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CDC Warns of Potential for Circulation of ‘Drifted’ Flu Viruses

Agency urges continued vaccination of unvaccinated patients

According to a new health advisory from the Centers for Disease Control and Prevention (CDC), influenza activity has increased slightly in most parts of the U.S. Surveillance data indicate that influenza A (H3N2) viruses have predominated so far, with lower levels of detection of influenza B viruses and even less detection of H1N1 viruses.

During the week ending November 22, 1,123 (91.4%) of the 1,228 influenza-positive tests reported to the 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 the CDC since October 1, 2014, 44 (52%) were significantly different (drifted) from A/Texas/50/2012 –– the U.S. H3N2 vaccine virus. Drifted H3N2 viruses were first detected in late March 2014, after World Health Organization (WHO) recommendations for the 2014–2015 Northern Hemisphere vaccine had been made in mid-February. At that time, a very small number of these viruses had been found among the thousands of specimens that had been collected and tested –– but these viruses have become more predominant over time, the CDC says.

Most of the drifted H3N2 viruses are A/Switzerland/9715293/2013 viruses, which is the H3N2 virus selected for the 2015 Southern Hemisphere influenza vaccine. According to the CDC, these drifted viruses will likely continue to circulate in the U.S. throughout the current flu season.

The CD further reports that all influenza viruses tested for resistance to neuraminidase inhibitors this season have shown susceptibility to both oseltamivir (Tamiflu, Genentech) and zanamivir (Relenza, GlaxoSmithKline).

Given the likelihood that the drifted influenza A (H3N2) viruses will continue to circulate this season, the CDC has issued the following recommendations for the use of influenza antiviral medications:

  • Clinicians should encourage all patients 6 months of age and older who have not yet received an influenza vaccine this season to be vaccinated against influenza. Several influenza vaccine options are available for the 2014–2015 flu season.
  • Clinicians should encourage all persons with influenza-like illness who are at high risk for influenza complications to seek care promptly to determine whether treatment with influenza antiviral medications is warranted.

When indicated, antiviral treatment should be started as soon as possible after the onset of illness, ideally within 48 hours of symptom onset. However, antiviral treatment might still have some benefits in patients with severe, complicated, or progressive illness and in hospitalized patients when started after 48 hours of illness onset.

Antiviral treatment with oseltamivir or zanamivir is recommended as early as possible for any patient who has confirmed or suspected influenza who is hospitalized; who has severe, complicated, or progressive illness; or who is at increased risk for influenza complications. This list includes:

  • Children younger than 2 years of age
  • Adults 65 years of age and older
  • Persons with chronic pulmonary (including asthma), cardiovascular (except hypertension alone), renal, hepatic, hematologic (including sickle cell disease), and metabolic disorders (including diabetes mellitus), or neurologic and neurodevelopment conditions (including disorders of the brain, spinal cord, peripheral nerve, and muscle, such as cerebral palsy, epilepsy [seizure disorders], stroke, intellectual disability [mental retardation], moderate-to-severe developmental delay, muscular dystrophy, or spinal cord injury)
  • Persons with immunosuppression, including that caused by medications or by human immunodeficiency virus (HIV) infection
  • Women who are pregnant or postpartum (within 2 weeks after delivery)
  • Persons aged younger than 19 years who are receiving long-term aspirin therapy
  • American Indians and Alaska Natives
  • Persons who are morbidly obese (i.e., with a body mass index equal to or greater than 40)
  • Residents of nursing homes and other chronic-care facilities

Clinical judgment, on the basis of the patient’s disease severity and progression, age, underlying medical conditions, likelihood of influenza, and time since onset of symptoms, is important when making antiviral treatment decisions for high-risk outpatients, the CDC says. Decisions about starting antiviral treatment should not wait for laboratory confirmation of influenza.

Oseltamivir is approved for the treatment of influenza in persons aged 2 weeks and older, and for chemoprophylaxis to prevent influenza in people 1 year of age and older, whereas zanamivir is approved for the treatment of persons 7 years of age and older and for the prevention of influenza in persons 5 years of age and older.

Because high levels of resistance to adamantane antiviral medications continue to be observed among circulating influenza A viruses, adamantanes (rimantadine and amantadine) are not recommended for the treatment or prevention of influenza.

Source: CDC; January 3, 2015.

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