Figures have shown that men are twice as likely as women to die from the disease but experts have been unable to determine why.
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Now researchers in Germany say that the male sex hormone may hold the answer.
Testosterone is key in how the male body regulates various immune responses, such as fighting viral infections, but women produce small amounts in their ovaries.
The scientists analysed the hormone levels of the first 45 confirmed Covid-19 patients admitted to intensive care at the University Medical Center Hamburg-Eppendorf.
These included 35 men and 10 women – of which nine of the men and three of the women later died.
Samples were taken from each patient and tested for 12 hormones, including testosterone and dihydrotestosterone.
They found that more than two-thirds – 68.6 per cent – of the male patients had low levels of testosterone.
And 60 per cent of the female patients had elevated testosterone levels.
The experts said that while low levels of testosterone cannot control the immune response in men, for women, higher testosterone levels were linked to a more significant inflammatory response.
Professor Gülsah Gabriel, from the Leibniz Institute for Experimental Virology in Hamburg, who was involved in the research, told MailOnline: "The majority of male Covid-19 patients had low testosterone levels.
"Of those male Covid-19 patients who died, the majority also had low testosterone levels.
"Thus, low testosterone levels in men seem to be a risk factor for severe and even fatal disease outcome in men upon infection with so-called 'cytokine inducing' respiratory viruses."
SARS-CoV-2, the virus which causes Covid-19, has been found to trigger the body into over producing cytokines – chemicals which flood to the site of infection to try and kill the pathogen.
But in Covid-19 patients, the immune system responds too aggressively and can cause the process to get out of control – known as a cytokine storm.
This overreaction by the body can cause severe lung damage, acute respiratory distress syndrome (ARDS) and even death.
In their study, the researchers wrote: "With SARS-CoV-2 continuing to infect humans worldwide, it was repeatedly reported that men with Covid-19 are at higher risk to develop severe and even lethal outcome compared to women, independent of age.
"Thus, it has become of utmost importance to understand why men are more likely to die from Covid-19 than women."
Their findings were published on MedRxiv – an online archive – and not in a journal as the research is yet to be peer-reviewed by other academics.
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