Influenza Update

November 30, 2023


Influenza activity is rising in Wisconsin and is dominated by influenza A (H1N1). Other than SARS-CoV-2, we are seeing significant levels of rhinovirus and parainfluenza viruses.   

Across the U.S. there were 2,584 (4.9%) influenza detections at clinical laboratories, from the 52,451 specimens collected last week. Of detected viruses at public health laboratories, 79.4% were influenza A and 20.6% were influenza B. Among influenza A viruses, 20.7% were H3 and 79.3% were H1. CDC has reported an excellent match between circulating and vaccine strains (H3: 100%; H1: 100%; B: 100%) for this season. As of 11/27/2023, CDC estimates that there have been at least 1.2 million illnesses, 12,000 hospitalizations, and 740 deaths from flu. The percentage of primary care patients with influenza-like illness (ILI) is 4.9% and is above baseline. ILI activity is high in 10 states. Three pediatric deaths, due to influenza A (1) and influenza B (2), have been reported for the 2023-2024 season to date.  

Wisconsin has recorded 70 hospitalizations for influenza this season, with 5 individuals admitted to ICU and one needing mechanical ventilation. This is higher than in 2019/2020 at this point (46) and lower than 2022/2023 (199).  

The use of rapid influenza testing at this time is tenuous at this time based on low levels of circulating virus.  RIDT(+) results should be confirmed using RT-PCR. Use of rapid COVID-19 testing is reasonable and positive results do not need confirmation.


Primary Care Snapshot

The most common viral causes of acute respiratory infections in Wisconsin primary care practices are rhinoviruses and RSV. For the week ending November 18, 2023, 14.3% of 13,025 specimens tested across Wisconsin and compiled by the Wisconsin State Laboratory of Hygiene and clinical labs were positive for SARS-CoV-2. About 1.3% of primary care patients are presenting with acute diarrheal illness. The most commonly identified gastopathogens are norovirus and Campylobacter.

Over the past 4 weeks the typical ARI case has been 19.8 years old; 83% of patients have been female. 86% of patients identified a sick contact 1-3 days before illness onset and they typically present to the clinic 3.9 days after illness onset. 56% of illnesses are characterized as mild, with 44% having moderate symptoms and 0% having severe symptoms.

Viruses in CirculationPercent* in statewide laboratory surveillancePercent** in primary care surveillance clinics
Influenza A14.414%
Influenza B2.5%0%
Seasonal Coronavirus***0.00%


The 7-day average for patients hospitalized for COVID-19 in Wisconsin is 348 (rising), with a 7-day average of 35 (10.1%) requiring intensive care.  


COVID-19 Vaccine

Across Wisconsin, 609,778 individuals (10.3% of the population) have received the updated 2023/2024 COVID-19 vaccine.


Across the US:

  • 7.7% of all deaths during week 46 (Nov. 12-18) were due to pneumonia, influenza, or COVID-19, and above the seasonal, epidemic threshold. 
  • Forecasting by the CDC indicates a stable level of COVID-19 hospitalization over the next four weeks. 
  • Variants: the national proportions of variants for the week ending November 25, 2023, include: HV.1 (31.7%); EG.5 (13.1%), BA.2.86 (8.8%), JD.1.1 (7.7%), HK.3 (7.4%), FL.1.5.1 (6.4%), and JG.3 (5.3%).  SARS-CoV-2 continues to be a rapidly diversifying virus.

* 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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