Seasonal COVID-19 vaccine in 2025/26 cut illness risk by half in Canadian study

Interim analysis from the Canadian Sentinel Practitioner Surveillance Network has estimated that the 2025/26 seasonal COVID-19 vaccine reduced the risk of symptomatic infection by 48 per cent at nine weeks post vaccination, with evidence of protection against circulating variants beyond the vaccine’s LP.8.1 target

A seasonal COVID-19 vaccine used in Canada for the 2025/26 respiratory virus season has reduced the risk of symptomatic illness by about half, according to a recently published interim analysis.

The study estimated that the updated vaccine was 48 per cent effective against medically attended COVID-19 at a median of nine weeks after vaccination. Researchers also found evidence of protection beyond the vaccine’s intended target strain – LP.8.1 – against other severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants that circulated during the same period.

Seasonal COVID-19 vaccination is recommended in Canada – as in many countries – for people who are at increased risk of severe illness and for all adults aged 65 years or older. For the 2025/26 respiratory virus season, the vaccine was updated to target LP.8.1, in line with World Health Organization advice.

However, several other SARS-CoV-2 lineages also circulated during the study period which allowed researchers to assess whether the vaccine offered broader protection.

The research was conducted through the Canadian Sentinel Practitioner Surveillance Network. It aimed to provide an early estimate of COVID-19 vaccine effectiveness for the 2025/26 season and to examine whether influenza and other respiratory infections could affect those estimates. This was particularly relevant because the analysis took place during an intense influenza season which could have influenced comparisons between COVID-19 cases and test-negative controls.

The research led by Dr. Danuta Skowronski, a clinical professor in the University of British Columbia School of Population and Public Health, and her team analysed respiratory samples from 5,400 patients aged 12 years or older who presented with acute respiratory illness between October 2025 and March 2026 in three Canadian provinces. COVID-19 vaccination status was obtained from provincial registries while samples were tested for COVID-19 and other respiratory infections.

The researchers used a test-negative study design, which is a standard approach in vaccine effectiveness research. They compared vaccination rates among patients with laboratory-confirmed COVID-19 and patients who had respiratory symptoms but tested negative for COVID-19. The primary analysis included 310 COVID-19 cases and 3,492 controls.

To identify the SARS-CoV-2 variants responsible for infection, the researchers performed whole genome sequencing on viral samples from 76 per cent of COVID-19 patients. This allowed the team to determine whether infection had been caused by LP.8.1 or by other circulating variants. The researchers also assessed the possible effect of influenza and other respiratory virus infections by recalculating vaccine effectiveness after the incremental exclusion of these infections from COVID-19 cases and/or controls.

Overall, vaccine effectiveness was estimated at 48 per cent, with a 95 per cent confidence interval of 21 to 66 per cent, at a median of nine weeks after vaccination. The estimate was slightly higher among adults aged 65 years or older, at 53 per cent, with a 95 per cent confidence interval of 21 to 73 per cent, compared with 44 per cent among those aged 12 to 64 years, with a 95 per cent confidence interval of 12 to 72 per cent. Because the confidence intervals overlapped, the apparent difference between age groups should be interpreted with caution.

Genetic analysis indicated that the 2025/26 vaccine offered protection against SARS-CoV-2 variants beyond the LP.8.1 lineage it was designed to target. This finding is important because seasonal COVID-19 vaccines are selected before or early in a respiratory virus season, while SARS-CoV-2 continues to evolve.

The exclusion of influenza infections from the control group had little effect on vaccine effectiveness estimates among people aged 65 years or older. However, among patients aged 12 to 64 years, the incremental exclusion of other respiratory infections increased estimated vaccine effectiveness. The authors suggested that future COVID-19 vaccine effectiveness studies should routinely test for other respiratory illnesses to help account for co-infections or other vaccine-preventable infections that may otherwise lead investigators to underestimate vaccine protection.

The findings provide an early indication that the 2025/26 seasonal COVID-19 vaccine has given meaningful protection in Canada, particularly among older adults and other groups at increased risk of severe disease. They also support the continued use of sentinel surveillance networks and variant sequencing to monitor vaccine performance during respiratory virus seasons in which several pathogens and SARS-CoV-2 lineages may circulate at the same time.

For further reading please visit: 10.2807/1560-7917.ES.2026.31.18.2600331