1. SEX DIFFERENCE IN PREVIOUS CORONAVIRUS OUTBREAKS
-HUMANS: Severe Acute Respiratory Syndrome Coronavirus 1 (SARS-CoV–1) showed a similar sex discrepancy in the 2003 epidemic. In Hong Kong, the case fatality rate was 13.2 % for females (95 % CI = 11.1, 15.3) and 21.9 % for males (95% CI = 19.0, 24.8), with an age-adjusted relative mortality risk ratio of 1.62 (95 % CI = 1.21, 2.16) for males 41. In a study in Singapore during the same outbreak, male sex associated with an odds ratio of 3.10 (95 % CI = 1.64, 5.87; p = <0.001) for ITU admission or death 42. A retrospective analysis of the Saudi Arabian Middle East Respiratory Syndrome (MERS) outbreak in 2013 - 2014 showed a case fatality rate of 52% in men and 23 % in women 43.
-ANIMALS: This sex bias is also observed in animal models of SARS infections. In a mouse model of SARS-CoV–1 infection, female mice had a lower case fatality ratio, and less lung inflammation and oedema than males. Moreover, ovarectomy and treatment with the oestradiol antagonist, ICI 182, 780, diminished the sex advantage of female mice, implicating oestradiol in this sex difference44. Studies in animals that are known vectors for human disease have not shown the same sex bias that is observed in humans. Seropositivity in these reservoir animals, which act as hosts for viral pathogens but generally do not display symptoms, has been investigated with sampling studies. Studies in wild bats, known reservoirs for pathogenic coronaviruses, have shown either similar seropositivity in males and females 45, or higher seropositivity in female animals46. Similar studies in dromedary camels, the main reservoir host for MERS-CoV, showed higher seropositivity in females 47,48. A possible explanation for the higher rate of seropositivity in females is the higher risk of exposure: the close proximity of female dromedaries to their highly susceptible calves puts them at repeated risk of infection 48, while pregnant bats collect to stay warm during roosting season46.
2. SEX DIFFERENCES IN OTHER INFECTIONS
Sex differences in response to infection occur at all ages with a generally higher burden of bacterial, viral, fungal and parasitic infections in human males. New-born males are more likely to die from infection than females49 and male children have higher rates of parasitic infections50. Tuberculosis and hepatitis B infection are more common in males, who are also more likely to die of sepsis than females51,52. An exception to this male predominance in infectious disease is urinary tract infections, where adult females are 40 times more likely than males to develop disease53. Females with human immunodeficiency virus (HIV) infection have less circulating viral RNA, but are 1.6 times more likely to progress to advanced immune deficiency syndrome (AIDS) at the same viral load as men54. Women are more susceptible to infections of the upper respiratory tract such as tonsillitis and sinusitis while men are more susceptible to lower respiratory tract infections i.e. community-acquired pneumonia55. Notably, the female sex bias in infection is mainly observed after puberty and before menopause, suggesting that sex hormones play an important role in this phenomenon53.
3. SEX DIFFERENCES IN INFLUENZA
Unlike COVID–19, mortality rates during previous influenza pandemics have typically been higher in females, despite a higher prevalence of infection in men. Studies from Japan during the most recent H1N1 pandemic found that during reproductive years morbidity rates were higher in females, but outcomes were worse for males under 20 and over 8056. Pregnancy is an established risk factor for influenza morbidity and mortality: in US data from the 2009 Influenza A (H1N1) pandemic, pregnant women represented 5% of all deaths despite comprising less than 1% of the population57. From the limited data about COVID–19 in pregnancy to date, there does not seem to be the same association with morbidity that is seen in influenza, but this may change as more information becomes available58. Data from influenza-infected animal models demonstrate no difference in viral load between males and females, but greater pro-inflammatory cytokine and chemokine production in females, suggesting that host-mediated inflammatory responses contribute to the disparity in morbidity between the sexes59. Analysis of the effect of sex hormones versus the sex chromosome complementon the response to influenza infection demonstrates that sex steroids are likely to be driving the differences observed59.
The stronger female response to influenza antigens that may underlie more severe disease is mirrored in the female response to vaccination. Women consistently report more severe local and systemic side effects and produce higher antibody titres in response to seasonal flu vaccinations60,61 than men. Females achieved equivalent protective antibody titres to males at half the dose of inactivated influenza vaccine 62. After influenza vaccination, female B cells produced more antigen-specific IgG, mediated by sex-based differences in gene expression within B cells 63.
4. SEX DIFFERENCES IN THE IMMUNE SYSTEM
Despite being a previously underappreciated biological variable, it is now well established that males and females mount different immune responses to infection. Whilst some differences are seen in the immune response from birth, certain differences are only observed after sexual maturity. This suggests that both sex chromosome complement as well as sex hormones influence the immune response. In very general terms, females tend to be skewed towards a more robust immune response towards pathogens with relatively decreased self-tolerance, whereas males tend to have better self-tolerance, but a less robust response to pathogens. This contributes to the clinical phenotype of females being relatively protected against infection and malignancy when compared to males, but more prone to developing autoimmune diseases.
SEX CHROMOSOMES AND SEX HORMONES: There is an over-expression of genes with an immune function on the X chromosome64, as evidenced by the existence of many X-linked immunodeficiency disorders65. There is emerging evidence that there may be variable inactivation and regulation of the inactive X chromosome in immune cells with subsequent bi-allelic expression of X-encoded immune genes in females 66,67. Many immune cells express oestrogen receptors alpha and beta 68. The effect of oestradiol seems to be dose-dependent, with low doses corresponding to a T helper 1 (Th1)-type response and cell-mediated immunity, and higher doses (such as in pregnancy) corresponding to T helper 2 (Th2)-type responses and humoral immunity 51,69. Oestradiol modulates CD4 T cells and CD8 T cells70, promotes T regulatory (Treg) cell expansion in vitro and in vivo 71 and decreases T helper 17 (Th17)/interleukin–17 (IL–17) production 72. Oestradiol is associated with increased antibody production, somatic hyper-mutation and class switching 73, abundance of neutrophils 74, and monocyte/macrophage cytokine production 75. In human T cells, almost half of the activated genes have an oestrogen response element in their promoter region76. Testosterone is generally thought to dampen the immune response; a lack of testosterone associates with increased inflammatory cytokines, antibody titres, CD4/CD8 ratios and natural killer cells along with a decrease in Treg cells 51,77.
INNATE IMMUNITY: In terms of innate immunity, limited data suggest sex-based differences in the expression and response of pattern-recognition receptors on various immune cells. For example, both neutrophils from human males and peritoneal macrophages from male mice express higher levels of toll-like receptor 4 (TLR4) 78,79 and produce higher levels of tumour necrosis factor alpha (TNFα) after lipopolysaccharide (LPS) stimulation than females 80.
There are important sex differences in the innate antiviral response that may be relevant to the sex discrepancy seen in COVID–19. There is a large body of evidence showing that females have a more robust production of type 1 interferon (IFN) upon sensing of viral RNA via toll-like receptor 7 (TLR7) 81–86. When exposed to HIV-derived RNA, plasmacytoid dendritic cells from females produce more type 1 IFN than males 87. This phenomenon has been shown to associate with sex hormone concentration but also the number of X chromosomes present 82 84.
ADAPTIVE IMMUNITY: The adaptive immune system mirrors the trend of females being skewed towards a more robust immune response, but poorer self-tolerance, and males having improved tolerance, but a less robust response to infections. Autoimmune regulator (AIRE) gene expression is decreased in female thymic tissue compared to males and females have improved thymic function when compared to males throughout life 88–90. At all ages, and even during HIV infection, females have more CD4+ T cells than males 91–96. Female T cells have more robust cytotoxic activity and upregulation of inflammatory genes than males75, while males have more Tregs than females97. Females have more B cells and produce more immunoglobulin than males 91,98.
5. SEX DIFFERENCE IN IMMUNE-AGING
There is a marked association between morbidity/mortality and advanced age in COVID–19. Although age-associated changes in immunity are beyond the scope of this review and are available elsewhere99, it is of note that age-associated changes in the immune system are also different between sexes. There is a male-specific, age-associated decline in B cells and a trend towards accelerated immune ageing in males 100,101 which may further add to the sex difference in disease phenotype.