Weekly Influenza Update

March 15, 2018


Wisconsin influenza activity is declining. Influenza A has been the dominant strain so far in Wisconsin, but the relative proportion of influenza B is increasing. Also, we are detecting more severe ARI in individuals with human metapneumovirus.

As of March 3, 2017, there had been 5,904 influenza-related hospitalizations since September 1, 2017; 65% of hospitalizations have been in individuals aged ≥65 years. There have been 742 admissions to ICUs, 58% were aged ≥65 years; and there have been 212 cases requiring mechanical ventilation, 54% aged ≥65 years. This level is well above last year at this time, and above the 4,546 hospitalizations reported at this time in 2014-2015.

The prevalence of influenza-like illness [fever of 100oF or higher and either cough or sore throat] in Wisconsin's primary care patients is at 2.5% and is declining.

9.8% of last week's primary care patients had all-cause respiratory infections.
The prevalence of acute diarrheal illness (ADI) in Wisconsin's primary care patients is at 1.5%

Primary Care Snapshot

The most commonly identified viral cause of Acute Respiratory infections (ARI) in Wisconsin is Influenza A. Over the past 4 weeks the typical ARI case presenting for primary care has been 34.1 years old and 61% of patients have been female. 63% of patients identified a sick contact 1-to-3 days before illness onset and typically present to the clinic 3.5 days after illness onset. 36% of illnesses are characterized as mild, with 57% having moderate symptoms and 3% having severe symptoms.

The typical symptoms reported include:

Typical SymptomsPercentViruses in CirculationPercent
fever63Influenza A53
cough86Influenza B7
sore throat61Coronavirus10
nasal congestion63RSV5
nasal discharge61Parainfluenza0

Clinical Notes


  • Influenza vaccine is recommended universally for everyone over the age of 6 months, including pregnant women
  • There is a good match of vaccine to the currently circulating viruses, but the current vaccine has an overall efficacy estimated at 36%
  • Pneumococcal vaccine PPSV23 is indicated for smokers, people with asthma and other chronic lung conditions as well as a number of other chronic conditions
  • ACIP routinely recommends PCV13 for individuals 65 years and older PPSV23 should be given 12 months after PCV13


  • Performance of Rapid Influenza Diagnostic Tests (RIDTs) depends on age and time from symptom onset
    • Higher sensitivities are attained at younger ages and within the first 3 days of symptoms
    • Clinical judgement is essential in diagnosis
  • Influenza infections are at high levels at this time
  • PPV of rapid antigen tests at this time is high
  • NPV of rapid antigen tests at this time is moderately high


Antivirals need to be started with 48 hours of symptom onset to be effective against influenza
Antivirals started after 48 hours may be effective for hospitalized patients with confirmed influenza

Resistance Patterns

  • 1,244 influenza A[H3N2], 566 influenza A[H1N1] and 568 influenza B viruses have been tested for antiviral resistance since 10/01/17.
  • High levels of adamantene antiviral resistance exist in influenza A isolates from around the world. Adamantanes include amatadine and rimantadine; they are ineffective for influenza B.


  • Human metapneumovirus and rhinoviruses are co-circulating in Wisconsin.
  • RSV activity is declining.

Across the US:

6,193 (17.7%) respiratory specimens during week 9 (February 25 – March 3) were positive for influenza.

For the 2017-2018 season to date (last week):

  • 72.7% (49.9%) of subtyped isolates have been type A
    • 87.3% (70.4%) of A viruses have been H3N2
      • 12.7% (29.6%) of all sub-typed A viruses have been 2009 H1N1
  • 27.3 % (50.1%) of isolates have been type B
    • 90.4% (87.3%) were of the Yamagata lineage
      • 9.6% (12.7%) were of the Victoria lineage

  • 8.8% of deaths during week 6 (February 4-10, 2018) were due to pneumonia or influenza [above the seasonally-adjusted epidemic threshold of 7.4%].
  • 119 pediatric deaths due to influenza have been reported this season: 23 deaths were due to A[H3N2], 23 deaths were due to A[H1N1], 34 deaths were due to an A virus for which no subtyping was performed, 38 deaths were due to influenza B, and one death due to an influenza for which type is unknown.
  • One out of every 270 elders (aged 65+ years) has been hospitalized for influenza. Overall, it has been estimated that 1 out of every 1,159individuals has been hospitalized for influenza this season.

Global News (from the WHO/CDC):

Zika: 5,673 cases have been reported in the U.S. with 2,418 cases in pregnant women. Wisconsin has had 4 cases so far, all associated with travel.

For up to date information, visit the zika page on CDC.

* The weekly influenza update is adapted from an email from Jon Temte, MD, PhD; Chair, Wisconsin Council on Immunization Practices; Professor, Department of Family Medicine and Community Health, University of Wisconsin School of Medicine and Public Health.

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