During periods of seasonal influenza activity, we found moderately active (1.5–2.9 METs/day) and active (≥3.0 METs/day) individuals to be approximately 15% less likely to have an influenza-coded physician office or emergency department visit compared to inactive individuals. When stratified by age, we observed similar findings among individuals <65 years but not ≥65 years. The various sensitivity analyses conducted demonstrated the robustness of our findings. Among individuals <65 years, moderately active and active individuals were not more likely than inactive individuals to visit physicians for non-influenza-related conditions such as dermatitis or periodic health examinations during influenza season, suggesting that the observed protective effects of physical activity against influenza-coded outpatient visits are not from underlying differences in health status or healthcare seeking behaviour. However, moderately active individuals ≥65 years were more likely to have periodic health examinations during the influenza season, which suggests the presence of “healthy user" bias for this outcome within this age group. This was confirmed in a post hoc analysis indicating that both moderately active and active individuals were more likely to have periodic health examinations during non-influenza periods.
The protective effects of physical activity among younger individuals but not older adults may be explained by age-related changes in immune function. Aging is linked to declines in the ability to defend against pathogens 
, and has been associated with increased morbidity and mortality from infectious diseases in the elderly 
. Additionally, age-related declines in immune response to influenza vaccines are well documented 
. The reduced immune function of the elderly may prevent them from receiving any immune system benefits from physical activity.
To our knowledge, this is the first epidemiologic study that has examined the relationship between physical activity and influenza-related morbidity during seasonal influenza epidemics. Previous studies have mostly focused on upper respiratory tract infections (URTIs) with an emphasis on athletes 
, and only a few focused on the general population 
. Our finding of a 15% reduction in influenza-coded outpatient visits is similar to the 20% reduction in URTIs observed in population-based studies, although those studies used self-reported outcome measures 
. Only one other study assessed the association between physical activity and influenza, and the outcome was influenza-associated mortality 
. Although a beneficial effect was found, our study suggests a protective effect at a much earlier stage than mortality.
This study had several limitations. First, our outcome measure was influenza-coded outpatient visits rather than laboratory-confirmed influenza infections, which would be the most ideal outcome measure but is challenging to incorporate in population-based studies because influenza infection is infrequently confirmed. Although in another study only 50% of outpatient visits coded as influenza were actually laboratory-confirmed to be positive for influenza 
, we found that in Ontario these visits have very high specificity and reasonably high positive predictive value for PCR-confirmed influenza infection. Moreover, since this misclassification is likely to have occurred equally across the physical activity groups, our estimates would be biased towards the null, and therefore the results of this study most likely underestimate the true protective effect of physical activity against influenza infection. A second limitation is that measurement of physical activity and certain covariates relied on self-report, and verification of subject responses was not possible. However, prior studies have demonstrated the validity of these measures 
. Also related to the survey data, these measures may have changed over the study period and could not be updated at the start of each influenza season for included participants. However, our decision to restrict the analyses to individuals who had completed a survey within three years prior to the start of an influenza season was guided by literature that suggests physical activity levels remain stable over three years 
. A fourth limitation is that this study focused mainly on leisure time physical activity. Data on work-related physical activity was limited as both the NPHS and CCHS had only one question that addressed physical activity at work. A fifth limitation is that we were underpowered to more finely stratify our age categories in order to determine which specific age groups (e.g., teenagers, young adults, middle-aged individuals) contributed most to our finding of reduced influenza-coded outpatient visits in those <65 years. Additionally, although the MET value is ubiquitous throughout the physical activity literature, its use across age groups, particularly in the elderly, has not been properly validated 
. Finally, as with all observational studies, the observed associations might be attributed to residual confounding.
Linking population-based physical activity data to health administrative databases was one major strength of our study. Doing so allowed us to determine the effect of physical activity on influenza at the population level using a more objective outcome measure (physician visit for influenza) rather than patient self-report of acute respiratory symptoms. Additionally, we included multiple influenza seasons in our study, which is important since the severity of influenza season varies between years.
Although the benefits of physical activity in preventing chronic conditions are well established, its impacts on infectious diseases have been less clearly defined. The results of this study suggest that moderate to high amounts of physical activity may be associated with reduced risk of influenza for individuals <65 years. Future research should ideally use laboratory-confirmed influenza outcomes to confirm the association between physical activity and influenza infection. Public health authorities and clinicians should work toward a common goal of increasing physical activity and the public’s awareness of its benefits. These actions may help to mitigate the health and economic burden caused by influenza.