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Distributed via the CDC Health Alert Network, February 1, 2016, 0850 EST (8:50 AM EST), CDCHAN-00387 #


Influenza activity is increasing across the country and CDC has received reports of severe influenza illness. Clini­cians are reminded to treat suspected influenza in high-risk outpa­tients, those with progres­sive disease, and all hospi­tal­ized patients with antiviral medica­tions as soon as possible, regard­less of negative rapid influenza diagnostic test (RIDT) results and without waiting for RT-PCR testing results. Early antiviral treat­ment works best, but treat­ment may offer benefit when started up to 4 – 5 days after symptom onset in hospi­tal­ized patients. Early antiviral treat­ment can reduce influenza morbidity and mortality.

Since October 2015, CDC has detected co-circu­la­tion of influenza A(H3N2), A(H1N1)pdm09, and influenza B viruses. However, H1N1pdm09 viruses have predom­i­nated in recent weeks. CDC has received recent reports of severe respi­ra­tory illness among young- to middle-aged adults with H1N1pdm09 virus infec­tion, some of whom required inten­sive care unit (ICU) admis­sion; fatal­i­ties have been reported. Some of these patients report­edly tested negative for influenza by RIDT; their influenza diagnosis was made later with molec­ular assays. Most of these patients were report­edly unvac­ci­nated. H1N1pdm09 virus infec­tion in the past has caused severe illness in some children and young- and middle-aged adults. Clini­cians should continue efforts to vacci­nate patients this season for as long as influenza viruses are circu­lating, and promptly start antiviral treat­ment of severely ill and high-risk patients if influenza is suspected or confirmed. 

  1. Clini­cians should encourage all patients who have not yet received an influenza vaccine this season to be vacci­nated against influenza. This recom­men­da­tion is for patients 6 months of age and older. There are several influenza vaccine options for the 2015 – 2016 influenza season (see http://​www​.cdc​.gov/​m​m​w​r​/​p​r​e​v​i​e​w​/​m​m​w​r​h​t​m​l​/​m​m​6430a3.htm), and all avail­able vaccine formu­la­tions this season contain A(H3N2), A(H1N1)pdm09, and B virus strains. CDC does not recom­mend one influenza vaccine formu­la­tion over another. 
  2. Clini­cians should encourage all persons with influenza-like illness who are at high risk for influenza compli­ca­tions (see list below) to seek care promptly to deter­mine if treat­ment with influenza antiviral medica­tions is warranted.
  3. Decisions about starting antiviral treat­ment should not wait for labora­tory confir­ma­tion of influenza. Clini­cians using RIDTs to inform treat­ment decisions should use caution in inter­preting negative RIDT results. These tests, defined here as rapid antigen detec­tion tests using immunoas­says or immuno­flu­o­res­cence assays, have a high poten­tial for false negative results. Antiviral treat­ment should not be withheld from patients with suspected influenza, even if they test negative by RIDT; initi­a­tion of empiric antiviral therapy, if warranted, should not be delayed.
  4. CDC guide­lines for influenza antiviral use during 2015 – 16 season are the same as during prior seasons (see http://​www​.cdc​.gov/​f​l​u​/​p​r​o​f​e​s​s​i​o​n​a​l​s​/​a​n​t​i​v​i​r​a​l​s​/​s​u​m​m​a​r​y​-​c​l​i​n​icians.htm).
  5. When indicated, antiviral treat­ment should be started as soon as possible after illness onset, ideally within 48 hours of symptom onset. Clinical benefit is greatest when antiviral treat­ment is admin­is­tered early. However, antiviral treat­ment might still be benefi­cial in patients with severe, compli­cated, or progres­sive illness, and in hospi­tal­ized patients and in some outpa­tients when started after 48 hours of illness onset, as indicated by clinical and obser­va­tional studies.
  6. Treat­ment with an appro­priate neuraminidase inhibitor antiviral drugs (oral oseltamivir, inhaled zanamivir, or intra­venous peramivir) is recom­mended as early as possible for any patient with confirmed or suspected influenza who 
    1. is hospi­tal­ized;
    2. has severe, compli­cated, or progres­sive illness; or
    3. is at higher risk for influenza compli­ca­tions. This list includes: 
      1. children aged younger than 2 years;
      2. adults aged 65 years and older;
      3. persons with chronic pulmonary (including asthma), cardio­vas­cular (except hyper­ten­sion alone), renal, hepatic, hemato­log­ical (including sickle cell disease), 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);
      4. persons with immuno­sup­pres­sion, including that caused by medica­tions or by HIV infection;
      5. women who are pregnant or postpartum (within 2 weeks after delivery);
      6. persons aged younger than 19 years who are receiving long-term aspirin therapy;
      7. American Indians/​Alaska Natives;
      8. persons who are morbidly obese (i.e., body-mass index is equal to or greater than 40); and
      9. residents of nursing homes and other chronic-care facilities.
  7. Antiviral treat­ment can also be consid­ered for suspected or confirmed influenza in previ­ously healthy, sympto­matic outpa­tients not at high risk on the basis of clinical judgment, especially if treat­ment can be initi­ated within 48 hours of illness onset.
  8. 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 outpatients.
  9. While influenza vacci­na­tion is the best way to prevent influenza, a history of influenza vacci­na­tion does not rule out influenza virus infec­tion in an ill patient with clinical signs and symptoms compat­ible with influenza. Vacci­na­tion status should not impede the initi­a­tion of prompt antiviral treatment.

Seasonal influenza contributes to substan­tial morbidity and mortality each year in the United States. In the most recent influenza season — the 2014 – 2015 season — CDC estimates that there were approx­i­mately 19 million influenza-associ­ated medical visits and 970,000 influenza-associ­ated hospi­tal­iza­tions [1]. The spectrum of illness observed thus far during the 2015 – 2016 season has ranged from mild to severe and is consis­tent with that of other influenza seasons. Although influenza activity nation­ally is low compared to this time last season, it is increasing; and some local­ized areas of the United States are already experi­encing high activity. Further increases are expected in the coming weeks. Typically, influenza seasons begin with increases in influenza-like-illness and the percent of respi­ra­tory speci­mens testing positive for influenza in clinical labora­to­ries. Those indica­tors are rising at this time. Increases in severity indica­tors tend to lag behind. At this time, national surveil­lance systems that track severity are not elevated, but CDC will continue to watch for indica­tions of increased severity from influenza virus infec­tion this season.

Labora­tory data so far show that most circu­lating flu viruses are still like the viruses recom­mended for the 2015 – 2016 influenza vaccines. CDC will continue to monitor circu­lating influenza viruses for changes that might impact vaccine effec­tive­ness and publish these data weekly in FluView (http:/www.cdc.gov/flu/weekly/summary.htm). CDC also is conducting epidemi­o­logic field studies to deter­mine vaccine effec­tive­ness this season. 

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