DOH and CDC Issue Alerts Regarding the Potential for Circulation of Drifted Influenza A Viruses

On Dec. 15, 2014, the Pennsylvania Department of Health (DOH) forwarded a Centers for Disease Control and Prevention (CDC) Health Advisory regarding the potential for circulation of drifted influenza A (H3N2) viruses. DOH sent the alert to those Pennsylvania physicians who subscribed to receive its alerts through its PA Health Alert Network (PA-HAN).

Alert Background

Influenza activity is currently low in the U.S. as a whole, but is increasing in some parts of the country. This season, influenza A (H3N2) viruses have been reported most frequently and have been detected in almost all states.

During past seasons when influenza A (H3N2) viruses have predominated, higher overall and age-specific hospitalization rates and more mortality have been observed. This was especially the case 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.

Influenza viral characterization data indicates that 48 percent of the influenza A (H3N2) viruses collected and analyzed in the U.S. from Oct. 1 through Nov. 22 were antigenically “like” the 2014-2015 influenza A (H3N2) vaccine component, but that 52 percent were antigenically different (drifted) from the H3N2 vaccine virus.

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).

Recommendations for Clinicians

Because of the detection of these drifted influenza A (H3N2) viruses, the CDC health advisory reminded all clinicians of the benefits of influenza antiviral medications and urged continued influenza vaccination of unvaccinated patients.

Specifically, the alert recommended that clinicians should:

  • Encourage all patients six months and older who have not yet received an influenza vaccine this season to get vaccinated. There are several influenza vaccine options for the 2014-2015 influenza season, which are outlined on the CDC’s website.
  • Encourage all people with influenza-like illness who are at high risk for influenza complications (list of those considered high-risk included in alert) to see care promptly to determine if the treatment with influenza antiviral medications is warranted.

The DOH alert also contained a summary of CDC recommendations for influenza antiviral medications.

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