Start of Influenza Season Declared in Ottawa 2025-2026

To: Ottawa Physicians and Nurse Practitioners
Date: December 4, 2025
From: Dr. Laura Bourns, Deputy Medical Officer of Health
Subject: Start of Influenza Season Declared in Ottawa

Dear Colleagues:

Ottawa Public Health (OPH) is now declaring the start of the 2025-2026 influenza season in Ottawa.

For the week ending November 29, the percent of respiratory lab specimens positive for influenza was 20.5.%. The week prior, the percent of respiratory lab specimens positive for influenza was 8.6%. Influenza season is declared when the percent positivity for influenza exceeds 5% two consecutive weeks. In addition to a rising proportion of tests positive for influenza, hospitalizations for influenza have also been increasing for the past three weeks.

Influenza Epidemiology- 2025-2026 Respiratory Season to Date:

• A total of 157 of laboratory-confirmed influenza cases have been reported in Ottawa to date this season. Of these cases 97.5% (153/157) have been influenza A
• Provincially and nationally, the majority of influenza cases to date are influenza A with some influenza B cases also reported. To date, influenza A(H3N2) is the dominant subtype, among sub-typed influenza A specimens.

Influenza vaccine effectiveness

• Initial estimates of influenza vaccine effectiveness in Canada, typically aren’t available until later in the season.
• Influenza genetic characterization results from the National Microbiology Lab (NML) show that of H3N2 samples characterized 74% (28/38) are subclade K, which this year’s vaccine may have reduced effectiveness against. Despite the potential mismatch of the H3N2 strain contained in the vaccine and the H3N2 subclade K, vaccination remains a key tool for protection against influenza, including severe outcomes from influenza.
• Even with a potential mis-match vaccination is likely to provide some cross-protection, and the vaccine contains a H1N1 component as well as an influenza B component.
• Early season estimates from the UK for, vaccine effectiveness against influenza-related emergency department attendances and hospital admissions were 72–75% in children and adolescents (< 18 years) and 32–39% in adults.

Influenza Resources

• A summary of influenza resources for health care professionals can be found on OPH’s Influenza webpage for Health Care Professionals, as well information on fall respiratory vaccines (influenza, COVID-19, and RSV) in a recent Public Health Alert.
• Ontario Health recently published a new guidance document: Recommendations for Antiviral Therapy of Seasonal Influenza included in their Influenza Resources page
• Up-to-date local respiratory surveillance is available at OttawaPublicHealth.ca/VirusReport
• Resources for patients and their families can be found at OttawaPublicHealth.ca/RespVirus.