The limited number of studies available to date reporting on patients' sex indicate a male preponderance, albeit subtle, for influenza-like illnesses or confirmed influenza infections, for example, as shown across all age groups in Spain during the influenza seasons from 2002–2003 to 2005–2006 .
Using hospitalization rates as an indicator for influenza severity, studies from Denmark and Canada confirm a higher influenza risk for males compared to females across all age groups during the H1N1 and H3N2 influenza virus outbreaks between 1995–19–2001 [15, 16].
Biological and physiological characteristics defining females and males may have differential impact on disease outcome upon pathogen exposition.
Females and males differ by their reproductive organs and thus, the corresponding sex hormone concentrations.
The interaction between sex hormones and the immune system is mediated by hormone receptors such as the estrogen receptors α and β, as well as androgen and progesterone receptors, which are expressed on immune cell subsets of the innate and adaptive immune response.