An interim analysis published on Eurosurveillance estimates that the COVID-19 vaccine for the 2025/26 season reduced the risk of illness in Canada by about half at about 9 weeks after vaccination, offering protection beyond the vaccine’s target strain.

Novel_Coronavirus_SARS-Cov-2_(50011008502)

Source: NIAID

Colorized scanning electron micrograph of an apoptotic cell (green) heavily infected with SARS-COV-2 virus particles (purple), isolated from a patient sample.

Seasonal COVID-19 vaccination is recommended in Canada as in several other countries for people older than 6 months with an increased risk of severe illness from the infection, and for all adults aged 65 or older. In the 2025/26 respiratory virus season, the vaccine was updated to target the LP.8.1 strain as recommended by the World Health Organization (WHO), but other strains were also circulating during that period.

The research was conducted through the Canadian Sentinel Practitioner Surveillance Network, which sought to perform an early estimate of COVID-19 vaccine effectiveness for the 2025/26 season and to explore the effect that influenza and other respiratory infections might have on these estimates.

Skowronski et al. analysed data from respiratory samples collected from 5,400 patients aged 12 years and older who presented with acute respiratory illness between October 2025 and March 2026 in three Canadian provinces. The COVID-19 vaccination status for patients was extracted from provincial registries, and the samples were tested for COVID-19 and other respiratory illnesses.

Vaccine effectiveness

The authors assessed vaccine effectiveness with a test-negative study design. They compared vaccination rates among patients seeking care for respiratory symptoms who had laboratory-confirmed COVID-19, with the rates of patients who also sought care but  tested negative and served as controls. This comprised a total of 310 COVID-19 cases and 3,492 controls.

To determine the SARS-CoV-2 variants involved in infections, whole genome sequencing of the virus was performed for 76% of COVID-19 patients. To judge the possible effect of influenza and other respiratory virus infections, vaccine effectiveness was also estimated by excluding these infections from findings for COVID-19 cases and/or controls, in light of a particularly intense influenza season.

Vaccine especially beneficial for older adults

The study found that the vaccine was most beneficial for the groups it targets. The vaccine was shown to be 48% effective (95% confidence interval: 21─66) at a median of 9 weeks after vaccination. Vaccine effectiveness was estimated to be slightly higher for those aged 65 years or older compared to those between 12 and 64, but with overlapping confidence intervals: 53% (95% confidence interval: 21─73) vs 44% (95% confidence interval: 12─72).

Genetic analysis also found that the 2025/2026 vaccine provided protection against virus variants that circulated beyond the vaccine’s target LP.8.1 lineage, including among older patients.

Excluding influenza infections from controls had little effect on effectiveness estimates among people aged 65 or older, but incremental exclusion of other respiratory infections increased vaccine effectiveness estimates among patients aged between 12 and 64. The authors suggest routinely testing for other respiratory illnesses in COVID-19 vaccine effectiveness studies, to help address the possible role of co-infections or other vaccine-preventable infections in the under-estimation of the protection offered by immunisation.

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