Influenza A(H1N2) Viruses in Wisconsin

By Carol Kirk, Wisconsin State Lab of Hygiene, Madison, WI.

The 2001-2002 influenza season in Wisconsin was marked by mild weather, a relatively late peak of
influenza activity, and by the detection of a reassortant influenza isolate - influenza
A/Wisconsin/12/2001 (H1N2). This isolate is believed to be a naturally occurring reassortment of the
currently circulating H1N1 and H3N2 influenza A viruses.

Through enhanced and routine surveillance, eight additional influenza A(H1N2) isolates have been
detected in Wisconsin to date. Seven of these were recovered from influenza isolates submitted for
subtyping by St. Vincent Hospital in Green Bay. The first isolate and one later isolate were recovered
from specimens submitted to the Wisconsin State Laboratory of Hygiene (WSLH) for culture
confirmation of positive rapid influenza test results.

The initial isolate was recovered from a specimen collected December 7, 2001 from a 6 month old;
the latter eight isolates were recovered from specimens collected between January 11, 2002 and
February 11, 2002, from patients aged 9 through 31 years. All of the Wisconsin H1N2 isolates to date
have been from two adjacent counties.

Influenza A viruses are classified by their subtypes based on their hemagglutinin (H) and
neuraminidase (N); surveillance incorporates subtyping of influenza A viruses to classify them by their
hemagglutinin. By convention, H1 viruses have been presumed to be H1N1 and H3 viruses have
been presumed to be H3N2., as the H1N1 and H3N2 subtypes have co-circulated among humans
since 1977.

Subtyping at the WSLH identified the original isolate as an influenza A(H1). The isolate was
forwarded to the CDC for further characterization, where it was confirmed as an influenza A(H1).
Additional testing by the CDC included sequencing of the genome, which identified the hemagglutinin
as H1, similar to currently circulating H1N1 viruses, but identified the neuraminidase as N2, similar to
currently circulating H3N2 viruses.

Two additional influenza A(H1N2) viruses were identified by the CDC from specimens collected
during July and September, 2001, in Texas and Nevada, respectively. In other parts of the world, the
Public Health Laboratory Service in the United Kingdom reported influenza A(H1N2) viruses isolated
in England, Israel and Egypt. Previously, 19 influenza A(H1N2)viruses had been identified in China
between 1988 and 1989.

According to the CDC, the current influenza vaccine is expected to provide good protection against
influenza A(H1N2) viruses because both the H1 and N2 proteins of the reassortant virus are similar to
those in the vaccine. It is unknown whether the A(H1N2) virus will continue to circulate, but it does
not appear to have caused unusual levels of disease.

For more information about influenza A(H1N2) viruses, see the CDC website at
http://www.cdc.gov/ncidod/diseases/flu/factsheetH1N2.htm.