Quantitative analyses on the global coronavirus pandemic

COVID-19 Infection and Mortality Rates as a Function of Age and Gender

With over 2.4 million COVID-19 cases and over 160,000 deaths globally, there is a lot of data that can be analyzed to better understand this virus as a function of age, co-morbidities, medical history (such as prior exposure to coronaviruses like SARS, and other vaccines like BCG), etc.  The two important factors that have been noted since the early days of this pandemic are age and gender and they continue to be the most important factors.  We look at the age and gender dependencies with the best single data set provided by the Spanish Health Department containing over 200,000 cases (or 0.4% of the total population) and over 20,000 deaths.  Spain has tested over 2.0% of its population which while short of a comprehensive sample does begin to give us some ideas about how the disease has affected the entire country. 

We look at the probability of a random person acquiring the disease as a function of age.  We know that this sample is not truly random nor comprehensive but it does give an intriguing result that should be examined further.  The figure below shows that an 80+ years old person has about 100 times the likelihood of getting infected compared to a person under 10 yrs old (the left axis measures percent of the population a certain age group has been infected with 0.4% of the population as a whole).  Some of this large difference may result from the fact that an 80+ years old person has a higher probability of getting approved for a test than a young person.  But if there is a real effect, it is all the more important that we perform random testing of a large sample of people to find the true infection rate by age.  The infection rate varies very little by gender.  Slightly more women than men get the disease, 53.6%, but Spain is 51.0% female.  

Another interesting question to ask is how and why the mortality for this disease varies so significantly by age and gender.  For regular seasonal flu, there is a modest mortality differential by age with older people more likely to die from it than younger people.  However, with COVID-19 someone older than 80+ has nearly 100 times the chance of dying than someone younger than 30 yrs old (see figure below where the mortality rate as a percentage is shown on the left axis).  One important note is that this age variation is continuous and not a step function as some people commonly assume.  Between 35 and 75 the mortality risk doubles for every 7 years increase in age.  There is no cliff at 65+ where your risk increases all of a sudden — the risk increases continuously for both men and women above the age of 35.  The graph does show that men (dashed grey line) are at a higher risk of dying from this disease than women (dotted red line), by about a factor of 2 for all ages from 35–75, with the gap widest at 2.6x in the 50–59 age group.   

The same mortality versus age trend was observed in Italy (shown as the solid blue line in the figure above).  Italian mortality was generally 20% higher than for Spanish most likely due to the fact that the sample taken as of March 30 was biased toward 1.25 times more males than females.  (However, the underlying population split was the same 51% female to total as Spain so it is unclear why Italian men are more likely to get infected than Spanish men vs women — probably a large sampling bias.)  The same mortality versus age and gender relationships were also observed in China.  So this age and gender effect seems to apply to this pandemic globally.

If both infection rate and mortality rate are strongly biased against the elderly it means that an 80+ years old man has 10,000 times higher chance of dying from COVID-19 than a kid — an astonishing disparity.


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  1. Anonymous

    So there appears to be strong association in the statistics. What are the causes for this fatality rate?

    • William Ku

      There is a very strong dependence of case fatality rate on age for COVID-19. No one knows the root cause of this behavior. Common seasonal flu has a similar but much less remarkable age dependence killing more older patients than younger patients by a ratio of 10:1 rather than 1000:1 seen for COVID-19. The Spanish flu of 1918 had a very different age dependence killing many young adults of working age.

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