Thursday, December 04, 2014

CDC HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus


# 9408


Late yesterday the CDC issued a HAN Advisory for clinicians on the possibility of seeing `drifted’ H3N2 virus infections during this year’s flu season, along with a reminder on the appropriate and timely use of antivirals. 


If this sounds familiar, we looked at this possibility at some length a little over a month ago in A `Drift’ In A Sea Of Influenza Viruses.


But briefly, over the summer – and since this year’s flu vaccine components were selected last February – a new, antigenically drifted H3N2 virus has begun to circulate more widely.


The rise of this new strain has already prompted  WHO to recommend a strain change for next year’s Southern Hemisphere vaccine formulation from the current A/Texas/50/2012 (H3N2)-like virus to a new A/Switzerland/9715293/2013 (H3N2)-like virus.


The most recent FluView report (week 47) shows that of the 85 H3N2 viruses tested since October 1st, 41 (48%) are a good match to the vaccine strain, while 52% were not.  This `mismatch’ is expected to reduce the vaccine’s effectiveness against this drifted strain, although some protection or a reduction in the severity of illness may still result.


With this year shaping up to be an H3N2-dominant year, and a possible vaccine `mismatch’ in the works, the CDC is reminding clinicians of value of prescribing neuraminidase inhibitor antiviral medications for severe influenza, or for patients with co-morbidities that raise their risk of seeing serious complications.


Despite the constant excoriation of Tamiflu ® and other NI antiviral medications in the press – based primarily on a series of Cochrane reports (see Revisiting Tamiflu Efficacy (Again)) – there are demonstrable benefits to using these medications, particularly when given early and for severe influenza (see The CDC Responds To The Cochrane Tamiflu Study).

Earlier this summer we saw a review in the journal Clinical Infectious Disease that suggested a serious Under Utilization Of Antivirals For At Risk Flu Patients, and a month ago we saw the UK’s PHE reiterate their Influenza Antiviral Recommendations.


Although you can never be sure what kind of flu season we will have until it is over, years in which H3N2 viruses have dominated tend to be rougher for the elderly, and the very young,  than years when H1N1 or influenza B dominate. 


Despite the expected reduced effectiveness of this year’s flu shot, there are still benefits to getting the vaccine if you haven’t done so already. 

  • There are two other strains (H1N1 & Influenza B) covered by the shot,
  • Half the H3N2 viruses tested so far are still a `match’  to the vaccine strain
  • There may still be some degree of cross-protection afforded against this new strain.

I’ve only posted the summary, so follow the link below to read the full HAN advisory, and the recommendations of antiviral use.


CDC Health Advisory Regarding the Potential for Circulation of Drifted Influenza A (H3N2) Viruses



Distributed via the CDC Health Alert Network
December 3, 2014, 16:00 ET (4:00PM ET)

CDC is reminding clinicians of the benefits of influenza antiviral medications and urging continued influenza vaccination of unvaccinated patients this influenza season.


Influenza activity is currently low in the United States 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, 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.

Influenza viral characterization data indicates that 48% of the influenza A (H3N2) viruses collected and analyzed in the United States from October 1 through November 22, 2014 were antigenically "like" the 2014-2015 influenza A (H3N2) vaccine component, but that 52% 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).

Because of the detection of these drifted influenza A (H3N2) viruses, this CDC Health Advisory is being issued 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.

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. Clinical trials and observational data show that early antiviral treatment can:

  • shorten the duration of fever and illness symptoms;
  • reduce the risk of complications from influenza (e.g., otitis media in young children and pneumonia requiring antibiotics in adults); and
  • reduce the risk of death among hospitalized patients.

(Continue . . .)


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