Weekly Influenza Update

November 8, 2018

Welcome to the 2018-2019 Influenza Season. Over the next several months, I will try to provide weekly updates on influenza’s passage across Wisconsin, enhanced by excellent surveillance data made available by the Wisconsin Division of Public Health, the Wisconsin State Laboratory of Hygiene, the Centers for Disease Control and Prevention, and the World Health Organization. I will make predictions based on trend data and my experience in reading the seasonal patterns of influenza. I will also provide some clinical guidance. Should you have any questions, please do not hesitate to contact me.


Wisconsin has low influenza activity at present.  Occasional detections occur, with the majority being Influenza A[H1N1].  There have been 21 influenza-related hospitalizations since September 1, 2018; 54.2% of hospitalizations have been in individuals age ≥ 65 years.

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 baseline.

7.4% 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 1.3%

Primary Care Snapshot

The most commonly identified viral causes of Acute Respiratory infections (ARI) in Wisconsin is rhinovirus, followed by parainfluenza. Over the past 4 weeks the typical ARI case presenting for primary care has been 30.9 years old and 73% of patients have been female. 54% of patients identified a sick contact 1-to-3 days before illness onset and typically present to the clinic 5.0 days after illness onset. 34% of illnesses are characterized as mild, with 64% having moderate symptoms and 0% having severe symptoms.

The typical symptoms reported include:

Typical SymptomsPercent
sore throat77
nasal congestion59
nasal discharge39

Clinical Notes


  • Influenza vaccine is recommended universally for everyone over the age of 6 months, including pregnant women
  • 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 low levels at this time
  • PPV of rapid antigen tests at this time is moderate
  • 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

  • 57 influenza A[H3N2], 70 influenza A[H1N1] and 44 influenza B viruses have been tested no viruses (0.0%) were resistant to oseltamivir, zanamivir or peramivir
  • High levels of adamantane antiviral resistance exist in influenza A isolates from around the world. Adamantanes include amantadine and rimantadine; they are ineffective for influenza B.


  • Rhinovirus and Parainfluenza viruses are co-circulating in Wisconsin

Across the US:

155 (0.9%) respiratory specimens during week 44 (October 28 - November 3) were positive for influenza.

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

  • 75.7% (74.2%) of subtyped isolates have been type A
    • 23.2% (11.1%) of A viruses have been H3N2
    • 76.8% (88.9%) of all sub-typed A viruses have been 2009 H1N1
  • 24.3% (25.8%) of isolates have been type B
    • 76.1% (100%) were of the Yamagata lineage
    • 23.9% (0%) were of the Victoria lineage
  • 5.4% of deaths during week 43 (October 21-27, 2016) were due to pneumonia or influenza [below the seasonally-adjusted epidemic threshold of 6.0%]

  • No additional pediatric deaths were reported last week. Two pediatric deaths have been reported this season due to influenza; one death was due to influenza A[H1N1] and one to an influenza A virus for which no subtyping was performed.

Global News (from the WHO/CDC):

Zika: 5,734 cases have been reported in the U.S. with 2,493 cases in pregnant women. Wisconsin has had no cases in 2018.

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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