Estimation of the attributable fraction of respiratory viruses detection to illness
We used unconditional logistic regression to estimate the attributable fraction (AF) of respiratory viruses-associated hospitalization and outpatient consultation by comparing the respiratory viruses detection rate among ILI or SRI cases to those of controls (outcome variables in the models). The attributable fraction was estimated from the virus specific odds ratio (OR) obtained from the multivariable regression models as follows:
[1]
Subsequently we estimated the individual respiratory viruses detection rate associated with illness among ILI and SRI cases (InflDetectRateIll ) from the observed detection rate (InflDetectRateObs ) as follows:
InflDetectRateIll = InflDetectRateObs*AF [2]
This analysis was implemented overall and within the following age categories: <5, 5-44 and ≥45 years of age. All estimates were obtained from multivariable models that included the different respiratory viruses investigated in this study, HIV infection, and underlying medical conditions as covariates as previously described [11Hellferscee O, Treurnicht FK, Walaza S, et al. The Fraction of Rhinovirus Detections Attributable to Mild and Severe Respiratory Illness in a Setting of High Human Immunodeficiency Virus Prevalence, South Africa, 2013-2015. J Infect Dis.2019 ;219(11):1697-1704. doi: 10.1093/infdis/jiy725.,22Tempia S, Walaza S, Moyes J, et al. Attributable Fraction of Influenza Virus Detection to Mild and Severe Respiratory Illnesses in HIV-Infected and HIV-Uninfected Patients, South Africa, 2012-2016. Emerg Infect Dis. 2017 ;23(7):1124-1132. doi: 10.3201/eid2307.161959.].