Influenza Update

Influenza activity is HIGH and declining in Wisconsin.

The CDC estimates that there have been at least 40 million illnesses, 520,000 hospitalizations, and 22,000 deaths from flu so far this season. Across the U.S. there were 18,329 (18.9%) influenza detections at clinical laboratories, from the 96,763 specimens collected last week. Of detected influenza viruses at public health laboratories, 96.0% were influenza A and 4.0% were influenza B. Among influenza A viruses, 45.6% were H3, 54.4% were H1, and 0% were H5. CDC has reported current match estimates between circulating and vaccine strains for this season (H3: 55%; H1: 99%; B: 100%). The percentage of primary care patients with influenza-like illness (ILI) has decreased to 4.9% and is well above baseline. ILI activity is very high in 9 states, high in 21 states, moderate in 10 states, low in 4 states; Wisconsin has high levels. 114 pediatric deaths (influenza A: 106); influenza B: 8) have been reported for the 2024-2025 season to date.  

Wisconsin has recorded 5,867 hospitalizations for influenza this season. This is above the level in 2023-2024 (3,900) at this point.  

The performance of rapid influenza testing is reasonable because of high levels of circulating virus.

• RIDT(+) results and RIDT(-) results can be trusted

The performance of rapid SARS-CoV-2 testing is moderate because of moderate levels of circulating virus.

• RSDT(+) results and RSDT(-) results can be trusted

• Only 33.7% of Wisconsinites have received influenza vaccine this season.
  • See: https://www.dhs.wisconsin.gov/immunization/influenza.htm
• Only 16.5% of Wisconsinites have received an updated COVID-19 vaccine this season.
  • See: https://www.dhs.wisconsin.gov/immunization/covid-19-vaccine-data.htm

Interim Estimate of Influenza Vaccine Effectiveness 

• See: https://www.cdc.gov/mmwr/volumes/74/wr/mm7406a2.htm
• Children & Adolescents - Outpatient ARI (32 - 60%); Hospitalization (63-78%)
• Adults - Outpatient ARI (36-54%); Hospitalization (41 - 55%)

There have been 70 confirmed and 7 probable cases (total = 77) detected in 14 states in 2024/2025. Cases have been linked to dairy cattle (42), poultry (29), other animals (2), and unknown (4). There have been 4 hospitalizations one death due to H5N1. The current public health risk is reported to be low.

https://www.cdc.gov/flu/hcp/antivirals/summary-clinicians.html

Antiviral treatment is recommended as soon as possible for any patient with suspected or confirmed influenza who:

• is hospitalized
• has severe, complicated, or progressive illness
• is at higher risk for influenza complications.

Decisions about starting antivirals for patients with suspected influenza should not wait for laboratory confirmation. Empiric antiviral treatment should be started as soon as possible in the above priority groups. Clinicians can consider early empiric antiviral treatment of non-higher-risk outpatients with suspected influenza based upon clinical judgment if treatment can be initiated within 48 hours of illness onset.

• For hospitalized patients with suspected or confirmed influenza, initiation of antiviral treatment with oral or enterically administered oseltamivir is recommended as soon as possible.
• For outpatients with complications or progressive disease and suspected or confirmed influenza (e.g., pneumonia, or exacerbation of underlying chronic medical conditions), initiation of antiviral treatment with oral oseltamivir is recommended as soon as possible.
• For outpatients with suspected or confirmed uncomplicated influenza, oral oseltamivir, inhaled zanamivir, intravenous peramivir, or oral baloxavir may be used for treatment, depending upon approved age groups and contraindications.

CDC Health Alert - Expanding Measles Outbreak

See: https://www.cdc.gov/han/2025/han00522.html

A large outbreak of measles is occurring in west Texas and New Mexico with 253 reported cases, a 14% hospitalization rate, and two deaths of in unvaccinated persons.

For more information on prevention and treatment, see: https://www.cdc.gov/han/2025/han00522.html

Please note: Consistent with guidance from the American Academy of Pediatrics, vitamin A may be administered to infants and children in the United States with measles as part of supportive management. Children with severe measles, such as those who are hospitalized, should be managed with vitamin A. Vitamin A should be administered under the supervision of a healthcare provider and is not a substitute for vaccination.

Viruses associated with acute respiratory infections in Wisconsin primary care practices have been dominated by Influenza A and RSV.  Influenza detections have peaked, RSV detections are stable, and SARS-CoV-2 detections are decreasing. For the week ending 2/22/2025, 4.7% of 18,461 specimens tested across Wisconsin by the Wisconsin State Laboratory of Hygiene and clinical labs were positive for SARS-CoV-2. The most commonly identified gastopathogen is norovirus.

Over the past 4 weeks the typical ARI case has been 44.2 years old. 86% of patients have been female. 30% of patients identified a sick contact 1-3 days before illness onset and they typically present to the clinic 4.4 days after illness onset.  25% of illnesses are characterized as mild, with 75% having moderate symptoms and 0% having severe symptoms.

 *estimate based on WSLH statewide data

** estimate based on primary care patients seen at five clinics in Dane County 

*** includes HKU1, NL63, 229E and OC43