Weekly Influenza Update

December 14, 2017


Wisconsin influenza activity is increasing. We are seeing outbreaks in schools and more cases in clinics. Influenza A[H3N2] has been the dominant strain so far in Wisconsin, comprising 86% of all tested viruses. There have been 210 influenza-related hospitalizations since September 1, 2017; 65% of hospitalizations have been in individuals aged ≥65 years. There have been 31 admissions to ICUs, 48% were aged ≥65 years; and there have been 7 cases requiring mechanical ventilation, 43% aged ≥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 2.5% and is increasing.

8.8% 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 causes of Acute Respiratory infections (ARI) in Wisconsin is rhinovirus and Parainfluenza. Over the past 4 weeks the typical ARI case presenting for primary care has been 34.0 years old and 60% 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 4.5 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
fever60Influenza A25
cough80Influenza B0
sore throat71Coronavirus0
nasal congestion56RSV0
nasal discharge47Parainfluenza13

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 the current vaccine had low overall efficacy during the 2017 Australian influenza outbreak
  • 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 moderate 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

  • 243 influenza A[H3N2], 48 influenza A[H1N1] and 59 influenza B viruses have been tested. No viruses (0.0%) were resistant to oseltamivir, zanamivir or 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.


  • Rhinovirus, parainfluenza and coronaviruses are co-circulating in Wisconsin.
  • RSV activity is increasing.

Across the US:

1355 (6.7%) respiratory specimens during week 48 (November 26-December 2) were positive for influenza.

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

  • 74.1% (76.3%) of subtyped isolates have been type A
    • 90.6% (90.5%) of A viruses have been H3N2
      • 9.4% (9.5%) of all sub-typed A viruses have been 2009 H1N1
  • 25.9% (23.7%) of isolates have been type B
    • 94.6% (95.2%) of B viruses have been of the Yamagata lineage
      • 5.4% (4.8%) of B viruses have been of the Victoria lineage
  • 5.7% of deaths during week 46 (November 12-18, 2017) were due to pneumonia or influenza (below the seasonally adjusted epidemic threshold of 6.6%).
  • Seven pediatric deaths has been reported this season due to influenza: 3 deaths were due to A[H3N12], 2 deaths were due to A[H1N1], one death due to an A virus for which no subtyping was performed, and one death due to influenza B.

Global News (from the WHO/CDC):

Zika: 5,601 cases have been reported in the U.S. with 2,311 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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