TABLE 4

Adjusted RH linear negative binomial regression models exploring the relationship of environmental factors with detection of influenza A and B viruses and all influenza virusesa

Demographic or climatic factorAdjusted IRR (95% CI) for the linear RH model
All influenza virusesInfluenza A virusInfluenza B virus
Age group (yr)
    65+1.001.001.00
    <11.29 (0.96–1.73)1.36 (0.94–1.97)*1.01 (0.59-1.72)
    1–42.8 (2.11–3.71)*2.55 (1.78–3.65)*2.76 (1.66-4.58)*
    5–196.49 (4.63–9.10)*4.80 (3.11–7.42)*6.98 (3.83-12.7)*
    20–641.65 (1.36–1.99)1.69 (1.32–2.17)*1.49 (1.06-2.10)*
Outbreak status
    Yes1.001.001.00
    No0.12 (0.09–0.16)*0.11 (0.07–0.15)*0.19 (0.11-0.32)*
RH0.99 (0.98–1.00)1.03 (1.02–1.04)*0.94 (0.93–0.95)*
Wind speed1.01 (0.99–1.04)1.00 (0.97–1.03)1.03 (1.00–1.07)
Temp0.93 (0.92–0.94)*0.91 (0.90–0.93)*0.94 (0.93–0.96)*
Temp fluctuation1.04 (1.02–1.05)*0.99 (0.97–1.02)1.09 (1.05–1.12)*
Temp · temp fluctuation1.00 (1.00–1.00)*1.00 (0.99–1.00)1.00 (0.99–1.00)
  • a AH, absolute humidity; RH, relative humidity; WS, wind speed; IRR, incidence relative risk, CI, confidence interval. The weekly median was used to measure AH, RH, WS, and temperature. The estimate is considered significant (*) when the 95% confidence interval does not cross 1. An incidence relative risk of 1.00 indicates that the category was used as a reference/comparison. The total weekly numbers of positive influenza A and B counts were used as dependent variables. The left column indicates all independent/predictable variables for which this model was adjusted. Temperature fluctuation represented the difference in median temperatures between the exposure day and the previous day. In this model, the weekly median of this measurement was included. Temperature · temperature fluctuation represents the interaction term for which the model was adjusted. These models include measurement of climatic factors and detection of influenza A and B viruses and all influenza virus-positive specimens for 293 unique weeks.