# Association between seasonal respiratory virus activity and invasive pneumococcal disease in central Ontario, Canada

## TL;DR

This study explores how seasonal respiratory viruses like influenza and RSV affect the risk of invasive pneumococcal disease in central Ontario.

## Contribution

The study is the first in Canada to examine interactions between influenza, RSV, and IPD risk.

## Key findings

- Influenza A and B both increase IPD risk independently.
- Co-circulation of influenza A and B reduces their individual impacts on IPD.
- RSV increases IPD risk only when influenza A or B is also active.

## Abstract

In central Ontario, influenza, respiratory syncytial virus (RSV), and invasive pneumococcal disease (IPD) follow similar seasonal patterns, peaking in winter. We aimed to quantify the independent and joint impact of influenza A, influenza B, and RSV on IPD risk at the population level.

We used a 2:1 self-matched case-crossover study design to evaluate acute effects of respiratory virus activity on IPD risk. This design ensures that effects are not confounded by within-individual characteristics that remain constant over short periods of time. We included 3,892 IPD cases occurring between January 2000 and June 2009. Effects were measured using univariable and multivariable conditional logistic regression. Multivariable models included environmental covariates (e.g., temperature, absolute humidity, and UV index) and interaction terms between viruses.

Influenza A activity and influenza B activity were both independently associated with increased IPD risk; however, co-circulation of influenza A and B reduced the impact of both viruses. RSV activity was positively associated with increased IPD risk only in the presence of increased influenza A or influenza B activity.

To our knowledge this represents the first study to consider the impact of interactions between these viruses on IPD risk in Canada. Our findings suggest that the prevention of IPD should be considered as a potential health benefit of influenza and RSV vaccination programs.

The online version contains supplementary material available at 10.1186/s12879-025-12419-8.