Influenza H3N2v


Even in the heat of summer, influenza is in the news. An outbreak of what is being termed H3N2v influenza has emerged in Indiana and Ohio, affecting as many as 130 people and counting.[1] The infection has been detected in people who had exposure to pigs that were sick with the H3N2v strain. While person-to-person infections have been detected, these seem to be limited and do not go beyond one or two people, according to the Centers for Disease Control and Prevention (CDC).[2]

The worry is that the H3N2v strain of influenza will mutate again and become more infectious between humans and without the need for pig exposure. Mutations in viruses in general and in the influenza virus in particular are not rare. In fact, mutations in the virus are the reason why a yearly vaccine is recommended. The mutations happen often enough that the flu virus may not be recognizable by the immune system from one year to the next. When these mutations are minor — and the immune system is somewhat prepared for the virus based on previous exposures to other viruses — there is said to have been a genetic drift in the viral genes. If the mutation is major — and the immune system is completely unprepared for the infection — a genetic shift has occurred. Of the three types of influenza (A, B, and C), type A influenza is the one that is most likely to shift suddenly from one year to another.

Influenza pandemics, i.e. worldwide epidemics, are triggered by genetic shifts in the virus. This translates to a majority of the world population being completely susceptible to influenza infection and disease, as was the case in 2009. That year, an H1N1 influenza strain that affected only pigs shifted its genes enough to become infectious to humans and to be contagious from person-to-person. Influenza surveillance in the United States, which was increased in response to the 2009 pandemic, has been detecting human cases of H3N2v, a strain that, until 2011, was detected only in pigs. The first human cases of this variant were detected in 2011.[3] The big distinction from the 2009 H1N1 situation is that the H3N2v strain appears to be limited in its ability to be contagious between humans.[4]

Surveillance for this and other strains of influenza — and even other diseases — will continue at all levels of public health. This means that more samples will be taken from sick people and analyzed in laboratories. It is very likely that the number of cases will continue to climb. Does that signal a pandemic has started? Not until experts in epidemiology and infectious diseases find that this or any other strain has shifted and is being spread easily between humans and between regions of the country and the world. If necessary, a vaccine may be prepared against H3N2v, or any influenza virus that poses a pandemic threat. Until then, it is an influenza outbreak with a variant strain (hence the “v” in “H3N2v”) that is happening during the summer and was detected very early because of the lessons learned in previous influenza epidemics.

Further Reading

●     Fact Sheet: Protect Yourself Against H3N2v – Centers for Disease Control and Prevention

●     H3N2v –

●     Health Tips for Ohio’s Fair Season – Ohio Department of Health


[1] “CIDRAP >> Variant H3N2 cases surge in Indiana, Ohio.” 2012. 9 Aug. 2012 <http//

[2] “CDC – Seasonal Influenza (Flu) – Information on H3N2 Variant …” 2011. 9 Aug. 2012 <”

[3] October, IO. “Update: Influenza A (H3N2)v Transmission and Guidelines — Five …” 2012. <

[4] “CDC – Seasonal Influenza (Flu) – Protect Yourself Against H3N2v.” 2012. 9 Aug. 2012 <