Influenza activity currently is low in the U.S., according to the CDC, but is on the rise in certain areas of the country. Drifted influenza A (H3N2) viruses have been reported most frequently and have been detected in almost all states, prompting the CDC to issue a health advisory to re-emphasize the importance of the use of neuraminidase inhibitor antiviral medications when indicated for treatment and prevention of influenza, as an adjunct to vaccination.
During past seasons when influenza A (H3N2) viruses have predominated, higher overall and age-specific hospitalization rates and more mortality have been observed, especially among older people, very young children and people with certain chronic medical conditions compared with seasons during which influenza A (H1N1) or influenza B viruses have predominated.
Influenza viral characterization data indicates that 48% of the influenza A (H3N2) viruses collected and analyzed in the U.S. from Oct. 1-Nov. 22 were antigenically like the 2014-2015 influenza A (H3N2) vaccine component, but that 52% were antigenically different (drifted) from the H3N2 vaccine virus, according to the advisory. In past seasons during which predominant circulating influenza viruses have been antigenically drifted, decreased vaccine effectiveness has been observed. However, vaccination has been found to provide some protection against drifted viruses. Though reduced, this cross-protection might reduce the likelihood of severe outcomes such as hospitalization and death. In addition, vaccination will offer protection against circulating influenza strains that have not undergone significant antigenic drift from the vaccine viruses (such as influenza A (H1N1) and B viruses).
The two prescription antiviral medications recommended for treatment or prevention of influenza are oseltamivir (Tamiflu) and zanamivir (Relenza). Evidence from past influenza seasons and the 2009 H1N1 pandemic has shown that treatment with neuraminidase inhibitors has clinical and public health benefit in reducing severe outcomes of influenza and, when indicated, should be initiated as soon as possible after illness onset.
According to background information in the advisory, as of Nov. 22, influenza activity has increased slightly in most parts of the U.S. During the week ending Nov. 22, 1,123 (91.4%) of the 1,228 influenza-positive tests reported to CDC were influenza A viruses and 105 (8.6%) were influenza B viruses. Of the 85 influenza A (H3N2) viruses collected by U.S. laboratories and antigenically or genetically characterized at CDC since Oct. 1, 2014, 44 (52%) are significantly different (drifted) from A/Texas/50/2012, the U.S. H3N2 vaccine virus. Drifted H3N2 viruses were first detected in March 2014, after World Health Organization recommendations for the 2014-2015 Northern Hemisphere vaccine had been made in mid-February. These drifted viruses will likely continue to circulate in the U.S. throughout the season. The CDC issued the following recommendations to remind clinicians of CDCs guidance for the use of influenza antiviral medications.
To read the CDC recommendation for clinicians, visit http://emergency.cdc.gov/han/han00374.asp