CDC Warns About Drifted Influenza A (H3N2), Recommends Antiviral Medications

December 08, 2014 07:12 pm Chris Crawford

Of the influenza virus strains that have been circulating during the 2014-15 flu season, influenza A (H3N2) viruses have been reported most frequently and have been detected in almost all states. That's according to a Dec. 3 CDC Health Alert Network advisory(, which reported that influenza viral characterization data indicate 48 percent of the H3N2 viruses collected and analyzed in the United States from Oct. 1 through Nov. 22 were antigenically "like" this year's H3N2 vaccine component.

Fifty-two percent, however, were antigenically different, or had drifted, from the H3N2 vaccine virus.

[Young woman laying in bed with piles of tissue paper]

In past seasons during which the predominant circulating influenza viruses have drifted, decreased vaccine effectiveness has been observed, the advisory noted.

However, the CDC still recommends administering the flu vaccine to patients because it 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," said the agency in its announcement. "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)."

In light of the H3N2 virus drift, the advisory went on to re-emphasize the importance of using neuraminidase inhibitor antiviral medications when indicated for treatment and prevention of influenza as an adjunct to vaccination. The two prescription antiviral medications recommended for flu treatment or prevention 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," the advisory noted.

Story Highlights
  • A Dec. 3 CDC Health Alert Network advisory said drifted influenza A (H3N2) viruses have been detected this flu season and recommended antiviral medication use when indicated.
  • The CDC still recommends administering the flu vaccine to your patients as it has been found to provide some protection against drifted viruses.
  • When antiviral treatment is indicated, the CDC recommends starting administration as soon as possible after illness onset, ideally within 48 hours.

AAFP liaison to the Advisory Committee on Immunization Practices Jamie Loehr, M.D., of Ithaca, N.Y., said the announcement actually doesn't change much for the average family doctor.

"We still recommend flu vaccine for everyone older than age 6 months; we still recommend meds for certain patients with flu, as well," Loehr told AAFP News. "What this says is that there is drift and that the drifted strains not covered by the vaccine are sensitive to the medicines (i.e., no resistance seen), so you should use the medicines in appropriate cases."

When antiviral treatment is indicated, the CDC recommends starting the medication as soon as possible after illness onset, ideally, within 48 hours.

Treatment with oseltamivir or zanamivir is recommended for any patient with confirmed or suspected influenza who

  • is hospitalized;
  • has severe, complicated or progressive illness; or
  • is at higher risk for influenza complications.

The CDC advisory reinforced the point that decisions about starting antiviral treatment should not wait for laboratory confirmation of influenza.

Oseltamivir is approved to treat influenza in patients age 2 weeks and older and as chemoprophylaxis to prevent influenza in patients age 1 year and older. Zanamivir is approved to treat flu in patients age 7 years and older and to prevent the illness in patients age 5 and older.

Because high levels of resistance to adamantane antiviral medications continue to be observed among circulating influenza A viruses, these drugs (rimantadine [Flumadine] and amantadine [Symmetrel]) are not recommended to treat or prevent influenza.

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