The 2025/2026 seasonal COVID vaccination was associated with a roughly 50% reduced risk for illness up to 9 weeks post-vaccination, especially in older Canadian adults, according to an interim analysis by the Canadian Sentinel Practitioner Surveillance Network. The seasonal vaccine, which was specifically formulated to target the LP.8.1 variant, also appeared to offer protection from other strains.

An important aspect of the 2025/2026 season was the simultaneous circulation of influenza and other respiratory viruses, which suggested the need to consider the role that respiratory virus co-circulation might play in estimating vaccine effectiveness (VE).

photo of Danuta Skowronski — Danuta M. Skowronski, MD

“This year, we were especially interested to check the effects of co-circulating respiratory viruses (eg, influenza and respiratory syncytial virus [RSV]) since COVID circulated at only low levels through the current season,” lead author Danuta M. Skowronski, MD, epidemiology lead for influenza and emerging respiratory pathogens at the BC Centre for Disease Control in Vancouver, told Medscape News Canada.

“From a [VE] perspective, it’s important to be cognizant of the potential impact of those co-infections to get to the true benefits that immunization might be providing,” she added. Skowronski also is clinical professor in the School of Population and Public Health at the University of British Columbia in Vancouver.

The study was published on May 7 in Eurosurveillance.

COVID VE in Robust Respiratory Virus Seasons

The researchers examined self-reported influenza, COVID, and RSV vaccination status at specimen collection in 5420 patients, 3802 of whom constituted the final sample for the analysis. Patients were aged 12 years or older and visited their physicians for acute respiratory illness in British Columbia, Ontario, and Quebec between October 2025 and March 2026. Provincial authorities provided information on COVID vaccine status.

The analysis relied on a test-negative design: a broadly used, observational, case-control strategy that allows researchers to compare patients who seek medical care and test positive for a certain disease with those seeking care for similar symptoms who test negative. VE is measured based on vaccination status.

“The main advantage of the test-negative design is that it standardizes for health-seeking behavior in ways that other study designs don’t,” said Skowronski. “Because we are comparing vaccine coverage in cases vs noncases that still have a respiratory infection, it inherently takes care of other confounders that might influence VE estimates, such as sex, social interconnectedness, or children in the household.”

VE was assessed overall and adjusted by age (ages 12-49 years, 50-64 years, ≥ 65 years), province, and calendar time (biweekly). Whole-genome sequencing was conducted for 76% of patients tested positive for COVID to determine SARS-CoV-2 variants. Because of the intense influenza season, VE estimates further excluded co-circulating respiratory viruses in sensitivity analyses.

Overall, 9% (29/310) of patients vs 17% (577/3492) of control individuals were vaccinated at least 2 weeks before illness onset (including 4% vs 9% of 12- to 64-year-olds and 21% vs 38% of ≥ 65-year-olds). Primary findings showed a 48% VE at median 9 weeks post-vaccination. VE was 44% in 12- to 64-year-olds and 53% among ≥ 65-year-olds). VE estimates remained stable and within 3% of primary analyses for the older patients when other respiratory viruses were considered and within 4% of the primary analyses for patients aged 12-64 years.

Excluding influenza virus and other respiratory infections from control individuals had minimal impact on VE estimates in ≥ 65-year-olds but increased COVID VE estimates among co-infected 12- to 64-year-olds.

Limitations of the study included low SARS-CoV-2 activity and vaccine coverage, which might have compromised statistical power and precision, especially in subset analyses. The sample of COVID-vaccinated individuals aged 12-64 years was small, so findings in this group should be interpreted cautiously.

A Call to Clinicians

“COVID has morphed into an annual virus that changes every year, so these ongoing assessments are needed to understand how well our COVID vaccines are working,” Allison McGeer, MD, professor of laboratory medicine and pathobiology at the Dalla Lana School of Public Health at the University of Toronto in Toronto, told Medscape News Canada. She was not involved in the research.

photo of Allison McGeer — Allison McGeer, MD

“All of us were a bit worried about the degree of protection against drifted SARS, COVID, two strains this winter, and the analysis can provide some additional data on how different strains can still be reasonably well covered by the vaccine,” said McGeer. The XFG lineage predominated, accounting for 70% (164/235) of sequenced viruses, followed by NB.1.8.1 (20%, 46/235) and no LP.8.1.

Though the interim analysis has shown that for the most part, COVID VE has remained stable over the past few years, substantial pockets of the Canadian population remain unvaccinated or have not received boosters.

“Healthcare providers have a radically important role in getting people vaccinated. I keep telling them that they don’t have to administer the vaccines; you can tell people at parties they should get their vaccine. It’s the single biggest influence on whether people get vaccinated,” said McGeer.

The interim analysis was funded by the BC Ministry of Health, Public Health Ontario, the Ministère de la Santé et des Services sociaux du Québec, the Canadian Institutes of Health Research, the Public Health Agency of Canada, and the Pacific Public Health Foundation. Skowronski reported having no relevant financial relationships. McGeer reported honoraria for advisory board participation and webinar presentation on behalf of AstraZeneca, GSK, Leyden Labs, Merck, Moderna, Roche, Pfizer, Sanofi, Seqirus, and Shionogi and participation on a data safety monitoring board or advisory board for Moderna and Ciara.

Liz Scherer is an independent health and medical journalist. She frequently reports on Canadian and European health news.

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Seasonal COVID Vaccine Linked With Halved Infection Risk