The age of a person may not decide how likely they are infected by SARS-CoV-2, which causes COVID-19, according to a study which found that development of symptoms, severity of the disease, and mortality are age-dependent.
It has been shown that elderly people disproportionately develop severe symptoms of COVID-19 and show higher mortality.
Scientists, including those from Hokkaido University, modelled available data from Japan, Spain and Italy to show that susceptibility to COVID-19 is independent of age, while occurrence of symptomatic COVID-19, severity and mortality is likely dependent on age.
According to the study, published in the journal Scientific Reports, causes of mortality in elderly individuals may be due to two factors.
The first is how likely they are to be infected due to their advanced age, which is reflected in the number of cases, and the second is how likely they will be affected by a severe form of the disease due to their advanced age, which is reflected in the mortality rate.
These factors are not fully understood for COVID-19, the researchers said.
They chose to analyse data from Italy, Spain and Japan to determine any relationship between age, susceptibility and severity as these countries 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, the researchers said.
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, they said.
The researchers 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 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.