Influenza virus activity is associated with climate; however, less is known about how climate affects influenza subtype activity. Using population-level influenza-associated hospitalization surveillance data, we assessed the relationship between predominant influenza subtype and average temperature in Colorado.
Methods
We analyzed laboratory-confirmed influenza-associated hospitalizations reported between October 1, 2009 - March 28, 2026*, excluding the 2020-2021 respiratory virus season due to minimal flu activity. We calculated Colorado weekly average temperatures and 30-year climate normals using National Oceanic and Atmospheric Administration data. Temperature anomaly was defined as the difference between average temperature and climate normals. Predominant seasonal subtype was defined as having more than a 10-percentage point difference in the proportion of subtyped influenza specimens. Spearman rank tests estimated the correlation between temperatures (weekly and anomaly) and influenza hospitalization rates and counts (overall and subtyped).
*Through May 2026 at presentation
Results
Between 2009 and 2026, nine influenza seasons were H3N2-predominant and six were H1N1-predominant. Of the H3N2-predominant seasons, five (55.55%) occurred during hotter-than-average seasons; three H1N1-predominant seasons (50.00%) occurred within colder-than-average seasons. Average weekly temperature showed a significant negative correlation with both H1N1 rates (⍴ = -0.342, p
