The impact of sex, gender and pregnancy on 2009 H1N1 disease

Abstract

Children and young adults of reproductive age have emerged as groups that are highly vulnerable to the current 2009 H1N1 pandemic. The sex of an individual is a fundamental factor that can influence exposure, susceptibility and immune responses to influenza. Worldwide, the incidence, disease burden, morbidity and mortality rates following exposure to the 2009 H1N1 influenza virus differ between males and females and are often age-dependent. Pregnancy and differences in the presentation of various risk factors contribute to the worse outcome of infection in women. Vaccination and antiviral treatment efficacy also vary in a sex-dependent manner. Finally, sex-specific genetic and hormonal differences may contribute to the severity of influenza and the clearance of viral infection. The contribution of sex and gender to influenza can only be determined by a greater consideration of these factors in clinical and epidemiological studies and increased research into the biological basis underlying these differences.

Figure 1

Rates of hospitalization with severe 2009 H1N1 are higher among females than males in a majority of published datasets. Female to male ratios of hospitalization with confirmed 2009 H1N1 were calculated using published datasets [5,6,14,16,27-30,32-37,43,134-136]. Pink bars = higher rates of hospitalization in females; blue bars = higher rates of hospitalization in males; grey bars = similar hospitalization rates in males and females. Details about sample sizes, time of data collection, and criteria for hospitalization are contained within each individual reference.

Figure 2

Rates of severe influenza disease among pregnant women and the general population. Estimated morbidity rates from April to December, 2009 for the 2009 H1N1 pandemic in select countries or geographic regions. Morbidity estimates are calculated based on datasets for the USA [27], Chicago, IL, USA [28], California, USA [43], New York, USA [49], Australia and New Zealand [37], Canada [5] and Brazil [14]. Estimates of the general population and pregnant woman are based on data from the US Census Bureau or the World Health Organization.

Figure 3

Why are pregnant women at increased risk for severe 2009 H1N1 disease? While 2009 H1N1 infection results in increased disease severity in pregnant women, the precise mechanisms responsible for this risk are not yet defined. The contribution of multiple biological factors to disease severity needs to be more thoroughly investigated.

Figure 4

Sex and gender biases in risk factors and co-morbidities for severe 2009 H1N1 disease. Several risk factors predispose patients to increased morbidity and mortality from 2009 H1N1. The likelihood of engaging in behaviors associated with increased exposure as well as the severity and prevalence of co-morbidities associated with severe 2009 H1N1 disease differ between males and females.