ACE2: Angiotensin Converting Enzyme 2; ARB: Angiotensin Receptor Blocker; TMPRSS2: Transmembrane Serine Protease 2

The novel Coronavirus disease outbreak has impacted the lives of millions of people around the world. As of June 10, 2022, according to the World Health Organization COVID-19 Dashboard, 6, 305, 308 deaths due to COVID complications have occurred worldwide with the number of cases still rising. Global data show that although incidence of disease is comparable, severity and mortality of disease is significantly higher in males than females [1]. The Global Health 50/50 dataset shows that across 181 countries, for every 10 female COVID-19 related cases, there are 10 male COVID-19 related cases, indicating equal incidence [2]. However, for every ten female COVID-19 related deaths, there are 13 male deaths [2]. This information presents the question: why are men more severely impacted by COVID-19 than women?

Recent studies have shown that disease severity is dependent on the expression of Angiotensin Converting Enzyme 2 (ACE2) receptor protein and transmembrane serine protease 2 (TMPRSS2). The Spike (S) protein, located on the Coronavirus surface, is primed by the cellular TMPRSS2 serine protease and is internalized by binding to ACE2 receptors expressed on cell membranes [3]. In a study done on human cell lines, antibodies against ACE2 blocked the entry of the COVID 19 virus [4]. Also, a prospective cohort study conducted in England showed that ACE inhibitor or Angiotensin Receptor Blocker (ARB) drug use is associated with a significantly lower risk of COVID-19 infection with a hazard ratio of 0.71 and 0.63, respectively, further supporting that ACE2 receptors play an important role in viral cellular entry [5].

Furthermore, studies have found that inhibition of androgen receptor signaling results in decreased ACE2 and TMPRSS2 protein levels. This indicates a potential androgen-mediated cause of increased ACE2 and TMPRSS2 and thus increased COVID-19 entry into human cells. Sex steroids, like testosterone and estrogen, are already known to cause different immune responses with estrogen causing stronger antiviral cellular and humoral responses [6].

This paper explores the plausibility of hormonal differences being the cause of the sex-based differences of COVID-19 severity and mortality. Other factors like gender-based lifestyle differences, including smoking, diet, and hygiene, as well as the presence of comorbidities are also analyzed in relation to increased disease susceptibility.

We performed a PubMed, Embase, JSTOR, and Google Scholar search for papers describing the pathophysiology of the recent outbreak of Coronavirus-19 and related viral infections. In addition, we searched through papers that were published in the last 20 years for the relationship between sex steroids (androgens and estrogens) and immunology. We also looked for differences in comorbidities and lifestyle factors such as diet, obesity, and smoking habits between males and females. Another crucial resource was the Coronavirus Situation report by the World Health Organization that provided key statistics on disease prevalence and mortality. Using the Global Health Dataset, we calculated odds ratios for male mortality vs. female mortality for several different countries and plotted the data in a forest plot. Only relevant articles that were published in English in scholarly journals were included.

Sex stratified differences in disease presentation and outcome

The Global Health 50/50 tracker contains sex-disaggregated data being reported in official governments websites in 181 countries [2]. We included countries that had reported data regarding COVID-19 cases, hospitalizations, mortality, and vaccinations, and calculated odds ratio of male mortality vs. female mortality (Table 1). Across 21 countries included, men are significantly more likely to lose their life to COVID-19 related complications than women. This data is illustrated in a forest plot (Figure 1) and further supported by other studies. A retrospective study by Meng et al. of 168 COVID-19 positive patients in Wuhan, China demonstrated that 12.8% of affected males died compared to 7.3% of women [7]. In addition, a multi-center cross-sectional study in Italy showed that most COVID-19 patients admitted to the ICU were men (74%) and concluded that sex is the primary determinant in COVID-19 disease severity [8].

The clinical COVID-19 disease also varies greatly between men and women. A European epidemiological study found significant data that men and women differ in their clinical presentation as well. Women were more likely to report loss of smell, headache, nasal obstruction, and throat pain, whereas men more commonly reported cough and fever [9]. In the retrospective study by Meng et. al, eleven laboratory markers for inflammation and kidney and liver function were significantly elevated in COVID-19 positive men compared to women. These parameters include hemoglobin, ferritin, C-reactive protein (CRP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), bilirubin, blood urea nitrogen (BUN), and creatinine [7].

The elevation of these markers can be suggestive of inflammation, liver damage, and kidney damage. The statistically significant difference between these values implies disparate mechanisms of disease onset and progression in men and women and should be further studied to determine possible differences in treatment approach.

Hormonal differences

Androgens, Estrogen, and Coronavirus: A significant amount of research exists regarding the relationship of sex hormones and the immune response. Estrogen has consistently been shown to be an immune boosting hormone in women compared to the immunosuppressive testosterone more predominant in males [10].

A murine study investigating the susceptibility to SARS-CoV infection demonstrated male mice are more vulnerable to the disease. Channappanavar et. al. found male mice to have elevated viral loads, enhanced vascular leakage, and alveolar edema as well as increased levels of inflammatory molecules in their lungs. Manipulating the mice via estrogen receptor antagonists as well as ovariectomy resulted in increased mortality in female mice infected with SARS-CoV. While 85% of ovariectomized mice died within 8 days of infection, 10-20% of female mice in the control group died. On contrary, there were no significant differences in morbidity or mortality of gonadectomized male mice infected with mice-adapted SARS-CoV when compared to control male mice. Estrogen receptor signaling thus was concluded to be protective and critical for protection against SARS-CoV in females [11]. This data from the mice supported the gender-based trends seen in SARS and MERS patients. Given that the COVID-19 virus is of the same virus family, such data on SARS and MERS is valuable in understanding the pathogenesis of COVID-19 and how it differs in men and women.

Researchers studying the effects of COVID-19 on cancer patients in Italy found that patients who are undergoing androgen-deprivation therapy (ADT) for prostate cancer are partially protected. The investigators found that such patients had a significantly lower risk of contracting COVID-19, as measured with positive SARS-CoV-2 tests, than prostate cancer patients who are not receiving ADT (OR 4.05; 95% CI 1.55–10.59) and an even lower risk than patients with any other type of cancer ((OR 4.86; 95% CI 1.88–12.56). Since androgen deprivation is known to decrease the levels of TMPRSS2 [12], the co-receptor needed for COVID viral entry, Montopoli et al. hypothesized that this may be the mechanism by which ADT plays a protective role [13].

In a study of potential therapeutic candidates for SARS-CoV-2, androgen signaling was revealed to be a key modulator of ACE2 levels. Previous studies showed that knock out of ACE2 resulted in decreased viral entry in murine lungs, and the investigators of this study analyzed various drugs to see which interaction would decrease ACE2 levels. They found that the mechanism of action of the drugs most effective in decreasing ACE2 protein levels was inhibition of androgen receptor signaling. Such drugs reduced ACE2 and TMPRSS2 on cellular membranes and resulted in reduced internalization of the SARS-CoV-2 within in vitro human primary alveolar epithelial cell lines [14].

In the same study, the researchers further assessed the role of androgens in COVID-19 disease manifestation. They found that among all male patients from recruitment centers with available COVID-19 testing in England, each standard deviation increase in free androgen index increased risk of a positive test (OR=1.14, 95% CI: 1.02–1.28, p-value 0.027), as well as severe COVID-19 infection (OR=1.15, 95% CI:1.01–1.31, p-value 0.037) independent of age, BMI, and the Townsend deprivation index (a census-based form of socioeconomic status measure used in the United Kingdom). The investigators concluded that the significant association between elevated free androgen levels and COVID-19 disease severity could be linked to the androgen-mediated ACE2 modulation discussed previously [14].

The difference in ACE2 expression due to differences in sex hormones was further supported by a study that assessed the effects of ovariectomy and orchiectomy in mice. While ovariectomy led to increased ACE2 activity in female mice, orchiectomy decreased ACE2 activity. Furthermore, testosterone was concluded to maintain high ACE2 levels in the heart and kidney among other organs while estrogen reduced ACE2 expression in the same organs [15]. The difference in interaction between sex hormones, sex specific organs and ACE2 expression could be a potential source for the disparate responses to COVID-19 in males and females, as ACE2 has been shown to be significant for viral entry.

In a small preliminary observational study in Spain, dermatologists found a high prevalence of male pattern hair loss among COVID-19 patients. Male pattern baldness is mediated by higher levels of androgens, leading the physicians to hypothesize that the high levels of androgens have an association with a higher risk of COVID-19 [16]. The study, however, is limited as it was done with a small cohort of patients.

A retrospective cohort study done in Hamburg, Germany analyzed sex hormones and cytokine responses in COVID-19 patients admitted to the ICU at the University Hospital Hamburg- Eppendorf, Germany. The study found that the majority of male patients presented with low testosterone (68.6% of patients) and low dihydrotestosterone (DHT) levels (48.6%), whereas 60% of the female patients had elevated testosterone levels without changes in DHT. This led to the conclusion that androgens are necessary in males to produce an appropriate immune response to the COVID-19 virus and implies an association between testosterone and disease manifestation in females [17]. This study, however, is also limited due to a small population. A recent United Kingdom study found that serum testosterone was lower (p = 0.0011, rank- sum test) in men with COVID-19 who died [median TT (IQR) = 2.0 (1.5–3.6) nmol/L] compared with survivors [median TT (IQR) = 5.0 (2.6–9.4) nmol/L]. Logistic regression found that the only factors that impacted mortality in the examined population were age and serum testosterone (odds ratio: 0.77, 95% confidence intervals [CI]: 0.64–0.91). This study similarly suggests that there may be an association between testosterone and COVID-19 disease outcome [18].

Autoantibodies & male susceptibility to COVID-19: A recent candidate gene study found autoantibodies to type 1 interferon (IFN) to be a significant cause of more severe COVID-19 disease. Of 101 hospitalized patients with life-threatening COVID- 19 pneumonia, 95 patients were found to have autoantibodies to interferons critical to mounting an immune response against viral agents. Although many autoimmune diseases are more common in women, 94% of these patients with autoantibodies were men. Comparatively, these autoantibodies were virtually absent in patients with mild or asymptomatic COVID-19. Overall, the researchers concluded that inborn genetic errors of type 1 IFN immunity accounted for the life- threatening pneumonia seen in at least 2.6% of women and 12.5% of men in the study [19]. Further, several of the genes linked to immune responses to viral infections were found to be on the X chromosome and mutations in the X chromosome are more likely to affect males given their single copy. This defect in innate immunity could provide an alternative explanation for the increased susceptibility to COVID-19 disease in men.

Lifestyle differences

Effects of smoking: An analysis of six cohort studies done in China suggested that the higher disease severity in males could be associated with the higher rates of smoking among males. The proportion of males suffering from such severe disease symptoms was 25%, while the proportion of females was 18.7%. Severe disease symptoms have been shown to be associated with smoking. Smoking was shown to increase ACE2 gene expression in a study that used transcriptomic data sets based on healthy lung tissue from patients with lung adenocarcinoma. After adjusting for age, gender, and ethnicity, current and former smokers were found to have higher pulmonary ACE2 expression than people who have never smoked. A further meta-analysis showed a statistically significant increase of 25% in pulmonary ACE2 expression in ever-smokers. The upregulation of ACE2, a SARS-COV-2 virus receptor, in smokers suggests an increased risk of infection and disease progression [20].

The data from the six studies were evaluated and smokers were found to be almost two times more likely (OR=1.92; 95% CI= 1.036-1.883) to experience severe COVID-19 symptoms than non-smokers. Given that approximately 62% of males and about 3.4% of females in China are smokers [21], the authors of this study suggested that more men suffer serious disease complications because more men smoke. Other countries like Ireland and Italy showed similar trends of higher cases of COVID-19 in males and higher rates of male smokers. However, this paper is limited by the lack of gender stratified data for smoking. This limitation prevents the conclusion of a cause-effect relationship between male gender and smoking with increased disease severity [22].

Influence of comorbidities on disease severity: COVID-19 disease severity is increased in patients with comorbidities. Existing research shows hypertension, diabetes, chronic lung disease, cardiovascular disease, and cerebrovascular disease to be the most common comorbidities in patients with severe COVID-19 outcomes [23,24]. A meta-analysis done on 61 cohort studies calculated an increased odds ratio of mortality from COVID-19 given a co-existing disease process such as chronic obstructive pulmonary disease (OR:3.7, CI 2.7–5.1), hypertension (OR:2.7, CI 1.7–4.4), diabetes (OR:2.5, CI 1.7–3.6), and cardiovascular disease (OR:4.7, CI 2.9–7.6). This data could serve as a prognostic tool for people with COVID-19 [25].

A retrospective study done on SARS-COV-2 positive patients in Wuhan, China demonstrated that patients with COVID-19 and history of Chronic Obstructive Lung Disease (COPD) were more likely to suffer severe disease outcomes. 67.9% of COVID patients with COPD were admitted to the ICU compared to 45.4% of COVID patients without COPD. The mortality rate was 78.6% in patients with both COVID and COPD compared to 36.0% in those without COPD. Furthermore, the proportion of males in the COVID with COPD patient group was significantly higher than the proportion of females. The researchers suggested this discrepancy was due to the 3-fold higher level of ACE2 expression in men, which makes them more susceptible to disease given the ACE2 receptor’s role in viral entry. Additionally, the researchers acknowledged that COPD is most common in China in men with a history of smoking [26].

Comorbidities were shown to be comparable between men and women in a case series analysis of COVID-19 mortality from Wuhan, China. Mortality was higher in males than in females across all ages, but gender-stratified data showed insignificant differences in the presence of comorbidities between the two groups. This implies the presence of comorbidities isn’t the cause of the gender discrepancy in COVID mortality. The data is, however, limited by the small study size [27,28].

Obesity and lifestyle: Obesity has been shown to be associated with increased risk of hospital admission and ICU care in COVID-19 patients less than the age of 60. The suggested mechanism of increased COVID severity in obese patients is related to the increased ACE-2 expression in adipose tissue. Since obese individuals have more adipose tissue, there is a larger amount of ACE2 receptor expression. Given that ACE2 receptors are necessary for viral entry, the increased expression is suggestive of higher susceptibility to viral penetration and infection [29,30]. Furthermore, adipose tissue can cause a decrease in chest wall compliance and thus compromise respiratory function if it accumulates around the diaphragm and ribs. Decreased total respiratory system compliance leads to an increased propensity to respiratory failure in the event of pulmonary injury. In addition, higher BMI is associated with exacerbated comorbidities like diabetes, hypertension, and cardiovascular disease - all associated with increased severity of COVID-19. All of these factors combined with the chronic proinflammatory state of obesity are suggested as mechanisms of greater COVID-19 susceptibility in patients with higher BMI [31].

A study done on 383 hospitalized COVID-19 patients in China demonstrated not only that obesity was associated with a higher rate of disease severity, but also that this association was stronger in men. Men with a BMI over 28 kg/m² were three times more likely to progress to severe COVID- 19 than men with a BMI below 28 kg/m². This association was not seen in women. However, the small population of obese women resulted in low power in the analysis of women and thus limited the study [32].

The role of gender norms in lifestyle habits has been suggested as another contributing factor to the higher rates of COVID-19 in men. Smoking and drinking are more prevalent in men, which result in increased comorbidities. Delayed health care seeking and less hand-washing are two other traits seen more in men. Both factors are acknowledged to be important preventative measures for disease. These negative habits are proposed as mechanisms for increased COVID susceptibility in men [33]. The role of women as caretakers both in and outside the home also poses an increased risk of disease among women, as about 70% of the world’s frontline healthcare workers are women. Yet, men continue to be more susceptible to mortality due to COVID-19.

Multiple research papers have been published regarding the varying effects of COVID-19 on men and women of all ages. Given the high infectivity rate and the impact COVID-19 has had on the world, the mechanisms behind the decreased mortality rates in women are imperative to understand. Doing so can help guide treatment and diagnostic plans for COVID patients. Here we re-establish the epidemiological data that confirms the increased morbidity and mortality rates of COVID-19 in men vs. women globally and review over 30 papers that have analyzed the differences in female and male biological susceptibility to viral entry, viral immune responses, respiratory function, comorbidities, and lifestyle risk factors in relation to COVID-19.

The literature presented makes clear that premenopausal women are relatively protected from disease via the strong immune responses by estrogen. Estrogen and progesterone are associated with increased nasal mucosa, implying better protection against foreign particles. In terms of immune function, estrogen induces more cytotoxic T-cell activity and virus-specific toll like receptor activity, both of which protect against viral infection, and are thus beneficial in the context of COVID-19. Ovariectomy studies of mice indicated that females also have less ACE2 expression in their lungs and ovaries, which suggests protection against COVID-19 entry into cells. Males, on the other hand, do not have strong nasal mucosal factors that can protect against particle entry and have high androgen mediated ACE2 expression. Further, testosterone is immune-suppressing and induces toll-like-receptors that are more effective at recognizing bacteria, which is not advantageous against the COVID-19 virus. Increased testosterone levels have been associated with increased COVID-19 infection rates. Low testosterone, however, is also associated with lower lung function and decreased testosterone levels were seen in men with severe COVID-19 disease progression, indicating that adequate testosterone is necessary for an appropriate physiological response. Men have also been found to have more auto-immunity to type 1 interferons, rendering them more susceptible to COVID-19. Androgen deprivation therapy (and possibly progesterone supplementation) could be a promising avenue of drug therapy for treatment and/or protection against COVID-19, as it is associated with decreased infection rates in pancreatic cancer patients. In the study done by Schroeder et al., COVID-positive women had high levels of testosterone. A study comparing COVID severity and outcome in female patients with and without Polycystic Ovarian Syndrome (PCOS) would provide a unique understanding of the effects of testosterone on disease progression in women. Similarly, an analysis of women with PCOS compared to men with similar levels of testosterone could help to further isolate the effects of testosterone from other gender-based differences.

However, it is unlikely that sex hormones and biology alone can explain the difference. Gender roles and lifestyle also play a role in determining disease severity. As shown in the section “Obesity and Lifestyle” higher rates of smoking, obesity, and comorbidities are all factors that result in more severe disease around the world. Analyses of men and women with similar rates of comorbidities showed higher mortality rates in men in the paper by Jin et. al [26] and higher mortality rates in women in the study by Hernandez-Garduno [27], thus suggesting unclear associations between comorbidities and their roles in gender stratified disease progression. Both smoking and obesity are associated with increased ACE2 expression suggesting more COVID-19 entry into cells. Many of these lifestyle factors, especially smoking and negative habits such as delayed health care seeking are higher among men indicating another way in which males are more susceptible to worse COVID-19 outcomes. Public health guidelines regarding habits to adopt to control the COVID-19 pandemic should take into account the differences between how men and women are affected by the disease.

In conclusion, acknowledging why men are more severely affected by COVID-19 can drive how we manage and prevent disease. Estrogen mediated enhanced viral clearance and decreased ACE2 receptor expression in females play a large role in protecting women relative to men. Potential avenues for therapeutics include emulating estrogen’s effect and blocking androgen-mediated ACE2 expression in men. The effects of testosterone in women could provide a better understanding of the hormone’s influence on disease susceptibility. More research needs to be done on COVID-19 and outcomes of viral infection in women with higher endogenous androgen levels. Further research into the area of which lifestyle factors affect COVID-19 outcomes would also be helpful in classifying which patients are at higher risk and which preventative measures to take. Stratified epidemiological data can help clarify associations between sex associated lifestyle differences and their influence on disease severity.

Limitations

This paper assumes that ACE2 receptors are the only point of entry for COVID-19 viral particles. However, there may be other factors that are not yet known or have not been considered. Global 50/50 dataset may have data that are under-reported. Many research papers that have been reported are largely limited by small study populations and lack of data on the COVID-19 virus’ mechanism. More research containing gender-stratified data is necessary to make stronger conclusions on the exact effects of sex and lifestyle factors influencing disease susceptibility.

The authors declare that they have no conflict of interests.

No funding was received for this study.