All over the world – in China, Italy, the United States and Australia – many are .
Why? Is it genes, hormones, the immune system – or behaviour – that makes men more susceptible to the disease?
俺也去俺也去电影网I see it as an interaction of all of these factors and it isn't unique to the SARS-Cov-2 virus – the different response of men and women is typical of many diseases in many mammals.
The grim figures
In deaths of men are more than double those of women. In men constitute about . Australia is shaping up to have , though here it's mostly in the .
俺也去俺也去电影网One major variable in severity of COVID-19 is age. But this can't explain the sex bias seen globally because the increased male fatality rate is the . Women also live on average , so there are more elderly women than men in the vulnerable population.
俺也去俺也去电影网The other major factor is the presence of chronic diseases, particularly , and . These are all more common in men than women, which .
俺也去俺也去电影网But then we must ask why men are more vulnerable to the diseases that put them at greater risk of COVID-19.
Men and women are biologically different
in their sex chromosomes and the genes that lie on them. Women have two copies of a mid-sized chromosome (called the X). Men have only a single X chromosome and a small Y chromosome that contains few genes.
One of these Y genes () directs the embryo to become male by kick-starting the development of testes in an XY embryo. The testes make male hormones and the hormones make the baby develop as a boy.
俺也去俺也去电影网In the absence of SRY an ovary forms and makes female hormones.
It's the hormones that control most of the obvious visible differences between men and women – genitals and breasts, hair and body type – and have a large influence on behaviour.
The Y chromosome and hormones
俺也去俺也去电影网The Y chromosome contains hardly any genes other than SRY but it is full of repetitive sequences ("junk DNA").
俺也去俺也去电影网Perhaps a "toxic Y" could lose its regulation during . This might hasten ageing in men and render them more susceptible to the virus.
俺也去俺也去电影网But a bigger problem for men is the male hormones unleashed by SRY action. Testosterone levels are implicated in many diseases, particularly heart disease, and .
俺也去俺也去电影网Men are also disadvantaged by their , which protects women from many diseases, including heart disease.
Male hormones also influence behaviour. have been credited with major differences between men and women in risky behaviours such as smoking and drinking too much alcohol, as well as reluctance to heed health advice and to seek medical help.
The extreme differences in in China (almost half the men smoke and only 2 percent of women) may help to account for their very high ratio of male deaths (more than double female).
Not only is smoking a severe risk factor for any respiratory disease, but it also causes lung cancer, a further risk factor.
俺也去俺也去电影网Smoking rates are lower and not as sex-biased in many other countries, so risky behaviour can't by itself explain the sex difference in COVID-19 deaths. Maybe sex chromosomes have other effects.
Two X chromosomes are better than one
The X chromosome bears more than 1,000 genes with functions in all sorts of things including routine metabolism, blood clotting and brain development.
The presence of two X chromosomes in XX females provides a buffer if a gene on one X is mutated.
XY males lack this X chromosome backup. That's why boys suffer from many sex-linked diseases such as (poor blood clotting).
The number of X chromosomes also has big effects on many metabolic characters that are separable from sex hormone effects, as .
俺也去俺也去电影网Females not only have a double dose of many X genes, but they may also have the benefit of two different versions of each gene.
This X effect goes far to explain why at every age from birth.
俺也去俺也去电影网And another man problem is the immune system.
俺也去俺也去电影网We've known for a long time that . This is not all good, because it makes women more susceptible to autoimmune diseases such as lupus and multiple sclerosis.
俺也去俺也去电影网But it gives women an advantage when it comes to susceptibility to viruses, as many show. This helps to explain why men are more susceptible to many viruses, including SARS and MERS.
There are at least 60 immune response genes on the X chromosome, and it seems that a higher dose and having two different versions of these gives women a broader spectrum of defences.
Sex differences in diseases – the big picture
俺也去俺也去电影网Sex differences in the frequency, severity and treatment efficacy for many diseases were . COVID-19 is part of a larger pattern in which males lose out – at every age.
This isn't just humans – it is true of most mammals.
Are sex differences in disease susceptibility simply the by-catch of genetic and hormone differences? Or were they, like many other traits, selected differently in males and females because of differences in life strategy?
俺也去俺也去电影网It's that male mammals spread their genes by winning competitions for mates, hence hormone control of risky behaviour is a plus for men.
俺也去俺也去电影网It's also suggested female mammals are selected for traits that enhance their ability to care for young, hence their stronger immune system. This made sense for most mammals through the ages.
So the sex bias in COVID-19 deaths is part of a much larger picture – and a very much older picture – of sex differences in genes, chromosomes and hormones that lead to very different responses to all sorts of disease, including COVID-19.
, Distinguished Professor of Genetics, .
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