Introduction

The coronavirus disease 2019 (COVID-19) pandemic has become one of the greatest public health challenges in modern times. Male sex, cardiovascular disease, diabetes mellitus and advanced age (>65 years) are predominant risk factors for a severe course and poor prognosis of COVID-19 (ref.1). Accordingly, men with COVID-19 die at twice the rate of women with COVID-19 (ref.2). Conversely, increasing evidence suggests that cis women (hereafter referred to as women) are at higher risk of developing long-term sequelae of the disease3 (Fig. 1). Given this disparity in clinical outcomes of COVID-19 between men and women, several theories have been proposed to explain this difference. Sociocultural (gender) differences in risk behaviours, such as smoking, hand-washing or delayed health-care seeking, have been suggested to contribute to this sex disparity as have male-specific comorbid conditions, including cardiometabolic disease. Additionally, biological (sex) differences in immune responses against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or expression levels of virus entry receptors have also been suggested to contribute to the differential outcomes of women and cis men (hereafter referred to as men)4.

Fig. 1: Sex differences in virus entry protein expression and clinical outcomes of COVID-19.
figure 1

ACE2, angiotensin-converting enzyme 2; COVID-19, coronavirus disease 2019; RAAS, renin–angiotensin–aldosterone system.

Full size image

The differential outcomes between women and men have raised great interest in the role of androgens in driving a less favourable prognosis of COVID-19 in men as compared with women and children. Higher androgen levels in men and the impact of androgens on genes encoding the SARS-CoV-2 entry receptors angiotensin-converting enzyme 2 (ACE2) receptor and cell surface transmembrane protease serine 2 (TMPRSS2) have been suggested to partially account for the higher risk of adverse outcomes observed in men with acute SARS-CoV-2 infection4. While these concepts are intriguing as they might offer potential antiviral strategies, many controversies regarding the impact of sex and sex hormones on the clinical course of COVID-19 remain. Consequently, research from the past year has questioned whether biological sex differences have a major effect on COVID-19 outcomes5,6, and it has been hypothesized that non-biological aspects of being a man or a woman (such as institutionalized gender and culturally entrenched roles and norms) could provide a better explanation for the observed sex dysbalance in disease outcomes7,8,9.

This Review summarizes the current understanding of the role of androgens, oestrogens and progesterone in ACE2 and TMPPRSS2 regulation and progression of COVID-19 as well as the therapeutic value of these hormones by discussing the latest data from both experimental and clinical studies.

ACE2 and TMPRSS2

The role of ACE2 in SARS-CoV-2 infection

SARS-CoV-2 preferentially infects type II alveolar cells (AT2) as opposed to type I alveolar cells (AT1) in the human lung. The virus enters AT2 cells by recognizing and attaching its spike glycoprotein to the membrane-bound ACE2 after the spike protein is cleaved and activated by TMPRSS2 (refs.10,11,12,13,14). The SARS-CoV-2 spike protein has two subunits, S1 and S2. The S1 subunit contains a receptor-binding domain that recognizes and binds to ACE2 or other cellular receptors while the S2 subunit undergoes priming and cleavage by TMPRSS2, thereby enabling S2 to mediate the fusion of the virus with the cellular membrane10. Accordingly, higher ACE2 expression correlates with increased viral load in human respiratory cells15. ACE2 is ubiquitously expressed in the endothelium, with highest levels being detected in the vasculature of the cardiovascular system, intestinal tract, kidneys and lungs16. Within the pulmonary tissue, ACE2 is abundantly expressed in bronchial transient secretory cells or AT2 cells, the main cellular target of SARS-CoV-2 (ref.17). Accordingly, a hexapeptide inhibiting the association between the S1 subunit and ACE2 was shown last year to reduce fever, inflammation and lung injury in SARS-CoV-2 spike S1-intoxicated mice18. Similarly, in a 2020 study, higher levels of ACE2 expression were detected in lung samples from patients with comorbidities that predispose them to severe COVID-19 as compared with healthy controls19. Based on these reports, it was concluded that overexpression of ACE2 might facilitate virus penetration and, hence, organ damage during SARS-CoV-2 infection. Accordingly, children, who appear to be less susceptible to SARS-CoV-2 infection than adults, were shown to express lower levels of ACE2 (refs.20,21). However, owing to its role as a key player in the renin–angiotensin–aldosterone system (RAAS), where ACE2 opposes the vasoconstrictor actions of angiotensin II by converting angiotensin II to vasodilatory angiotensin 1–7, the involvement of ACE2 in COVID-19 seems to be more complex than initially thought. In addition, ACE2 also plays a role in both innate and adaptive immune responses by regulating T lymphocyte responses and secretion of pro-inflammatory cytokines, resulting in anti-inflammatory effects as well as in inhibition of cancer cell growth and tumour angiogenesis22,23. Finally, the ability of SARS-CoV-2 to downregulate ACE2 expression in infected cells adds further complexity to the role of ACE2 in COVID-19. The downregulation of membrane-bound ACE2 results from endocytosis of the receptor into endosomes alongside viral particles and from the enhanced activity of disintegrin and metalloproteinase domain-containing protein 17 (ADAM17). ADAM17 activity leads to shedding of ACE2, release of soluble ACE2 (sACE2) and accumulation of vasoconstrictive angiotensin II. Angiotensin II then stimulates the release of pro-inflammatory cytokines via liberation of membrane-bound precursors of tumour necrosis factor, IFNγ and IL-4, resulting in detrimental effects, including enhanced vascular permeability, multiorgan inflammation, and pulmonary and/or myocardial injury24. sACE2 is enzymatically active and seems to exert moderate antiviral activity25; however, it is currently unclear whether its protective antiviral activities predominate in COVID-19.

Taken together, the bivalent role of ACE2 hampers research efforts exploring its therapeutic utility in SARS-CoV-2 infection. However, two trials are currently testing the use of human recombinant sACE2, administered either as an aerosol26 or intravenously, for COVID-19 (refs.27,28). One of them, a clinical phase II trial testing the ability of intravenous recombinant human ACE2 to sequester SARS-CoV-2 particles in the circulation while activating the protective axis of the RAAS, was completed in 2021 (NCT04335136)28. Preliminary data published by the sponsor (Apeiron Biologics) in March 2021 show an improvement in mechanical ventilator-free days and a reduction in viral RNA load as compared with the placebo group; however, no further results have since been published29.

The role of TMPRSS2 in SARS-CoV-2 infection

TMPRSS2 is abundantly expressed in the prostate epithelium and its expression in the prostate increases in response to androgens through direct transcriptional regulation by the androgen receptor30. TMPRSS2 is one of the most dysregulated genes in prostate cancer. Recurrent gene fusions of the 5′ untranslated region of TMPRSS2 to the transcription factor ERG (encoding transcriptional regulator ERG) is the most frequent genomic alteration in early-stage and late-stage prostate cancer and results in overexpression of ERG31. TMPRSS2 is also detected in epithelial cells throughout the entire respiratory tract, including the lungs, bronchi, larynx, trachea, nasal mucosa and respiratory sinus, where its normal physiological function remains unknown17. Evidence of TMPRSS2 expression was found in different cells of the lung and bronchial branches17,32, with higher levels of TMPRSS2 being detected in AT2 cells as compared with AT1 cells17. Consistent with an involvement of TMPRSS2 in viral spike protein priming, a case-control study conducted in 2021 has demonstrated that the ratio of TMPRSS2 to ACE2 mRNA but not the level of TMPRSS2 mRNA alone outperforms ACE2 mRNA expression as a predictor for COVID-19 respiratory outcomes. This finding indicates that the functional activity of TMPRSS2 on viral fusion depends on ACE2 (ref.33). Accordingly, TMPRSS2 inhibitors have been shown to block entry of SARS-CoV-2 in vitro34,35,36. In mice sensitized to SARS-CoV-2 infection, treatment with the serine protease inhibitor nafamostate before or shortly after infection resulted in lower viral replication and mortality compared with untreated mice37. Likewise, patients with mild COVID-19 treated with the serine protease inhibitor camostat mesylate over 7 days had a more rapid resolution of COVID-19 symptoms and amelioration of the loss of taste and smell than patients in the placebo group in a clinical trial currently published as a preprint38. Conversely, results from a phase II trial conducted in 2021 show that addition of nafamostate to standard of care did not change time to clinical improvement in the overall population of 104 patients and led to faster recovery only in a small subset of 36 patients with COVID-19 at high risk and requiring oxygen treatment39.

Sex-specific expression of ACE2

It has been hypothesized that women have higher levels of cell-bound ACE2 than men, and thus have a potentially larger reservoir for the maintenance of RAAS equilibrium and tissue protection after viral entry of SARS-CoV-2. Indeed, genes coding for ACE2 and angiotensin II receptor 2 are located on the X chromosome. Although one of the two X chromosomes in females is transcriptionally silenced during late blastocyst stage to ensure a balanced gene expression between sexes, approximately 15–30% of X-linked genes escape the inactivation40. Given that the majority of genes escaping X inactivation are located on the short arm (p) of the X chromosome and ACE2 maps at band p22.2 (ref.41), it has been suggested that a higher expression of ACE2 occurs in women compared with men. Conversely, the SRY gene family located on the male Y chromosome has been shown to upregulate the activity of components of RAAS that decrease ACE2 promoter activity42 (Fig. 1). In addition, gender-specific environmental factors might influence epigenetic mechanisms such as DNA methylation by DNA methyltransferases at the CpG sites, resulting in altered ACE2 gene expression in women and men43.

Tissue expression of ACE2

Studies assessing pulmonary ACE2 expression in humans and experimental models have yielded highly conflicting results (Table 1). In fact, an analysis conducted in 2020 comparing expression levels of ACE2 RNA across 31 human tissues found no statistically significant sex difference44. Likewise, other studies report similar expression levels of ACE2 in women and men in a variety of tissues17,45,46. Conversely, a large meta-analysis comprising 31 single-cell RNA sequencing studies revealed a cell type-specific association between sex, age and smoking status, with higher ACE2 expression levels in AT2 cells being associated with male sex and increasing age47. A higher ACE2 gene expression in lung epithelial cells and airway smooth muscle cells was also found in men as compared with women in previous investigations48,49. Single-cell RNA sequencing of ACE2 in the adult human heart demonstrated a higher myocardial ACE2 RNA expression in women as compared with men50. On the contrary, several preclinical studies in mice agree that ACE2 activity as well as expression is increased in males as compared to females in different tissues, mainly under pathological conditions51,52,53,54 (Fig. 1). Overall, these conflicting data make it difficult to draw any conclusion regarding a sexual dimorphism in ACE2 expression. Species differences, difficulties in precisely measuring ACE2 expression at the tissue level, confounding variables, such as smoking, obesity and pre-existing cardiac conditions, as well as counter-regulatory effects of oestrogens on RAAS could account for the high variability of reported data.

Table 1 Sex-specific pulmonary ACE2 expression
Full size table

Soluble ACE2

Older women show higher sACE2 serum activity than younger women55, and a longitudinal study conducted in 2020 demonstrated that serum levels of sACE2 protein increases more in boys than in girls, resulting in sex differences in adulthood56. Accordingly, a genome-wide association study identified three loci associated with increased plasma concentrations of sACE2 in men but not in women57, and higher circulating levels of ACE2 have been described in men with type 1 diabetes mellitus compared with healthy men and healthy women as well as in men with renal disease as compared with women with renal disease58. Finally, in women and men with type 1 diabetes mellitus, circulating sACE2 activity increases with increasing vascular tone and in the presence of microvascular or macrovascular atherosclerotic disease59.

Overall, male sex and certain disease states seem to have an enhancing effect on sACE2 levels (Fig. 1). However, numerous questions remain unresolved regarding the equilibrium of membrane-bound ACE2 and sACE2 during SARS-CoV-2 infection. In fact, current evidence suggests that the role of RAAS-associated molecules changes dynamically during the course of COVID-19 and cannot be seen as straightforwardly positive or negative. As such, it is likely that serum levels of sACE2 mirror increased endocytosis of ACE2 by SARS-CoV-2 and disruption of the RAAS equilibrium, which, in turn, is associated with an attenuation of immune responses and an increased risk of multiorgan injury. On the other hand, increased sACE2 activity might protect against the high levels of inflammation associated with a cytokine storm. The role of sex and sex hormones in ACE2 endocytosis and shedding is currently unknown and will have to be explored by further studies.

Regulation of ACE2 by sex hormones

In humans, oestradiol, in combination with androgen deprivation therapy (ADT), resulted in an increase in ACE2 expression and a higher amount of ACE2-expressing Sertoli cells in trans women60, while an oestrogen and progesterone combination statistically significantly reduced ACE2 expression in testicular tissue in this population as compared with cis men61. In an experimental study, however, ACE2 expression in various tissues remained unaltered in multiparous 12-month-old mice, which displayed statistically significantly higher progesterone levels as compared to nulliparous mice62. Additionally, stimulation of the androgen receptor resulted in upregulation of ACE2 in mouse lung epithelial cells, and exposure to testosterone for 24 h increased ACE2 expression in isolated male and female human airway smooth muscle cells46,49. Accordingly, a moderate decrease of pulmonary ACE2 expression was observed in mice after administration of the androgen receptor antagonist enzalutamide32. Treatment with anti-androgenic drugs reduced ACE2 expression in human embryonic stem cell-derived cardiac cells and protected human embryonic stem cell-derived lung organoids against SARS-CoV-2 infection63.

Experimental data indicate that oestrogens can disrupt glycan–glycan and glycan–protein interactions between ACE2 and SARS-CoV-2, thereby blocking alveolar uptake of the SARS-CoV-2 spike protein64. However, the interaction between oestrogen and ACE2 is complex and seems to be organ and/or context specific. While oestrogen appears to reduce myocardial and renal ACE2 expression in vivo65, no alteration of ACE2 mRNA was found in VERO E6 cells, an in vitro model for SARS-CoV-2 infection, following 17β-oestradiol treatment66. Analysis of public genomic data showed that oestrogen upregulates ACE2 in mouse thymus and human lung epithelial adenocarcinoma cells20,60. In contrast, in bronchial epithelial cells and airway smooth muscle cells, ACE2 seems to be moderately downregulated by 17β-oestradiol49,67.

Taken together, sex hormones seem to modulate ACE2 expression, with an upregulation of ACE2 by testosterone being consistently shown by several studies (Fig. 2). More research effort is needed to clarify the effect of oestradiol on ACE2 expression.

Fig. 2: Effects of sex steroid hormones on SARS-CoV-2 susceptibility.
figure 2

Effect of testosterone (upper part), progesterone (middle part) and oestradiol (lower part) on virus entry receptors (left), immune responses (middle) and coronavirus disease 2019 (COVID-19) course (right). ACE2, angiotensin-converting enzyme 2; TMPRSS2, transmembrane protease serine 2; TH, T helper cell; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Full size image

Sex-specific expression of TMPRSS2

The androgen-dependent regulation of TMPRSS2 and its involvement in SARS-CoV-2 spike protein priming has led researchers to suspect that TMPRSS2 has a potential detrimental role in COVID-19 outcomes in men. Consistent with this hypothesis, oral epithelial cells in men had higher expression levels of TMPRSS2 compared with oral epithelial cells in women68. Along that line, a meta-analysis conducted in 2021 based on human single-cell RNA sequencing data revealed higher levels of TMPRSS2 expression in AT1 cells in men as compared with AT1 cells in women47. However, the authors did not detect any sex differences in TMPRSS2 gene expression in AT2 cells, the main cellular target of SARS-CoV-2 (ref.47). In addition, further studies reported similar levels of TMPRSS2 in lung tissue of women and men based on single-cell transcriptomics and protein expression data derived from the Genotype-Tissue Expression (GTEx) project and the Human Protein Atlas GTEx data, respectively45,69. These findings were confirmed in a study conducted in 2021, which reported no sex difference in TMPRSS2 protein expression in alveolar epithelial cells32.

In conclusion, there is currently only weak evidence for an increased expression of TMPRSS2 in the male lung with no data supporting sex discordance of TMPRSS2 expression in AT2 cells. Table 2 lists studies reporting sex-disaggregated data of TMPRSS2 expression.

Table 2 Sex-specific pulmonary TMPRSS2 expression
Full size table

Regulation of TMPRSS2 by sex hormones

The androgen-mediated regulation of TMPRSS2 and the higher case fatality rates seen in men infected with SARS-CoV-2 than in women has brought questions to the forefront about the role of sex hormones in COVID-19 severity and whether their modulation could serve as a treatment option for SARS-CoV-2 infection. As previously outlined, TMPRSS2 transcription in the prostate gland is regulated by androgenic ligands and an androgen receptor-binding element in the promoter31. Accordingly, an increased expression of TMPRSS2 was observed following androgen treatment in a human lung adenocarcinoma-derived cell line70 (Fig. 2). Similarly, androgen receptor antagonism by enzalutamide downregulated TMPRSS2 and reduced cell entry of SARS-CoV-2 in multiple human lung cell lines as well as in mouse lung epithelial cells71. Further, androgen deprivation by gonadectomy or treatment with anti-androgens attenuated spike-mediated cellular entry of SARS-CoV-2 in mice72. If this link proves correct, it could pave the way to novel strategies for the treatment of COVID-19. Therefore, these strategies are the subject of several clinical trials (listed in Supplementary Table 1); to date, however, many of these trials have been withdrawn due to lower-than-planned accrual or limited resources, and those who have been published report mixed results (Table 3). Additionally, more recent studies offer little hope that repurposing of androgen synthesis inhibitors or androgen receptor antagonists will become a valuable treatment option in COVID-19. TMPRSS2 expression in lung cancer cell lines, mouse lung tissue and human lung organoids remained unaltered following treatment with enzalutamide, suggesting that TMPRSS2 regulation in the lung differs from the clear androgen-dependent regulation in prostatic tissues32,73.

Table 3 Overview of ongoing and completed randomized controlled clinical trials assessing the safety and efficacy of treatments targeting sex hormone pathways on COVID-19 outcomes
Full size table

Notably, there is a lack of data regarding the effect of progesterone on TMPRSS2 signalling, and only few studies have assessed an association between oestrogen levels and TMPRSS2 expression. In fact, two reports indicate that prostate cancer cells expressing the TMPRSS2:ERG fusion gene might be responsive to oestrogen signalling30,74. Consistent with this observation, treatment with 17β-oestradiol resulted in a reduction in TMPRSS2 mRNA in VERO E6 cells66 (Fig. 2).

Sex steroids and immune responses

Both, innate and adaptive immune responses differ between men and women. Women and female animals usually mount stronger immune responses against pathogens than men and male animals as the number of innate immune cells, including monocytes, macrophages and dendritic cells, is higher in females75,76,77. Women also exhibit higher cytotoxic T cell activity78, higher immunoglobulin levels (both at baseline and following infection or vaccination79,80), higher CD3+ and CD4+ cell counts, and more robust T helper (TH) cell activation than men81,82,83,84. Several mechanisms could account for these sex differences, including an imbalance in the expression of genes encoded on the X and Y chromosomes85, polymorphism in autosomal genes86, epigenetic modifications87, and direct effects of sex hormones on immunological pathways. Indeed, sex steroids, particularly testosterone, oestradiol and progesterone, have all been shown to influence the function of immune cells by binding to their specific receptors, which are expressed in various lymphoid tissue cells as well as in circulating lymphocytes, macrophages and dendritic cells88 (Fig. 2). Testosterone is known to suppress TH2 and TH17 cell function89,90 and to alter the production of cytokines, including IFNγ91. Additionally, many pro-inflammatory and antiviral genes have oestrogen response elements in their promoters78, and immune responses to viruses have been shown to vary depending on female hormone status (for example, menopause, pregnancy, contraception and hormone replacement therapy (HRT))92. Accordingly, treatment with raloxifene, a selective oestrogen receptor modulator, resulted in an increase in white blood cell numbers alongside an accelerated viral clearance in patients with SARS-CoV-2 infection in a clinical trial conducted in 202293 (n = 61; Table 3).

Although stronger cytokine responses to viral infections are usually seen in women, higher levels of pro-inflammatory cytokines, such as IL‐8 and IL‐18, have been observed in men infected with SARS-CoV-2 compared with infected women94. These higher pro-inflammatory cytokine levels are seen in individuals with severe COVID-19 culminating in a systemic inflammatory syndrome and lung injury95. Poorer COVID-19 outcomes are also seen in people with weak T cell activation, which was more commonly observed in men than in women94. Men with COVID‐19 also have a lower lymphocyte count and higher neutrophil‐to‐lymphocyte ratios and serum concentrations of C‐reactive protein as compared with women, all of which have been associated with a poor prognosis in COVID-19 (ref.96). Conversely, women seem to clear the SARS-CoV-2 virus faster than men given that the virus is detected for a longer period in men than in women97,98. In support of this observation, clinical trials testing the anti-androgenic drugs dutasteride (a 5α-reductase inhibitor used to treat benign prostatic hyperplasia and male pattern hair loss) and proxalutamide (an androgen receptor antagonist) have reported an accelerated viral clearance, reduced viral shedding and reduced C-reactive protein levels as compared with placebo in patients with mild to moderate COVID-1999,100 (n = 87 and n = 236, respectively; Table 3).

Impact of androgens on COVID-19 severity

Consistent with a detrimental role of androgens in immune responses to viral infections, several studies conducted during the early phase of the pandemic reported a link between the androgen-mediated phenotype of androgenetic alopecia and COVID-19 severity101,102. Although these studies were lacking a control group, and hair loss can be the consequence of physical shock or treatment side effects, these studies have strengthened the hypothesis of a potential role of male sex hormones in COVID-19. This hypothesis was further supported by observational studies reporting reduced COVID-19 incidence and case fatality rates in patients with prostate cancer receiving ADT103,104. Likewise, men exposed to 5α-reductase inhibitors were less likely to develop severe COVID-19 as observed in two studies from 2021, one of which is a clinical trial conducted in 87 men99,105 (Table 3). Similarly, a study conducted in 2021 in individuals with gender dysphoria described that the risk of contracting COVID-19 was 3.46 times higher in individuals with female-to-male gender dysphoria (who had received testosterone therapy) than in individuals with male-to-female gender dysphoria (who received oestrogen and anti-androgen therapy)106. However, it should be noted that there is substantial risk that the effect size reported in this study is overestimated and that undetected confounders might have influenced the study results given its small sample size and large confidence intervals (1.01–11.84)106.

Finally, the results of several clinical trials testing proxalutamide in women and men with COVID-19, which are currently available as preprints, suggest that clinical endpoints as well as inflammatory and immunological markers are improved in the treatment arm107,108,109,110,111,112 (Table 3). However, of note, these trials have all been conducted by the same authors and have recently been criticized for irregularities113,114 (Table 3). Additionally, a clinical trial in 268 men infected with SARS-CoV-2, which reported reduced hospitalization rates by 91% in the proxalutamide arm, was retracted last year115. The same compound administered at a higher dose (300 mg) and for a longer treatment duration (14 days) resulted in a 121% higher recovery rate and a reduction of all-cause mortality by 80% in 778 men and women with COVID-19 (ref.112). However, this trial has also been criticized for methodological issues (Table 3).

Additionally, more recent observational studies from last year have refuted an association between increased androgen activity and COVID-19 outcomes116,117. Moreover, a clinical trial in 96 hospitalized patients with COVID-19 testing the anti-androgenic compound degarelix demonstrated that androgen suppression did not result in amelioration of COVID-19 severity118, and a trial testing the androgen receptor antagonist enzalutamide in a small cohort of 42 patients was terminated early because patients treated with enzalutamide required longer hospitalization than patients in the placebo group119. These trial data were supported by epidemiological data from 7,894 patients with prostate cancer where no preventive effects of ADT on COVID-19 outcomes was observed119. Similarly, in women with conditions that are associated with androgen excess (polycystic ovarian syndrome, acne cystica and hirsutism), no excess morbidity related to COVID-19 was reported120. Moreover, testosterone replacement therapies were not associated with worse disease outcomes in men infected with SARS-CoV-2 in an observational study conducted in 2020 (ref.121). Given these inconsistencies, it is important to note that testosterone levels might change during acute illness and that the relationship between circulating androgen levels, androgen sensitivity and COVID-19 severity is not straightforward122 (Fig. 2). Indeed, low serum levels of testosterone characterize the hormonal milieu in seriously ill individuals and predict organ injury and poor prognosis in men infected with SARS-CoV-2 (refs.123,124,125,126,127,128,129,130).

As outlined previously, increasing evidence also suggests that a blunted immune response in men with normal testicular function can occur during the early phase of infection, resulting in low viral clearance and a high risk of systemic illness97,98. On the other hand, lower androgen activity, frequently seen in critically ill older men with hypogonadism, might negatively affect endothelial cell functioning and increase the risk of an aggressive inflammatory response with the release of large amounts of pro-inflammatory cytokines, known as a ‘cytokine storm’131,132. A cytokine storm is associated with an increased risk of severe lung injury, multiorgan failure and an overall unfavourable prognosis133 (Fig. 1). Consistent with this hypothesis, an association between COVID-19 complications and androgen imbalance, defined as high serum levels of luteinizing hormone, low levels of testosterone and/or increased levels of oestradiol, has been shown in men infected with SARS-CoV-2 (refs.63,127,134). Accordingly, we have shown, in 2022, that a higher ratio of testosterone to oestradiol was linked to a favourable prognosis in hospitalized patients with COVID-19 (ref.135).

Taken together, the multifactorial nature of COVID-19 infection and hormonal regulation in men does not currently allow us to draw any definitive conclusions. However, existing data emphasize that consideration of sex, reproductive age, disease state and comorbidities seem to be crucial when exploring the effect of androgens on COVID-19 outcomes. Indeed, the health status of individuals infected with SARS-CoV-2 is often complex, and the effect of these comorbidities on hypogonadism, treatment responses and outcomes should be given special attention. While many clinical trials have been initiated to assess the effect of sex steroids on clinical outcomes of COVID-19, many of those trials have never been completed, most probably reflecting the complexity of this research field and the lack of a clear mechanism as well as the challenges associated with drug repurposing for COVID-19 treatments (Table 3). An overview of studies addressing the different roles of androgens in COVID-19 prognosis is given in Supplementary Table 1. The complex impact of age on COVID-19, sex hormones, virus entry proteins and disease outcomes is summarized in Fig. 3.

Fig. 3: Alterations of hormone levels, immune responses, virus entry protein expression and clinical outcomes according to age in men.
figure 3

This figure depicts the relationship between increasing age and testosterone levels during severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, cardiovascular risk factors, the risk of critical illness and viral clearance during SARS-CoV-2 infection (upper part). The risk of a cytokine storm according to age and testosterone levels is also shown (middle). There is only a weak association between age and pulmonary angiotensin-converting enzyme 2 (ACE2) expression and no clear association between age and testosterone-dependent regulation of transmembrane protease serine 2 (TMPRSS2) expression in the lung (lower part).

Full size image

Female sex steroids and COVID-19 severity

Oestradiol and progesterone play distinct roles in modulating innate and adaptive immunity and have been associated with an attenuated inflammatory response during acute infection136,137. Moreover, a protective role of oestrogen against SARS infection in mice has been previously reported, highlighting the potential for oestrogens to modulate COVID-19 susceptibility and progression138. Accordingly, two studies reported higher SARS-CoV-2 infection rates139 and longer hospital stays140 in women in postmenopause as compared with women in premenopause. The same authors found that circulating levels of 17β-oestradiol and anti-Müllerian hormone, a marker for ovarian reserve, were higher in women with mild COVID-19 disease course than those with a more severe disease course140. A cohort study comprising 68,466 people infected with SARS-CoV-2 reported a reduction of fatality risk by 50% in women aged >50 years receiving postmenopausal HRT (n = 439)141. These results were confirmed by preliminary data from a study conducted in 2022 that described a lower likelihood of COVID-19-related mortality in women using HRT than in women not using HRT in a sample of 1,863,478 women142. In addition, the fact that women with hormone-driven cancers, who are often treated with anti-oestrogen therapies, seem to encounter an increased risk of SARS-CoV-2 infection and critical illness than women without these cancers suggests a protective role of oestrogens in COVID-19 (ref.143).

However, more recent studies from 2021 on the impact of oestrogen on COVID-19 outcomes have provided highly controversial results: while both the ablation of oestrogens by selective oestrogen receptor modulators in women with breast or ovarian cancers143 and the use of exogenous oestrogen in the form of the combined oral contraceptive pill139 was associated with a reduced incidence of COVID-19 in women, HRT in women in postmenopause did not show consistent correlations with COVID-19 incidence and disease severity139. Additionally, no statistically significant differences in sex hormone levels were detected in women critically ill with COVID-19 as compared to women with mild disease127. The lack of information on hormone treatment type, route of administration, duration of treatment and potential confounders, such as comorbidities, most likely accounts for the current controversies regarding the effect of female sex steroids on SARS-CoV-2 infection. In addition, the variation of endogenous oestrogen levels and the predominance of different oestrogen subtypes depending on age and reproductive status further complicates assessment of its role in COVID-19144 (Fig. 2).

Finally, progesterone has been shown to exert broad anti-inflammatory effects by decreasing leukocyte activation and production of pro-inflammatory mediators through inhibition of NF-κB145,146 (Fig. 2). Due to its multifaceted function, progesterone has been suggested to play a potential beneficial role in SARS-CoV-2 infection in the events of immune dysregulation147. Accordingly, in a pilot trial conducted in 2021, it was demonstrated that subcutaneous administration of progesterone improved the clinical status of 20 critically ill men infected with SARS-CoV-2 (ref.148). Although this preliminary work suggests that progesterone could be beneficial in patients with COVID-19 with pathophysiological indications of immune dysregulation, severe symptoms and critical illness, the small sample size of the trial along with marginal P values does not permit any definite conclusions to be drawn. Further studies in larger populations are warranted to confirm these findings.

Importantly, progesterone levels fluctuate throughout the cycle and reach high levels during pregnancy. Hence, pregnant women and those in premenopause in the mid-luteal phase of the cycle display the highest progesterone levels149. Although several small studies reported no adverse outcomes of pregnancy and/or COVID-19 in pregnant women infected with SARS-CoV-2 (refs.150,151), surveillance data from 8,207 pregnant women published by the US Centers for Disease Control and Prevention (CDC) showed an increased risk of hospitalizations, intensive care unit admissions and mechanical ventilation in this population152. Further, increased risks of pre-eclampsia, preterm birth and other adverse pregnancy outcomes were reported by a large meta-analysis comprising 42 studies involving 438,548 pregnant women153. Further data from China demonstrated that most severe cases in pregnant women occurred after delivery154, when progesterone levels rapidly decline. However, it should be noted that pregnancy complications related to COVID-19 might occur for reasons other than hormonal changes as a direct link between female sex steroids and pregnancy complications in women positive for SARS-CoV-2 has never been established. In conclusion, further mechanistic studies are warranted to corroborate or refute the protective effect of progesterone in men and women with COVID-19 and potentially establish a mechanistic link between progesterone and molecular key variables such as TMPRSS2 or ACE2.

Post-COVID-19 syndrome

Increasing evidence suggests that SARS-CoV-2 causes a protracted disease course beyond acute illness. Several studies from different populations indicate that the prevalence of this ‘post-COVID-19 syndrome’ and the number of persistent symptoms is higher in women than in men3 (Fig. 1). Given that the largest group of patients with post-COVID-19 syndrome appears to be in their early 50s3, an age at which menopause occurs, a link between alterations in ovarian steroid hormone production and post-COVID-19 syndrome has been hypothesized155. An alternative explanation for the higher incidence of post-COVID-19 syndrome in women involves sex differences in immunomodulation. Indeed, a specific immune signature, which has been previously observed following viral infections, has been suggested to account for the persistence of fatigue following acute COVID-19 (ref.156), and preliminary data (currently available as a preprint) indicate that dysfunctional immune cells with an autoimmune phenotype are present in patients with post-COVID-19 syndrome157. Thus, the higher prevalence of autoimmune disorders in the female population as well as known sex differences in immune responses to SARS-CoV-2 (ref.94) might predispose women to a potential long-term hyper-inflammatory state. Further, preliminary data from Newson et al. indicate that 74% of women with post-COVID-19 syndrome report changes in their menstrual cycle as well as worsening of symptoms during menstruation when hormone levels are at their lowest155. The authors suggest that involvement of the angiotensin–(1–7)–Mas receptor–ACE2 axis in follicle maturation and disturbance of ovarian steroid hormone production following SARS-CoV-2 infection might account for these associations155.

Conclusions

Despite intense research efforts, there is currently no consensus on whether sex differences in ACE2 and TMPRSS2 expression drive the higher disease burden of COVID-19 among men than in women and children. Additionally, the precise mechanisms by which female sex hormones might provide protection against SARS-CoV-2 infectivity remain unknown. Conflicting data have been reported regarding pulmonary ACE2 and TMPRSS2 expression in humans, making it difficult to draw conclusions regarding the sexual dimorphism of the two receptors. Clinical studies exploring the effect of sex hormones on COVID-19 outcomes report inconsistent effects of androgens and oestrogen in COVID-19, and there is currently no evidence supporting the administration of oestradiol or deprivation of androgens in patients with COVID-19. Thus, although variation in sex steroid levels in men and women might, to some extent, explain the sex disparities in susceptibility to SARS-CoV-2, the underlying mechanisms are still open to speculation. Current evidence is hampered by the lack of mechanistic studies as well as the lack of clinical trials reporting sex-disaggregated data. Finally, there is strong evidence that sociocultural gender influences a person’s lived experience and, therefore, exposure to SARS-CoV-2 and access to care7,8,9. Hence, investigating the influence of both biological (sex) and sociocultural (gender) differences on COVID-19 manifestations will be necessary to understand and manage this pandemic.