Weekly Influenza Update

March 21, 2019

Influenza peaking


Wisconsin is at near peak influenza activity at present.  There has been a shift in Wisconsin from influenza A[H1N1] to A[H3N2].  Also, there has been a shift in influenza A[H3N2] viruses to a clade (smaller genetic clustering of viruses) that is not well covered by this year’s vaccine.  As of March 9, 2019, there had been 1,638 influenza-related hospitalizations since September 1, 2018; 46% of hospitalizations have been in individuals age ≥ 65 years; 295 individuals have been admitted to ICUs (38% with age ≥ 65 years) and 85 have required mechanical ventilation (40% with 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 3.4% and is nearing its peak.

10.6% 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 1.8%; the most common pathogen is norovirus.

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 31.7 years old and 56% of patients have been female. 55% 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. 25% of illnesses are characterized as mild, with 65% having moderate symptoms and 8% having severe symptoms.

The typical symptoms reported include:

Typical SymptomsPercentViruses in CirculationPercent
fever69Influenza A27
cough83Influenza B0
sore throat60Coronavirus32
nasal congestion60RSV14
nasal discharge65Parainfluenza0

Clinical Notes


  • Influenza vaccine is recommended universally - keep vaccinating 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 high
  • NPV of rapid antigen tests at this time is high


Antivirals need to be started within 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

  • 519 influenza A[H3N2], 791 influenza A[H1N1] and 167 influenza B viruses have been tested. Two viruses (0.3%) were resistant to oseltamivir and peramivir, but not zanamivir.
  • 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 coronavirus are co-circulating across Wisconsin. RSV activity is declining.

Across the US:

10,591 (25.8%) respiratory specimens during week 10 (March 3-9) were positive for influenza.

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

  • 96.3% (97.2%) of subtyped isolates have been type A
    • 37.2% (61.3%) of A viruses have been H3N2
    • 62.8% (38.7%) of all sub-typed A viruses have been 2009 H1N1
  • 3.7% (2.8%) of isolates have been type B
    • 52.0% (20.0%) were of the Yamagata lineage (not in the trivalent vaccine)
    • 48.0% (80.0%) were of the Victoria lineage
  • 7.2% of deaths during week 9 (February 24 - March 2) were due to pneumonia or influenza [below the seasonally-adjusted epidemic threshold of 7.3%]

  • 4 additional pediatric deaths were reported last week. 68 pediatric deaths have been reported this season due to influenza; 34 deaths were due to influenza A[H1N1], six due to A[H3N2], 25 due to an influenza A virus for which no sub-typing was performed, and three due to influenza B.

Global News (from the WHO/CDC):

Measles Alert

From January 1 to March 14, 2019, 268 individual cases of measles have been confirmed in 15 states. The states that have reported cases to CDC are Arizona, California, Colorado, Connecticut, Georgia, Illinois, Kentucky, Michigan, Missouri, New Hampshire New Jersey, New York, Oregon, Texas, and Washington. According to the American Academy of Pediatrics, “at least 20 states have introduced bills this year that would broaden the reasons why parents can exempt kids from getting vaccines even if there isn’t a medical need.”


As the Democratic Republic of the Congo Ebola outbreak is unfolding in a war zone, security incidents and pockets of community resistance continue to impact civilians and front line workers, requiring the response to continually adapt to the situation. This is now the 2nd largest Ebola outbreak in history.

Latest numbers as of 19 March 2019

Total cases: 980 (confirmed = 915, probable cases = 65)
Deaths: 610 (confirmed = 545, probable deaths = 65)

Case Fatality Rate: 62.2%

* 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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