Impact of case definition on respiratory virus surveillance: findings of a 12-year prospective study

Issue

In Québec, Canada, hospitalisations and complications due to acute respiratory infections (ARI) are monitored through the HospiVir program, a prospective hospital-based surveillance network with systematic multiplex PCR testing among admitted patients since 2012-13. In response to the COVID-19 pandemic, the case definition (CD) was revised to incorporate SARS-CoV-2-specific symptoms alongside those of other respiratory viruses (RV). Evaluation of RV surveillance systems is essential to ensure the validity of data used to inform public health decision-making and immunization policies.

Problem description

We assessed the impact of the CD change on viral positivity, overall and by age group, by using logistic regression models with adjustment for age, sex, comorbidities, living environment and surveillance period. Viral positivity was compared before (2012-13 to 2018-19) and after (2022-23 to 2024-25) the change in CD (i.e., from influenza-like illness to an adjusted ARI definition from the Canadian Nosocomial Infection Surveillance Program).

Results

28,165 participants (median age 63; 49% female) were included. Overall positivity (any RV) declined from 70% to 58%. The revised CD was associated with 2-fold lower odds of positivity (adjusted odds ratio=0.48, 95% CI:0.44–0.53). Similar declines were observed in all age groups. Decreased positivity was found for most viruses, notably influenza A H3N2 and B (4-fold decreases) and RSV (2.5-fold). It remained unchanged for influenza A H1N1 and showed only modest reductions for coronavirus and rhino/enterovirus.

Lessons

As expected, changes in CD significantly influence viral positivity, with a differential impact on various RVs. For public health practice, CDs should align with surveillance objectives, and methodological changes must be carefully evaluated, as they can substantially affect the interpretation and comparability of overall and pathogen-specific RV trends over time.

Key messages

• Methodological changes in ARI surveillance, particularly case definition, substantially alter measured viral positivity independent of real epidemiologic trends.

• Routine evaluation of surveillance definitions is critical to ensure data used to inform public health policy remain valid, comparable, and aligned with evolving epidemiologic contexts.