Weekly Influenza Update

January 10, 2019

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 continues to have low, but increasing influenza activity at present. The majority of detections (63%) have been Influenza A[H1N1].  As of December 29, 2018, there had been 137 influenza-related hospitalizations since September 1, 2018; 49% of hospitalizations have been in individuals age ≥ 65 years. (As a point of reference, Wisconsin had tailed 1,328 hospital admissions at the same point last year).

The prevalence of influenza-like illness [fever of 100oF or higher and either cough or sore throat] in Wisconsin's primary care patients is 2.1% and is slowly rising.

11.2% 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 1.7%; the most common pathogen is norovirus.

Primary Care Snapshot

The most commonly identified viral causes of acute respiratory infections (ARI) in Wisconsin is parainfluenza. Over the past 4 weeks the typical ARI case presenting for primary care has been 35.5 years old and 71% of patients have been female. 60% of patients identified a sick contact 1-to-3 days before illness onset and typically present to the clinic 4.3 days after illness onset. 24% of illnesses are characterized as mild, with 71% having moderate symptoms and 2% having severe symptoms.

The typical symptoms reported include:

Typical SymptomsPercentViruses in CirculationPercent
fever67Influenza A20
cough86Influenza B10
sore throat57Coronavirus0
nasal congestion69RSV0
nasal discharge49Parainfluenza30

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 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

  • 106 influenza A[H3N2], 240 influenza A[H1N1] and 43 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, coronavirus and parainfluenza co-circulating in Wisconsin. RSV activity is increasing.

Across the US:

3,636 (13.7%) respiratory specimens during week 52 (December 23-29) were positive for influenza.

For the 2018-2019 season to date (last week):

  • 92.7% (97.1%) of subtyped isolates have been type A
    • 18.6% (10.6%) of A viruses have been H3N2
    • 81.4% (89.4%) of all sub-typed A viruses have been 2009 H1N1
  • 7.3% (2.9%) of isolates have been type B
    • 63.3% (25.0%) were of the Yamagata lineage (not in the trivalent vaccine)
    • 36.7% (75.0%) were of the Victoria lineage
  • 6.1% of deaths during week 51 (December 16-22) were due to pneumonia or influenza [below the seasonally-adjusted epidemic threshold of 6.9%]

  • Two additional pediatric deaths were reported last week. Thirteen pediatric deaths have been reported this season due to influenza; eight deaths were due to influenza A[H1N1], one due to A[H3N2], three due to an influenza A virus for which no subtyping was performed, and one due to influenza B.

Global News (from the WHO/CDC):

Ebola: On 1 August 2018, the Ministry of Health of the Democratic Republic of the Congo declared a new outbreak of Ebola virus disease in North Kivu Province. As this is unfolding in a war zone, security incidents and pockets of community resistance continue to impact civilians and frontline workers, requiring the response to continually adapt to the situation. This is now the 2nd largest Ebola outbreak in history.

Latest numbers as of 27 December 2018

Total cases: 627 (confirmed = 579, probable cases = 48)
Deaths: 382 (confirmed = 334, probable deaths = 48)

* 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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