The age of an individual does not indicate how likely they are to be infected by Covid-19, but development of symptoms, progression of the disease, and mortality are age-dependent.
A study published in Scientific Reports modeled available data from Japan, Spain and Italy to determine if any relationship between age, susceptibility and severity. These three countries were chosen as they have well recorded, publicly available data.
As of May 2020, the mortality rate (number of deaths per 100,000) was 382.3 for Italy, 507.2 for Spain and 13.2 for Japan. However, despite the wide disparity in mortality rates, the age distribution of mortality (the proportional number of deaths per age group) was similar for these countries.
Variation of susceptibility to symptomatic infection by age
Causes of mortality in older individuals may be due to two factors: how likely they are to be infected due to their advanced age (age-dependent susceptibility), which is reflected in the number of cases; and, how likely they will be affected by a severe form of the disease due to their advanced age (age-dependent severity), which is reflected in the mortality rate. These factors are not fully understood for Covid-19.
The scientists developed a mathematical model to calculate susceptibility in each age group under different conditions. They also factored in the estimated human-to-human contact level in each age group, as well as varying restriction levels for outside-home activities in the three countries.
The model showed that the susceptibility has to be unrealistically different between age groups if they assume age does not influence severity and mortality. On the other hand, the model indicated the age should not influence susceptibility but should negatively influence severity and mortality, to explain the fact that the age distribution of mortality is similar between the three countries.