Update on Seasonal Influenza Immunization

As the 2025-2026 respiratory virus season begins, influenza infections and transmissions are expected to increase around the community, significantly impact human health, and place increased strain on the health care system. Based on trends that are being observed globally, Influenza A(H3N2) may circulate in Canada at higher levels than the previous two seasons. In addition, an influenza A(H3N2) subclade K, has emerged and is showing signs of being a mismatch to the vaccine strain with immune evasion potential, but the extent of the impact of this subclade remains uncertain. Regardless of what influenza strain is circulating in any given season, vaccination remains the best protection against influenza-associated morbidity and mortality.

Background

• Although influenza A(H1N1)pdm09 and A(H3N2) viruses have co-circulated for the past couple of influenza seasons, Canada has not had a major influenza A(H3N2) epidemic since the early 2022- 2023 season.
• After the World Health Organization (WHO) selected a J.2 subclade for A(H3N2) component of the 2025-2026 northern hemisphere influenza vaccine, a further evolved A(H3N2) variant called subclade K emerged during the tail-end of the southern hemisphere 2025 influenza season.
  • The subclade K has multiple mutations that may reduce vaccine effectiveness but do not directly affect virulence or response to antiviral medications.
  • Antigenic characterization data are limited to date, but of the subclade K viruses characterized all have been considered antigenically mismatched relative to the 2025-26 vaccine strain.
• Early surveillance signals include an early sharp rise in A(H3N2) detections reported from the United Kingdom (UK), Japan and other countries. In the UK more than 85% of sequenced A(H3N2) viruses have been subclade K this season.
• The BCCDC Public Health Laboratory reported that from Aug 24 to Oct 19, 75% of subtyped influenza A samples have been H1; however, in recent weeks, the proportion of H3 increased with sequencing of a small subset to date confirming subclade K is present in BC. Influenza Strain Characterization in Preparation for the 2025/2026 Influenza Season
• A(H3N2) is typically associated with higher case numbers and may cause additional impact on human health and health care system. The addition of a vaccine mismatched A(H3N2) subclade K with immune evasion potential may further increase the number of cases this season. The extent of the impact of this subclade remains uncertain as it is too early in the season to have sufficient data to assess.

Implications for the 2025/26 respiratory season

• Influenza seasons are challenging to predict, but global and BC trends so far signal that increased circulation of Influenza A(H3N2), combined with the emergence of the A(H3N2) subclade K, may lead to more overall infections and corresponding increase in the absolute number of severe outcomes compared to prior seasons.
• Because of their generally greater immunological susceptibility and social contacts, children typically experience the highest attack rates.
• In general, older adults often experience worse outcomes during A(H3N2) epidemics while people of all ages with underlying health conditions remain at increased risk for severe outcomes.

Recommendations to prevent influenza infection and transmission

We are asking clinicians to:

• Continue recommending influenza vaccination, especially to those at high risk for severe complications and individuals who may transmit influenza to them
  • The influenza vaccine protects against three strains (influenza A(H1N1), A(H3N2) and B), and even a mismatched vaccine can provide some cross-protection, especially against severe outcomes. Despite the emergence of A(H3N2) subclade K, vaccination remains the best defense against influenza this season.
  • Discuss with patients the benefits and possible side effects of the influenza vaccine, the risks of not being vaccinated, and address the questions they may have about A(H3N2) subclade K.
  • Recommend other vaccines that protect against respiratory disease at the same time, including COVID-19 vaccine and pneumococcal vaccine, as appropriate.
• Receive influenza vaccine yourself
• Encourage the use of other personal protective measures, which can include:
  • Cleaning hands regularly
  • Covering coughs and sneezes
  • Masking when in crowded indoor spaces or symptomatic
  • Staying home when sick
  • Regularly cleaning and disinfecting high-touch surfaces

Resources for More Information

• BCCDC, 2025. Respiratory Virus Data
• BCCDC, 2025. Influenza Strain Characterization in Preparation for the 2025/2026 Influenza Season
• Fraser Health, 2025. 2025-2026 Seasonal Influenza and COVID‐19 Vaccine Program