, state epidemiologist for Maine, said the 2 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 influenza 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 [in guinea pigs].
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 7 or 8 years, 1st 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 [the proteins encoded] in these 2 genes.
[Byline: Helen Branswell]
[There are now 7 cases (6 children and an adult) of infection with an
influenza virus that has been sporadically jumping to people from pigs
in the United States; 2 in Maine, 3 in Pennsylvania and 2 in Indiana.
Although these cases appear to be independent events, most of those
infected had close contact with pigs.
At present there is no evidence that the virus is spreading from
person to person and there has been no report of influenza among
It seems likely that the swine H3N2 virus probably picked up its M
gene (by sub-unit reassortment) from the pandemic H1N1 virus when a
pig was co-infected with swine H3N2 and the H1N1 strain. Model
experiments with guinea pigs suggest that the presence of the M gene
from the 2009 H1N pandemic virus favours transmissibility of the the
H3N2 reassortant virus.
The infections have been mainly mild, but as a precaution the
influenza division of the CDC has already developed a seed strain for
a vaccine that would protect against this virus and supplied it to
A map of the states of the United States of America can be accessed
http://www.50states.com/cap.htm. - Mod.CP]