Research has documented an excess of male relative to female deaths in the SARS-CoV2 (COVID-19) pandemic [1, 2] . This sex-dependent pattern has been observed also for other pathogenic coronaviruses (CoVs), including the severe acute respiratory syndrome (SARS)-CoV and the Middle East respiratory syndrome (MERS)-CoV [3] . The excess of severe male cases may provide clues as to the pathogenic effects of and the immunological responses to these effects. Males tend to generate less robust immune responses than females and are more susceptible to a variety of infectious agents including RNA viruses [4] [5] [6] . This has led to the suggestion that testosterone (among other sex-steroids) may have a negative effect on the human immune system [7] .
Here we consider the relationship between COVID-19 case fatality rates (CFR; a measure of the national severity of the disease) and digit ratio (2D:4D), a proxy for prenatal sex-steroid levels that correlates negatively with prenatal testosterone and positively with prenatal estrogen [8, 9] . National CFRs from COVID-19 vary considerably as do the percent male deaths per country [10, 11] . We examined the relationship between CFRs and percent male deaths from cases of COVID-19 and mean national values of 2D:4D in males and females.
J o u r n a l P r e -p r o o f Digit Ratio and Covid- 19 3 A straightforward hypothesis would be that high prenatal testosterone (low 2D:4D) is linked to immune suppression and high CFRs. However, strong immune responses in females may also lead to immunopathology, resulting in fatal outcomes [4] .
Another possibility is that testosterone facilitates cell entry by SARS-CoV2 and is one of the driving factors of the epidemic ("androgen-driven COVID-19 pandemic theory") [15, 16] . Infectivity of COVID-19 depends on priming of the spike proteins by transmembrane protease, serine 2 (TMPRSS2) [13] , and TMPRSS2 may cleave angiotensin converting enzyme 2 (ACE2) for augmented viral entry [14] . Importantly, androgen receptor activity is a requirement for the transcription of the TMPRSS2 gene, suggesting that testosterone facilitates SARS-CoV2 cell entry [15, 16] . The androgendriven COVID-19 pandemic theory concerns circulating levels of testosterone and has some support from the relationship between age-related COVID-19 mortality rates. For example, it may explain why children are more resistant to infections before adrenarche and puberty [15, 16] . Thus, high mortality from SARS-CoV2 in men should be related to hypergonadism and to low (masculinized) 2D:4D.
The androgen-driven theory would predict that low 2D:4D (high prenatal testosterone) correlates with high severity of COVID-19 cases. However, there are some theory-inconsistent anomalies with the relationship between age, testosterone, and COVID-19 in males. If high testosterone is associated with mortality, one should expect a mini-peak during the activation of the hypothalamic-pituitary-gonadal axis in the 6-8th
week after delivery of male neonates. In this period, the levels of sex steroids are similar to early-middle pubertal levels [17] . There is no support for elevated frequencies of COVID-19 at this age. Moreover, in men COVID-19 mortality rates increase with age [1] J o u r n a l P r e -p r o o f Digit Ratio and Covid- 19 4 but testosterone levels decrease [18] . The expression of ACE2 is sex-dependent with a higher expression in females compared to males, and in the latter, levels of ACE2 decline with age [19] . Paradoxically, ACE2 is necessary for SARS-CoV2 cell entry, but its expression correlates negatively with mortality from the virus. Thus, there is the possibility that the severity of COVID-19 relates to male hypogonadism and not hypergonadism and that high (feminized) 2D:4D in men relates to increased severity of cases.
We examined the association between mean national values of 2D:4D and the CFRs, and percent of male mortality across 41 nations, thus assessing the two competing views of the relationship between testosterone and prognosis in cases of COVID-19. A negative association would support the androgen-driven COVID-19 pandemic theory, whereas a positive association would indicate that male hypogonadism relates to high severity of COVID-19 cases.
The BBC Internet Study was a multi-national and multiethnic online survey that included around 200 questions concerning sex-dependent aspects of demographics and behavior, along with self-measurement of the lengths of index finger (2D) and the ring finger (4D.) Reimers [19] reports the details of recruitment and ethical issues in the Study. A sample of 255116 participants from >100 countries completed all sections of the Study. The most commonly represented nationalities were the United Kingdom [20] have examined the effects of sex and ethnicity on 2D:4D in the Study. Means of 2D:4D from the Study showed the expected sex difference (males < females) and this extended across all ethnic groups. Previous research including the Study data (e.g., [20, 21] ) restricted the age of participants to ≥ 18 years and the 2D:4D range of 0.80 to 1.20.
This had the effect of reducing SD's to about 0.05. In the present study, we adopted these restrictions, thus leaving for analysis 2D:4D of 103482 men and 83366 women.
The COVID-19 CFR statistics were obtained on April 21st from the World Health Organization (WHO) reports [10] and gender-segregated data from Global Health 50/50
[11]. The WHO reports CFRs by country but without ethnic details. Therefore, we included all ethnic groups in the national means for 2D:4D.
There were 41 nations in the sample (Table 1) Advocates of the androgen-driven theory have suggested treatment with androgen blockers such as spironolactone [15, 16] . This seems reasonable given the substantial excess in male mortality associated with the disease [22] . However, there is support for the counter-theory that treatment of COVID-19 should involve an increase the amount of ACE2 in the lungs [23] . ACE2 protects from lung injury, but ACE2 is also the critical SARS-CoV receptor. The severity of COVID-19 could be explained by SARS-CoV spike protein binding to ACE2, which leads to endocytosis of the virus but the loss of ACE2 from the surface of the cell. This establishes a circle of viral infection and local loss of lung protection. In support of this model, Monteil et al. [24] showed that soluble human ACE2 molecule could significantly inhibit SARS-CoV-2 infections and reduce viral load by a factor of 1,000-5,000.
Our findings support an association between low prenatal testosterone (high 2D:4D) and high severity of COVID-19, and high mortality in males. They are in accord with the knowledge of the variation in expression and regulation of the SARS-CoV2 receptor ACE2 gene. The ACE2 gene is more strongly expressed in females compared to males, in young males compared to old males and in healthy men compared to those with several co-morbidities including type 2 diabetes [23] . These associations suggest a negative correlation between ACE2 expression and CovID19 fatality at both population and molecular levels, which will be instrumental when designing potential prevention and treatment strategies for ACE2 binding coronaviruses in general [23] . The downregulation of ACE2 may therefore be associated with poor prognosis from COVID-19.
The finding that soluble ACE2 reduces viral load substantially supports this position [24] .
Digit Ratio and Covid-19 9
Hence, ACE2 [24] and alternatives to ACE2 such as spironolactone [22] may constitute treatments for COVID-19.
ACE2 is also expressed in the Leydig cells of the testes, which indicates a regulatory function in spermatogenesis [25] [26] [27] . Research suggests that testosterone in men and estrogen in women up-regulates ACE2 [23] . A comparison of testicular function in COVID-19 patients and healthy controls showed similar testosterone levels in both groups but higher LH in the former, suggesting that the patients' pituitary has to work harder than that of the controls to maintain testosterone levels. This may be evidence that COVID-19 damages testicular function, however the same results would be obtained if the patients had compromised gonadal function before infection [28] . It may be relevant that men who have few or no sperm and low testosterone have high 2D:4D [29, 30] .
Moreover, the severity of COVID-19 is associated with cardiovascular disease and high 2D:4D correlates with early heart attacks in men [31] . Such correlations support an association between high 2D:4D and high CFRs.
In conclusion, we have found that mean male 2D:4D per nation correlates positively with CFR and percent of male deaths due to COVID-19. The effect was independent of mean female 2D:4D. Thus, high prenatal testosterone (low 2D:4D) in men may be protective of the serious effects of COVID-19. Our findings suggest that an excess of male deaths resulting from COVID-19 is not the product of high prenatal Tables   Table 1. COVID 