Weekly Influenza Update

April 28, 2018


Wisconsin influenza activity is at baseline. Influenza A has been this season’s dominant strain in Wisconsin, but the relative proportion of influenza B is increasing, now representing 84% of cases.

As of April 14, 2018, there had been 7,229 influenza-related hospitalizations since September 1, 2017; 65% of hospitalizations have been in individuals aged ≥65 years. There have been 930admissions to ICUs, 58% were aged ≥65 years; and there have been 259 cases requiring mechanical ventilation, 53% aged ≥65 years.

Wisconsin's influenza vaccination rate to date: 35.6% (data from the Wisconsin Immunization registry)

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 1.4% and is at baseline.

7.2% 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 1.6%

Primary Care Snapshot

The most commonly identified viral causes of Acute Respiratory infections (ARI) in Wisconsin are Influenza B. Over the past 4 weeks the typical ARI case presenting for primary care has been 41.2 years old and 69% of patients have been female. 41% of patients identified a sick contact 1-to-3 days before illness onset and typically present to the clinic 4.7 days after illness onset. 32% of illnesses are characterized as mild, with 60% having moderate symptoms and 6% having severe symptoms.

The typical symptoms reported include:

Typical SymptomsPercentViruses in CirculationPercent
fever68Influenza A6
cough79Influenza B52
sore throat66Coronavirus0
nasal congestion61RSV6
nasal discharge53Parainfluenza0
Symptoms in Patients with PCR-Confirmed Influenza (n=183 cases for season)Percent
sore throat64
nasal congestion61
nasal discharge66

Clinical Notes


  • Influenza vaccine is recommended universally for everyone over the age of 6 months, including pregnant women
  • Keep vaccinating as an estimated 1.0% of influenza cases are yet to come based on historical patterns and influenza B is continuing to circulate
  • There is a good match of vaccine to the currently circulating viruses, but the current vaccine has an overall efficacy estimated at 36%
    • 25% against illness caused by influenza A(H3N2)
    • 67% against A(H1N1) viruses
    • 42% against influenza B viruses
  • 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 moderately high
  • NPV of rapid antigen tests at this time is 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

  • 1,955 influenza A[H3N2], 969 influenza A[H1N1] and 887 influenza B viruses have been tested for antiviral resistance since 10/01/17.
  • 10 influenza A[H1N1] viruses (1.0%) were resistant to oseltamivir and peramivir
  • All influenza A[H3N2] and influenza B viruses have been sensitive to oseltamivir, zanamivir and peramivir
  • 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:

2,056 (10.9%) respiratory specimens during week 15 (April 8-14) were positive for influenza.

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

  • 68.5% (34.3%) of subtyped isolates have been type A
    • 85.5% (59.6%) of A viruses have been H3N2
      • 14.5% (40.4%) of all sub-typed A viruses have been 2009 H1N1
  • 31.5% (65.7%) of isolates have been type B
    • 86.8% (85.6%) were of the Yamagata lineage
      • 13.2% (14.4%) were of the Victoria lineage

  • 7.1% of deaths during week 13 (March 25-31, 2018) were due to pneumonia or influenza [below the seasonally-adjusted epidemic threshold of 7.2%].
  • 156 pediatric deaths due to influenza have been reported this season: 31 deaths were due to A[H3N2], 26 deaths were due to A[H1N1], 42 deaths were due to an A virus for which no subtyping was performed, 55 deaths were due to influenza B, one death was due to influenza virus co-infection, and one death due to an influenza for which type is unknown.
  • One out of every 224 elders (aged 65+ years) has been hospitalized for influenza. Overall, it has been estimated that 1 out of every 964 individuals has been hospitalized for influenza this season.

Global News (from the WHO/CDC):

Zika: 5,672 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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