Scientists have estimated that an individual’s age does not indicate how likely it is to be infected with SARS-CoV-2.

However, the development of symptoms, disease progression, and mortality are age dependent.

There have been large numbers of deaths from the ongoing Covid-19 pandemic, and the elderly have been shown to develop disproportionately severe symptoms and have higher mortality rates, Medical Daily reported.

A team of scientists, including Associate Professor Ryosuke Omori of the Zoonosis Control Research Center at Hokkaido University, modeled available data from Japan, Spain and Italy to show that susceptibility to Covid-19 is independent of age while onset of symptomatic Covid-19 occurs, severity and mortality likely depend on age.

The causes of death in the elderly can be attributed to two factors: how likely is it that they are infected due to their advanced age (age-related susceptibility), which is reflected in the number of cases; and how likely it is that they will have a severe form of the disease due to their advanced age (age-related severity), which is reflected in the death rate.
These factors are not fully understood for Covid-19.

The scientists analyzed data from Italy, Spain, and Japan to see if there was a relationship between age, susceptibility, and severity. These three countries were chosen because they have well-recorded, publicly available data.

In May 2020, the death rate (number of deaths per 100,000) was 382.3 for Italy, 507.2 for Spain and 13.2 for Japan. Despite the large differences in death rates, the age distribution of mortality (the proportional number of deaths per age group) was similar for these countries.

The scientists developed a mathematical model to calculate the susceptibility in each age group under different conditions. They also took into account the estimated level of person-to-person contact in each age group, as well as different restrictions on outside activities in the three countries.

The model showed that the susceptibility must be unrealistically different between the age groups if it is assumed that age has no influence on severity and mortality. On the other hand, the model showed that age should not affect susceptibility, but should negatively affect severity and mortality to explain the fact that the age distribution of mortality is similar between the three countries.

Ryosuke Omori, from the Zoonosis Control Research Center at Hokkaido University, specializes in epidemiological modeling: the use of mathematics and statistics to understand and predict the spread of disease. Since the Covid-19 outbreak, he has struggled to determine the true extent of the spread of the pandemic in Japan and abroad.