As of November 22, 2014, influenza activity has begun to increase in most regions of the United States. Surveillance data indicate that infection with influenza A (H3N2) viruses has predominated so far, with lower detection rates of influenza B and H1N1 viruses. Genetic characterization of Influenza A (H3N2) isolates has revealed that 48% of viruses were antigenically similar to the A/Texas/50/2012 virus included in the 2014-2015 influenza vaccine but that 52% had significantly drifted. Most of the drifted H3N2 viruses resemble A/Switzerland/9715293/2013 viruses. These drifted viruses are expected to circulate in the United States throughout this flu season.
Vaccination will offer protection against circulating influenza strains that have not undergone significant antigenic drift from the vaccine viruses, but will not be as effective antigenically drifted H3N2 viruses. Cross-protection may reduce severity of illness and lessen the likelihood of hospitalization and death. All influenza viruses tested for resistance to neuraminidase inhibitors this season have shown susceptibility to both oseltamivir and zanamivir.
During past seasons when influenza A (H3N2) viruses have predominated, higher overall and age-specific hospitalization rates and increased mortality have been observed, especially among older people, very young children, and persons with certain chronic medical conditions compared with seasons during which influenza A (H1N1) or influenza B viruses have predominated.
Three different types of tests are available for detection of influenza virus. Rapid antigen testing detects and differentiates both influenza A and B. Some rapid tests may not detect influenza A (H3N2). The major disadvantage of rapid antigen testing is its low sensitivity of 60-80%.
Many laboratories offer PCR testing for influenza viruses. In addition to influenza B, flu A/B PCR detects influenza A H1 and H3 subtypes and differentiates 2009 H1N1. Sensitivity averages 90% with specificity near 100%. Nasopharygeal or nasal swabs submitted in viral transport media or nasal washes are the only acceptable specimen types for flu A/B PCR.
For critically ill patients, a respiratory PCR panel detects influenza A, influenza B, coronavirus, human metapneumovirus, rhinovirus, parainfluenza, RSV, adenovirus, Bordetella pertussis, Mycoplasma pneumoniae, and Chlamydophila pneumoniae. Respiratory PCR panel can be performed on bronchoscopy specimens in addition to nasal swabs or washes.
For all 3 tests, a nasopharyngeal specimen should be collected with a flocked swab and sent to the laboratory in viral transport media.