First H3N2v Outbreak of 2013 Reported; CDC Continues to Urge High Risk People to Avoid Swine at Fairs

The first cases of H3N2 variant virus infection for 2013 were reported this week. H3N2 variant virus, or “H3N2v,” is a non-human influenza virus that normally circulates in pigs but which has infected humans. The 4 cases reported by the state of Indiana this week were associated with fair attendance and related swine exposure. Reportedly pigs at the fair also tested positive for H3N2 infection. This outbreak may foreshadow a number of such outbreaks this summer based on what happened last summer when multi-state outbreaks resulted in 309 reported H3N2v cases, including 16 hospitalizations and one death. Genetic sequencing by CDC on one of the Indiana samples has confirmed that the H3N2v virus in Indiana is 99% similar to the H3N2v viruses detected last summer.

The epidemiology of this outbreak in Indiana so far seems consistent with what was seen last summer, when most cases reported exposure to pigs, particularly at agricultural fairs. While most illness was mild, serious illness also was observed; however none of the cases in Indiana this week were hospitalized and there were no deaths.

Symptoms of H3N2v have included fever, sore throat, cough and body aches. No sustained human-to-human spread of the virus has been detected, though sporadic limited spread of this virus has occurred in the past.

Steps to make a vaccine against H3N2v have been taken, but no vaccine is currently available for H3N2v. The 2012-2013 and 2013-2014 seasonal flu vaccines are not formulated to provide protection against H3N2v, but are formulated to protect against seasonal flu viruses that circulate widely each season. However, the same influenza antiviral drugs used to treat seasonal flu can treat H3N2v. The currently recommended drugs – oseltamivir and zanamivir - are available by prescription only. Early treatment works better and is especially important for people with a high risk condition.

The risk of this virus triggering a full-blown pandemic is considered relatively low, however, because serology studies have suggested that significant numbers of adults have some existing immunity against this virus. Children younger than about 10 years old, however, have little to no immunity against H3N2v virus. Given this, a more likely scenario if H3N2v were to become more transmissible among people would be localized outbreaks in pockets of the population that do not have immunity against this virus, for example, in day care or school settings.

Source and additional information: CDC Influenza Spotlights, June 28, 2013